behavior and ecology of pacific white-sided dolphins...
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BEHAVIOR AND ECOLOGY OF PACIFIC WHITE-SIDED DOLPHINS (Lagenorhynchus obliquidens) IN MONTEREY BAY, CALIFORNIA
A thesis submitted to the faculty of San Francisco State University
in partial fulfillment of the requirements for the
degree
Master of Science in
Marine Science
by
Nancy A. Black
Pacific Grove, California
December, 1994
Copyright by Nancy A. Black
1994
BEHAVIOR AND ECOLOGY OF PACIFIC WHITE-SIDED DOLPHINS
(Lagenorhynchus obliquidens) IN MONTEREY BAY, CALIFORNIA
Nancy A. Black San Francisco State University
1994
Between 1 987 and 1991, the distribution, relative abundance, behaviors,
and food habits of Pacific white-sided dolphins (Lagenorhynchus obliquidens)
were investigated in Monterey Bay, California and surrounding waters.
Relative abundance of Pacific white-sided dolphins was greatest near
the shelf-break and up to 10 km beyond it, in Carmel Bay, and near the northern
rim of Monterey Submarine Canyon. Mean (±SO) group size of dolphins was
203±395.4, 50.6% of groups contained 50 or fewer dolphins. During the
upwelling season (Mar-Jul), relative individual and group abundance was low
and group sizes were small. During the oceanic season (Aug-Oct), relative
group abundance was high, and dolphins often fed and milled in dispersed
subgroups. During the Davidson Current season (Nov-Feb), relative individual
abundance was high, and group sizes were large.
Pacific white-sided dolphins were observed milling, 33.3% of the time,
feeding 23.9%, traveling 21.9%, socializing 17.9%, and resting 3.0%. Pacific
white-sided dolphins were observed feeding significantly more in shallower
depths, closer to the shelf-break, and in areas with greater bottom relief
compared to other behaviors. Dolphins were observed traveling significantly
more often in deeper depths, further from the shelf-break, and in the largest
cohesive groups. Pacific white-sided dolphins had the greatest coefficient of
association values with northern right whale dolphins. Risso's dolphins, and
California sea lions.
Three radio-tagged Pacific whlte-sidecJ dolphins exhibited a mean (±SO)
dive duration of 23.5±1 .92 sec, mean (±SO) respiration rate of 2.5±.32, with a
mean (±SO) speed of 7.6±2.1 9 km/hr.
Fifteen anomalous-colored Pacific white-sided dolphins were
photographically identified, thirteen of which were predominantly white in color.
These dolphins were resighted from one to eight times during particular oceanic
seasons, as well as among oceanic seasons in different years which indicated
particular dolphins frequented the Monterey area over variable periods of time,
rather than new groups continually moving through.
Relative individual and group abundance, and sigrting distance to the
shelf-break were positively correlated to temperature and the near-shore fronte.l
gradient. When sea surface temperature anomaly was high, dolphins were
more abundant and occurred closer to the shelt-·break.
Pacific whiting, plainfin midshipman, northern anchovy, Sebastes sp.,
Gonatidae, Loligo, and Onychoteuthis were the most importullt prey of Pacific
white-sided dolphins found dead along the central California coast.
Pacific white-sided dolphins occurred year-round, were seasonally
abundant, were not randomly distributed, and were frequently observed
feeding. The;efore, certain locations in the Monterey Bay area are important for
these dolphins, providing a predictable and abundant toad source.
ACKNOWLEDGMENTS
I thank my committee members, Dr. Bernd WOrsig who initially
encouraged and advised me during t!;le initial phases of this project, Dr. James
Harvey who greatly assisted me during the final phases, and Dr. Gregor
Cailliet.
Tracy Thomas, captain of the AN Ricketts from Moss Landing Marine
Laboratories, spent many days at sea searching for dolphins with me, and I
thank Tracy as well as Mike Prince for allowing me extensive boat time on the
Ricketts as well as the Boston Whalers.
I especially thank Debra Shearwater of Shearwater Journeys for
generously providing me with many days of boat lime on her natural history
trips, for her interest in this project, and for often remaining with dolphins long
enough for me to photograph them. I also especially thank Richard Ternullo, •'
captain of the MN Pt. Sur Clipper, for his exceptional ability to maneuver
around dolphins, extensive knowledge and insight of animals inhabiting the
Bay, and for his encouragement throughout. Richard also provided me with
additional sighting data and reviewed drafts of this work.
I thank Alan Baldridge for sharing his vast knowledge about marine
mammals of Monterey Bay, and for his advise and interest in this project. I
greatly appreciate help from Tom Kieckhefer who offered invaluable
assistance with all aspects of this project. Tom as well as Pamela Byrnes and
vi
Tom Jefferson reviewed and improved drafts of this work.
I also thank Sheila Baldridge, Dr. Randy Wells, Dr. John Hall, Susan
Kruse, Dawn Goley, Carol Keiper, David Lemon, Charlie Denney, Steve
Bailey, Ron Branson, Eric Dorfman, Dave Ekdahl, Peter Pyle, Francisco
Chavez, Steven Ramp, and Dave Husby. I also thank the many people who
assisted with surveys and the radio-tracking efforts. I thank my parents for their
encouragement and support through this effort.
Robert Jones (U.C. Berkeley), Long Marine Laboratory, Moss Landing
Marine Laboratories, and California Academy of Sciences collected stranded
dolphins and provided stomach contents. William Walker, of the National
Marine Mammal Laboratory, Eric Hochberg, of Santa Barbara Museum of
Natural History, and James Harvey assisted with identification of cephalopod
beaks and fish otoliths.
This work was supported in part by Earl and Ethyl Meyers
Oceanographic Foundation, Monterey and Los Angeles chapters of the
American Cetacean Society, Cetacean Society International, American
Museum of Natural History, National Geographic Society, and Moss Landing
Marine Laboratories, Sam's Fishing Fleet.
vii
TABLE OF CONTENTS
List of Tables............................................................................................................ x List of Figures ........................................ : .................................................................. xi Chapter 1 Introduction ................................................................................................................. 2 Methods 7
siui:iy.Area::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::.:::::::::::::::::::::::::::::::::::::.".7 Surveys ........................................................................................................... 9 Distribution and Relative Abundance Analysis ..................................... 13 Behavior Analysis ....................................................................................... 14 Radio-Tag and Track .................................................................................. 17 Environmental Data Analysis and Correlations ................................... 18
RMU~ 20 ofstrihutlaii··a.iic:i·FieiaHvei.Ai:iiiiiCiaiice······················································2o Behavior Observations ...................................................... 34 Environmental Factors ·a.iid.8eiiavfar······················································s4 Social Factors and Behavior ······················································ 41 M u It i-S pecies Associations_.::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::: 45 Calves 50 Photo-iCie·ri-tiilcatiC!ri···················································································· 51 Radio-Track ·····················································································53 Environ me ntiil. Carre iaifo ii s:: :::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::: 55
Discussion 60 Distrfi:iiitfaii··································································································· 60 SeasonalitY:::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::: 64 Behavior 68 Photo-ldeiitlflcatfaii·····················································································7s Radio-Track ..................................................................................... 73 Environmenta"l.ca·riCiftfoiis::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::: 75
Chapter 2 lntroduction ............................................................................................................. -88 Methods .................................................................................................................... 91
viii
Results ......................................................................................................................... 93
Discussion ................................................................................................................ 1 02
References ............................................................................................................... 115
ix
LIST OF TABLES
Table Page
1. Mean depth (m), distance to the shelfbreak (km), and contour index
for quadrats with predominant behaviors exhibited by Pacific white-
sided dolphins ................................................................................................. 39
2. Coefficient of association values for all cetacean multi-species groups
encountered during the study period .......................................................... 46
3. Anomalous colored Pacific white-sided dolphins, including dolphin
number, date, group size, associated species and their group size ..... 54
4. Summary of radio-track data ......................................................................... 56
5. General oceanographic characteristics of the three seasons as
related to Pacific white-sided dolphin occurrence and relative
abundance ....................................................................................................... 76
6. Mean (±SO) of eight environmental variables for the three
oceanographic seasons during the study period .................................... ..77
7. Prey of 16 Pacific white-sided dolphins collected off central
California. 94 ----·····································································································
8. Percent frequency of occurrence of Pacific white-sided dolphin prey
from southern California, Monterey Bay, northern California, and
Washington .................................................................................................... 1 04
X
LIST OF FIGURES
Figures Page
1. Study area encompassing Monterey Bay, California and surrounding
waters .............................................................................................................. .
2. (A) Survey effort: number of km traversed in each quadrat during
dedicated Pacific white-sided dolphin surveys and opportunistic
8
surveys. (B) Additional effort covered by sport fishing vessels .............. 12
3. Relative abundance of Pacific white-sided dolphins represented as
number of dolphins per km per quadrat. .................................................... 21
4. Relative group abundances of Pacific white-sided dolphins represented
as number of dolphin groups per km per quadrat. ................................... 22
5. Number of occurrences of the distance Pacific white-sided dolphin ..
groups were sighted from the shelfbreak (km) ......................................... 23
6. Observed and expected percent frequency of occurrence of Pacific
white-sided dolphin sightings among five temperature categories ...... 25
7. Observed and expected frequency of occurrence of Pacific white-
sided dolphin sightings among five contour index classes .................... 26
8. Number of occurrences of Pacific white-sided dolphin group sizes. ____ 27
9. Mean group size of Pacific white-sided dolphins per quadrat. 28
xi
10. Pacific white-sided dolphin sightings by season and group size ......... 30
11. Percent frequency of occurrence of Pacific white-sided dolphin
group sizes for each oceanographic season ............................................ 31
12. Mean Pacific white-sided dolp~in gourp size (±SE) for each month
and oceanographic season ......................................................................... 32
13. Relative group abundance of Pacific white-sided dolphins, represented
as the mean (±SD) number of dolphin groups per km each month and
oceanographic season ................................................................................. 33
14. Relative abundance of Pacific white-sided dolphins, represented as
the number of dolphins per km for all months and seasons during the
study period ..................................................................................................... 35
15. Mean depth (m), mean distance to the shelfbreak (km), and mean
contour index for Pacific white-sided dolphins engaged in milling,
travellin~, socializing, and feeding ............................................................. 36
16. Quadrats where Pacific white-sided dolphins exhbited predominant
behaviors (mill, social, feed, travel) ............................................................ 38
17. Percent frequency of occurrence of behaviors exhbited by Pacific
white-sided dolphins during morning, mid-morning, and afternoon .... 40
18. Mean Pacific white-sided dolphin group size for each behavior .......... 42
19. Percent frequency of occurrence of Pacific white-sided dolphin group
size categories among behaviors ............................................................... 43
xii
20. Percent frequency of occurrence of group cohesiveness, aerial
behavior, and multi-species associations among behaviors exhibited by
Pacific white-sided dolphins ................ : ...................................................... 44
21. Mean group size of Pacific white-.sided dolphins, northern right whale
dolphins, and Risso's dolphins for single species groups and multi-
species groups when associated with one another ................................ 47
22. Percent frequency of occurrence for behaviors that Pacific white-sided
dolphins were engaged in while associating with other species. ________ 49
23. Example of anomalous-colored Pacific white-sided dolphins ......... _____ 52
24. Dive duration histogram, mean dive duration, percent pattern type
for radio-tagged Pacific white-sided dolphins ......................................... 57
25. Mean evironmental variables and dolphin measurements .................... 58
26. Mean estimated standard lengths (em) and estimated weights (g) for
" eight fish species ............................................................................................ 95
27. Mean estimated mantle lengths (em) and estimated weights (g) for
13 cephalopod genus/species ........................................ ~---····· .. ····-------···· 96
28. Frequency histograms of estimated standard lengths (em) for four
fish species found in Pacific white-sided dolphin stomachs ................. 98
29. Frequency histograms of estimated weight (g) for four fish species
found in Pacific white-sided dolphin stomachs ........................................ 99
xiii
30. Frequency histograms of estimated mantle lengths (em) for eight
cephalopod genus/species found in Pacific white-sided dolphin
stomachs ........................................................................................................ 1 00
31. Frequency histograms of estimatec[ weights (g) for eight cephalopod
genus/species found in Pacific white-sided dolphin stomachs ........... 1 01
xiv
CHAPTER 1
DISTRIBUTION, RELATIVE ABUNDANCE, AND BEHAVIOR OF PACIFIC WHITE-SIDED DOLPHINS, IN MONTEREY BAY, CALIFORNIA
1
2
INTRODUCTION
Distribution, movements, and behavior of many species of terrestrial
mammals are correlated with several environmental variables. Food type,
availability, and distribution directly influences the ecology of these vertebrates.
Terrestrial social mammals, such as baboons (Papio cynocephalus; Dunbar
and Dunbar 1 975), chimpanzees {Pan troglodytes; Goodall 1986), lions
{Panthera leo; Schaller 1972), and elephants (Loxodonta africanus; Moss
1 988), living in areas with seasonal variations in food supply generally exhibit
flexible group structures and associations. However, the core social unit,
usually matriarchal linkages, remains intact regardless of environmental
conditions. In contrast, social mammals with stable year-round food supplies,
such as mountain gorillas (Gorilla gorilla beringei) have relatively fixed social
groups with small overlapping home ranges and no territorial defense (Schaller
1963, Fossey 1983).
Similar ecological correlations occur with cetaceans, although describing
patterns in the marine environment is difficult. Cetacean prey often are
unknown, or clumped, and spatially and temporally variable. The occurrence
and distribution of baleen whales within feeding areas is non-random and
related to oceanographic features (e.g. fronts, eddies, upwellings, and
physiography; Gaskin 1982, Brown and Winn 1 989), where specific prey
species concentrate (Murison and Gaskin 1989, Piatt et al. 1989, Reilly and
Thayer 1990, Schoenherr 1991 ). The characteristics and types of prey also
influence behavior and aggregations of whales (Jurasz and Jurasz 1979,
Wursig et al. 1984, WOrsig et aL 1986, Dolphin 1987, Guerrerro 1989,
Kieckhefer 1992).
3
In contrast to baleen whales, which migrate seasonally to specific
feeding areas, most small cetaceans exhibit more subtle seasonal changes in
distribution, abundance, and behavior (Wursig 1989). The interpretation of their
ecological patterns, however, can be difficult, because dolphins feed on diverse
and poorly understood fish and cephalopod prey. The availability and
distribution of food resources, predation pressure, physical characteristics of the
environment, sex and age class segregation, and reproductive status influence
the ecology of small cetaceans (Miyazaki and Nishiwaki 1978, Norris and Doh I
1 980b, Wells et aL 1980, Myrick et al. 1986, Wells 1991 ).
Seasonal changes in oceanographic conditions within specific habitats
commonly affect prey and their predators. Distribution and seasonal movements
of odontocetes are related to temperature (Gaskin 1968, Evans 1975,
Leatherwood et al. 1980, Wursig and Wursig 1980), bathymetric features
(Evans 1971, Hui 1979, Doh! et al. 1986, Kenney and Winn 1 986, Selzer and
Payne 1988), currents and water masses (Gaskin 1968, Miyazaki et al. 1974,
Kasuya and Jones 1984, Au and Perryman 1985, Smith et al. 1986, Reilly
1990), and a combination of environmental factors (Smith and Gaskin 1983,
Watts and Gaskin 1985, Dorfman 1990). In a few cases, the occurrence of
odontocetes has been directly correlated with prey type and availability (WOrsig
and Wursig 1980, Shane 1984, Kenney and Winn 1986, Selzer and Payne
1988, Scott et aL 1990, Felleman et aL 1991).
Behavior and group structure of coastal dolphins is related to depth
(WOrsig and WOrsig 1980, Shane 1990, Cipriano 1992), distance to shore
(Cipriano 1992), physiography (Norris et a1. 1994, Heimlich-Baran 1988, Scott
et al. 1990, Felleman et al. 1991, Cipriano 1992), time of day (Brager 1993),
season (Hui 1979, Saayman and Taylor 1979, Shane 1990, Cipriano 1992,),
and prey patterns (WOrsig and Bastida 1986).
4
The behavioral ecology of pelagic dolphin species is generally poorly
known because they are usually not visible from shore. Exceptions include
common dolphins, Delphinus spp. off southern California (Evans 1971, Evans
197 4, Evans 1975, Hui 1979, Dahl et al. 1986), and in the northwestern Atlantic
(Kenney and Winn 1986, Seizer and Payne 1988), Stene/la spp. in the eastern
tropical Pacific (Perrin et al. 1973, Perrin et al. 1979, Au and Perryman 1985,
Polacheck 1987, Reilly 1990), Atlantic white-sided dolphins (Lagenorhynchus
acutus) in the northwestern Atlantic (Kenney and Winn 1986, Seizer and Payne
1988), and Risso's dolphin (Grampus griseus) off central California (Kruse
1989). In some cases, methods used to study coastal species, such as photo
identification, radio-tracking, and stomach content analysis, have been used
successfully with a few pelagic species.
The Pacific white-sided dolphin (Lagenorhynchus obliquidens) is one of
the most abundant pelagic species of dolphins endemic to the temperate North
Pacific (23 oN to 61 oN; Leatherwood et al. 1984). There is a smaller northern
form and a larger southern form that cannot be distinguished at sea, with a
distributional overlap in the Southern California Bight (Walker et al. 1986).
These dolphins commonly occur in groups of less than several hundred but can
5
form herds containing thousands of individuals, often in association with
northern right whale dolphins (Lissodelphis borealis). They feed
opportunistically on a variety of schooling fishes and cephalopods (Stroud et al.
1981). Calving occurs from May through September (Brown and Norris 1956,
Ridgway and Green 1967, Harrison et al. 1969, Dohl et al. 1983) corresponding
to a mid to late summer breeding season (Ridgway and Green 1967).
Off California, Pacific white-sided dolphins inhabit productive continental
shelf and slope waters (Fiscus and Niggol1965, Dahl et al. 1983, Leatherwood
et al. 1984) generally within 185 km of shore (Barlow in press). Although
seasonal movements are not well documented, Dohl et al. (1983) found that this
species was the most abundant cetacean off central and northern California,
with greatest abundance during fall and winter and lowest during spring. Near
northwestern Baja, California, southern California and Monterey Bay, dolphins
are seen year-round and some may be resident with seasonal changes in
abundance (Leatherwood et al. 1984).
Aside from knowledge of Pacific white-sided dolphin general distribution
and abundance, factors influencing their behavior, local distribution, and
occurrence are not well known. Because Pacific white-sided dolphins were
known to frequent the Monterey Bay area year-round in the near-shore, but
deep waters of the Monterey Submarine Canyon (A. Baldridge, R. Ternullo,
pers. comm.), and were relatively accessible, a detailed study was conducted
between 1987 and 1991. This study investigated the importance of Monterey
Bay to Pacific white-sided dolphins, based on the null hypotheses that dolphins
are distributed randomly throughout the bay, are year-round residents with daily
behavioral patterns, and are not influenced by variations in oceanographic
conditions. Therefore, the study objectives were to: (1) determine year-round
distribution and relative abundance; {2) identify behavioral patterns; and (3)
relate abundance, distribution, and behavior to physiography and
environmental variables.
6
7
METHODS
Study Area
The study area included 1 ,062 km2 off the central California coast
between 36.2 °N and 37.0 °N (Fig. 1). with water depth of 10m to 2,800 m. The
area east of a line drawn between Santa Cruz and Pt. Pinos was considered
Monterey Bay, west of the line was considered outer Bay waters. The Monterey
Submarine Canyon is the most prominent bathymetric feature in the area. It
begins 100 m off Moss Landing and extends 82 km offshore. The canyon
divides the bay on a roughly east-west axis into two shallow shelves. The shelf
break occurs at 150 m in most areas. Associated canyons include the
Ascension Canyon complex in the north; Soquel Canyon, a branch of the main
Monterey Canyon, and Carmel Canyon in the south. Carmel Canyon
approaches shore within 0.5 km of Pt. Lobos, where the shelf-break occurs at a
depth of 100m.
Monterey Bay is influenced by the California Current, an eastern
boundary current that transports subarctic water towards the equator (Mar-Sep),
and the poleward California Undercurrent, which occurs at depths below 150 m,
and is termed the Davidson Current when it surfaces during winter (Nov-Feb). A
cyclonic gyre begins north of the bay, flows towards Pt. Pinos in the south, and
then curves towards the north inside the Bay (Breaker and Broenkow 1994 ).
During strong northwest winds, coastal upwelling occurs north of the bay at Pt.
Ano Nuevo and south at Pt. Sur, with advection of cold, upwelled waters
entering the Bay from the north (Broenkow and Smethie 1978, Rosenfeld et al.
. I I I I I I I I I I I I
1500
I I 5 km I 1 ~ 1ooo
~- ------~-----Figure 1. Study area (dashed line) encompassing Monterey Bay, California and surrounding waters. Study area was approximately 1 ,062 km2•
8
9
in press). During wind relaxation there is shoreward advection and surtace
warming. Frequent eddy-like features are located west of the Bay, and internal
waves are common in the Canyon (Brdenkow and Smethie 1978, Shea and
Broenkow 1982, Koehler 1990, Breaker and Broenkow 1 994). Three distinct
oceanographic seasons occur in Monterey Bay. The upwelling period (Mar-Jul)
is characterized by strong northwest winds, low surtace temperatures, high
surtace salinities, a steep rise in isotherms, and strong coastal fronts between
cold upwelled and warm offshore waters. The oceanic period (Aug-Oct) occurs
when winds relax and warmer California Current water approaches the shore,
producing near-shore thermal gradients, increased surface stratification and
deeper isotherms. The Davidson Current period (Nov-Feb) is dominated by
southerly winds, low surtace salinities and temperatures, reduced horizontal
and vertical temperature gradients, and a deep mixed layer (Skogsberg 1936,
Bolin and Abbott 1963, Broenkow and Smethie 1978, Chavez et al. 1991,
Breaker and Broenkow 1994). The onset of each season is variable from year to
year, usually with an abrupt "spring transition" into the upwelling period (Huyer
et al. 1990). These three periods were used for seasonal analyses, with the
onset of each period determined by surtace and vertical temperature changes
(Chavez et al. 1991, F. Chavez, pers. comm.) throughout the study period.
Surveys
Vessel surveys were conducted from June 1987 to June 1991, although
most surveys occurred from 1987 to 1989. Sixty-six dedicated surveys were
conducted approximately twice monthly aboard the 10.7-m RN Ed Ricketts and
'( ?:
10
a 4.6-m Boston Whaler. One-hundred fifty opportunistic surveys were conducted
on various natural history/research cruises, mainly aboard the 16.5-m MN Pt.
Sur Clipper. Twenty-three percent of st.Jrveys occurred during the upwelling
season, 30% during the Davidson season, and 47% during the oceanic season.
One to three dedicated observers were present on all cruises, and one (the
author) was consistent through all surveys. Additional sighting data were
obtained from a network of sport-fishing vessels in the area (R. Ternullo, pers.
comm.), and additional effort and sighting data were obtained from individuals
conducting harbor porpoise (Phocoena phocoena) surveys in near-shore Bay
waters (Dorfman 1990}.
During dedicated dolphin surveys, the RN Ed Ricketts departed from
Moss Landing Harbor and usually surveyed areas where water depths were
greater than 1oo·m. The Boston Whaler was limited to surveying the southern
Bay to Pt. Lobos. These vessels headed towards locations of reports of dolphins
from other vessels or, if none, searched waters throughout the Bay. Observers
equipped with binoculars were on constant watch during surveys when sea
states were Beaufort 4 or less and visibility was 3 km or more. The effective
sighting distance was up to 1 km from the vesseL During these surveys; time,
position, course, Beaufort sea state, weather conditions, and sea surface
temperature were recorded every half hour, at course changes, and when
cetaceans were sighted. When Pacific white-sided dolphins were sighted,
additional data recorded included estimate of group size for all cetaceans,
associated species of marine mammals and birds, general behavior state of the
Pacific white-sided dolphins, degree of group cohesiveness, occurrence and
1 1
type of aerial activity, and presence of calves. These data were recorded at the
initial sighting and thereafter every 15 min until observations of that group
ceased. Surveys were conducted durimJ daylight periods (generally 0700 to
1500 h) for seven to eight hours duration. Dolphins with distinct markings on
their dorsal fins and anomalous-colored individuals were photographed for
identification (WOrsig and Jefferson 1990).
Opportunistic surveys were conducted in conjunction with bird
watching/natural history trips (Shearwater Journeys), a study of blue whales
(Balaenopte.ra musculus; Earthwatch, R. Wells and S. Kruse), and gray whale
(Eschrichtius robustus) watching trips. All vessels departed Monterey Harbor
and spent seven to eight hours per day (0700 to 1500) at sea. Excluding gray
whale watching trips, courses were haphazard, and usually covered 90 to 140
km per day. Depending on trip type, either inner bay shallow waters, waters
overlying the Canyon, or offshore waters were traversed. Vessels stopped for
periods of up to one hour during marine mammal or bird observations. Data
were collected similar to dedicated dolphin surveys.
The study area was divided into a grid containing 256, 4x4 km2 quadrats.
The number of km surveyed in each quadrat was calculated and divided into
four categories of effort which were 0-10, 11-149, 150-299, and 300+(Fig. 2a).
Additional dolphin sightings made by persons aboard 5 fishing vessels
operating in the Monterey Bay area were collected and compiled by R. Ternullo,
aboard the MN Pt. Sur Clipper on a year-round basis, and were screened for
reliability based on observer experience. Data collected were date, time,
position, temperature, estimate of Pacific white-sided dolphin group size, and
Figure 2. (a) Survey effort: number of km traversed in each quadrat (4x4km2) during dedicated Pacific
white-sided dolphin surveys and opportunistic surveys. Quadrats are shaded relative to four effort categories. Numbers in lower left of quadrats indicate number of km surveyed in each quadrat. (b) Additional effort covered by charter vessels. Fishing destinations are shaded. Approximate effort represents shaded areas and transit to these areas from Monterey harbor (source: R. Ternullo).
presence of associated species. Effort was not quantified but was fairly
localized and constant year-round. Sightings of cetaceans from these vessels
occurred while transiting to fishing locations and while on site (Fig. 2b). ~
Distribution and Relative Abundance Analysis
13
Number of dolphins per km {relative individual abundance), number of
groups per km (relative group abundance), mean depth, distance to shelf-break,
and Contour Index value were calculated for each 4x4 km2 quadrat. Pacific
white-sided dolphin sightings were categorized by depth: shelf {0·200 m), slope
{201-1000 m), and oceanic {>1000 m) waters; distance to shelf-break, from_.::; 5
km inside to > 15 km beyond the break; temperature, from 9.0°C to 19.0°C; and
Contour Index (CI), or degree of bottom relief. Contour index was calculated
(Evans 1975, Hui 1979, Selzer and Payne 1988) according to the formula:
Cl=1 OO{M-m) M
where m=minimum depth, and M=maximum depth within a quadrat. Contour
Indices ranged from nearly no slope (0.01) to a steep slope (1.0). Index values
were grouped into five equal categories from 0.01 to 99.99 (1 to 5). Observed Cl
values, or frequency of occurrence of Cl categories in which dolphins occurred,
were compared to expected values, or the frequency of occurrence of Cl
categories, assuming dolphins were evenly distributed across all categories.
Chi-square analysis was used to compare the observed vs. expected frequency
of occurrence categories among the Cl, depth, distance to shelf-break, and
temperature classes.
14
To determine if dolphins were distributed differentially according to group
size, mean group size for dolphin sightings was calculated for each quadrat
with more than 20 km of effort, and shaded relative to five group size categories.
This was evaluated visually to assess trends. Group size means were
compared by season and month with a Kruskai-Wallis test. Dolphin occurrence
and relative abundance were compared by season and month with a Kruskai
Wallis test to determine any seasonal patterns.
Behavior Analysis
Group size, behavior, degree of group cohesiveness, occurrence and
type of aerial activity, and presence of multi-species groups were recorded
during 15-min scan samples (Altman 1974). Behavior was observed between
0800 to 1400 hrs. Group size was the number of dolphins sighted within a 2 km
area. This entire group often was composed of distinct subgroups of 5 to 25
dolphins. Dolphin behavior was categorized into five general states: feed,
travel, social, mill, and rest (Shane 1990). Dolphins synchronously diving in a
localized area, often with flukes emergent before diving, were considered
feeding. Other factors that indicated feeding included detection of prey by the
vessel's depth sounder and aggregations of scavenging seabirds near the
dolphins. Surface feeding dolphins were easily classified as they pursued and
caught fishes. A dolphin group moving in a single direction was considered
traveling. Social activity was characterized by physical and/or sexual contact
between two or more individuals, including rubs, nudges, chases, and object
play (usually kelp). Milling behavior, or frequent direction changes, were those
activities not classified as feeding, traveling, socializing, or resting. Resting
dolphins were tightly grouped and highly synchronous in their respiration .
15
patterns {Norris and Dohl1980a, Wursig and Wursig 1980, Norris et al. 1994).
Mean sighting depth, distance to shore, and Contour Index were calculated for
each behavior and compared with Kruskai-Wallis or Chi-square tests.
Differences in frequency of behaviors {mill, travel, socialize, or feed)
among quadrats were totaled for all quadrats with more than 60 min. of
behavioral observations. Frequency of each behavior in each of these quadrats
was compared with Chi-square analysis, and where significant, the primary
behavior was noted. Quadrats were grouped according to primary behavior.
Mean depth, distance to the shelf-break (from center of the quadrat), and
Contour Index values were determined for each group of quadrats and
analyzed with a Kruskai-Wallis test to determine environmental differences
among these quadrats. Chi-square analysis was used to determine if frequency
of behaviors differed by time of day.
Mean group size of dolphins engaged in each of the five behaviors was
compared with a Kruskai-Wallis test. The observed number of occurrences of
dolphins engaged in each behavior among four group size categories was
compared to the percentage of total observations for all behaviors per group
size category and multiplied by the total observations lor each behavior
{expected) with a Chi-square test.
Group characteristics of each behavior were compared; group
cohesiveness, aerial behavior, and multi-species categories with a Chi-square
test. Expected values were 50% of total frequency of observations for each
16
category. Group cohesiveness was scored at each 15 minute sample as (1)
scattered • inter-individual distance greater than 100 m, or (2) tight - inter
individual difference less than 100m. Aerial behavior occurred when all or part
of the dolphin's body emerged above the water's surface, excluding normal
respiration. Two types of aerial behaviors were distinguished (Norris and Dahl
1980a, Wursig and Wursig 1980): (1) single aerial leaps, creating little splash
and (2) percussive repetitive leaps, characterized by rapid repetition of a
particular leap creating distinct splashes. Aerial behavior was scored by type
and occurrence within the previous 15-minute interval.
A multi-species group was considered as two or more species of marine
mammals either intermixed as a single group, or In close proximity (within 0.5
km) of each other. The coefficient of association of mutli-species groups was
calculated according to the formula:
Coefficient of association- Nab (Na+Nb+Nab)
where Nab=number of occasions species a and species b were seen together,
Na=number of occasions species a was seen without species b, and Nb=
number of occasions species b was seen without species a. Scores range from
0 (no association) to 1.0 (complete association; Martin and Bateson 1986). To
determine if there was any difference in multi-species associations among
Pacific white-sided, Risso's, and northern right whale dolphins by season,
coefficients of association were compared with a Chi-square test. Observed
values were the number of occurrences of each multi-species group per
season. Expected values were the percentage of Pacific white-sided dolphin
17
sightings (with no other species) per season multiplied by the total (all seasons)
number of occurrences of a particular multi-species group. This was based on
the assumption that if multi-species groups occurred equally throughout the
seasons, the number of occurrences of these groups should be proportional to
the number of Pacific white-sided dolphin sightings per season. Mean group
sizes for the three species, when alone and in association with each other, were
compared with a Mann Whitney U test. All combinations of multi-species groups
also were evaluated with Chi-square analysis to determine if associations were
significantly related to specific behaviors.
Radio-Tag and Track
Three dolphins {01, 02, 03) were captured and radio-tagged in
Monterey Bay. Tohe first dolphin was captured by B. WOrsig using a tail-grab
device {WOrsig 1982). The other two dolphins were captured on the same day
within the same school by J. Hall using a break-away hoop net (Evans 1974).
The radio-tags {148 MHz) were 8.0 by 1.5 em, with a 35.0 em whip
antenna, a battery life of one month, and an estimated reception range of up to
10 km from a boat, 30 km from shore, and about 100 km from a plane (B.
WOrsig, pers. comm.). Tags were attached to the dolphins' dorsal fin by boring
two 0.5-cm holes through the fin. Tags were secured with two corrodible
magnesium nuts, expected to break apart within two to four weeks. Dolphins
were tracked from boats, shore, and planes.
Pulses from the radio-tags were only detected when the dolphin
surfaced. Time and number of pulses, therefore, were recorded for each surface
18
period. When possible, the behavior of the dolphin group associated with the
tagged dolphin was recorded. Two dive variables were analyzed and compared
to behavior: 1) dive duration - the interval b-etween pulses; 2) respiration rate •
the number of pulses or sequence of pulses per min. Two dive patterns were
distinguished and compared to behavior: 1) regular dives • relatively consistent
dive durations; 2) clumped -relative short dive durations interspersed by
relatively long durations (30 sec+). Distances traveled and travel speeds were
calculated for each hour and for each behavior.
Environmental Data Analysis and Correlations
To determine how Pacific white-sided dolphins may be influenced by
oceanographic conditions, nine environmental variables were assessed by
month and season for each year during the study. Mean sea surface
temperature was obtained from NOAA buoy #46042 (36°45'N, 122°25'W,
Fig.1 ). Sea surface temperature gradient was calculated by subtracting the
greatest mean temperature from the lowest mean temperature from four
locations within the study area. Locations of these temperature readings
included Santa Cruz (36°57.5'N, 122°01.0'W), Hopkins Marine Station in
Pacific Grove (36°37.3'N, 121 °54.2'W), Granite Canyon (36°25.9'N,
121°55.0'W), and NOAA buoy #46042. Sea surface temperature gradient (or
relative position of thermal fronts) was calculated with eight temperatures at 50
km intervals from a line extending west of Pt. Pinos {NOAA-Oceanographic
Monthly Summary). Each temperature value was subtracted from the adjacent
one to the west, creating seven temperature gradient values (numbered 1 to 7
19
extending east to west for reference). The thermal front was located where the
greatest temperature difference occurred. Relative intensity of the near-shore
thermal gradient was the difference between the near-shore and 50 km offshore
value. Mean coastal upwelling indices were obtained from NOAA buoy #46042
(David Husby/NMFS). Mean salinities were obtained from Granite Canyon
(CDFG). Mean depth of mixed layer was obtained from F. Chavez (MBARI).
Mean temperature anomaly was obtained from the Pacific Grove site (Surface
Water Temperatures, Salinities, and Densities at Shore Stations, Marine Life
Research Group, Scripps Institution of Oceanography). The percentage of days
with intense upwelling was calculated by dividing the number of days with an
upwelling index value of 100 or greater by the total number of days within a
season.
Relative individual and group abundance, mean group size, and mean
distance to the shelf-break for each season were correlated to the nine
environmental variables with Spearman's rank correlation.
20
RESULTS
Distribution and Relative Abundance
Ninety-three percent (n=239) of 256 quadrats were surveyed, totaling
19,338 km of trackline. Pacific white-sided dolphins were observed in 108
(42.2%) surveyed quadrats. Dolphins were sighted on 133 days ( 201 separate
sightings), or during 63% of dedicated and opportunistic surveys. In addition,
268 dolphin sightings collected by other observers resulted in 469 total
sightings for the study period.
Pacific white-sided dolphins relative individual abundance (number
dolphins/km) was greatest off Carmel Bay, across the outer Bay waters, and
near the northern Monterey Canyon rim (Fig. 3).
Pacific white-sided dolphins relative group abundance (number of
groups/km) was greatest near the canyon edge from Pt. Pinos to Pt. Lobos over
water 50 m to 1,500 m deep (Fig. 4).
Dolphin sightings were not equally distributed among depth
(Chi2=34.314, df=2, P<0.001), distance to the shelf-break (Chi2=227.397, df=7,
P<0.001), temperature (Q.!J.[2=173.5, df=4, P<0.001), and Contour Index
categories (Qbl2=10.228, df=4, P<0.05). Most sightings (43.7%) occurred over
inner slope (201-1 000 m) waters, with fewer sightings over shelf (34.6%; 0-200
m) and outer slope waters (21.7%; 1001+ m). Mean (±SD) water depth for all
sightings was 450.9±451.15 m and ranged from 50 to 2,012 m. Dolphins were
most frequently sighted over the shelf-break and up to 2.0 km beyond it (Fig. 5).
Mean (±SD) distance to the shelf-break for all sighlings was 4.7±6.63 km,
-------
#dolphinslkm
oo D o.1-5.o 0]5.1-10.0
l!llill10.1-15.0
11111115.1+
21
Figure 3. Relative abundance of Pacific white-sided dolphins represented as number of dolphins per km per quadrat. Quadrats with more than 10 km effort are shaded relative to four abundance categories. Numbers in the lower left ~orner of each quadrat represent actual number of dolphins per km per quadrat. Numbers were rounded to the nearest 0.1.
#groupslkm
oo fill 0.01-0.04
II o.os+
36"20'N
22
Figure 4. Relative group abundance of Pacific white-sided dolphins represented as number of dolphin groups per km per quadrat. Quadrats with more than 10 km effort are shaded relative to two occurrence categories. Numbers in the lower left corner of each quadrat represent actual number of dolphin groups per km per quadrat. Numbers were rounded to the nearest 0.01.
160 n=469
140 en Q) 120 u c: Q) ... 100 ... :::1 u u
80 0 -0 ... 60 Q) .c E 40 :::1 z
20
0 + ..... ..... 0 0 0 0 + ..... C\1 0 C\1 l[) 0 l[) ..... u) ' ' ..... ..... u) ' 0 .f!, 0 0 ..... - - 0 0 - 0 ..... - -0 0 ..... .....
l[) C\1 C\1
' ' l[) 0 .....
Distance to Shelf-break (km)
Figure 5. Number of occurrences of the distance Pacific white-sided dolphin groups were sighted from the shelf-break (km). Negative values represent distances inshore of the shelf-break. 1\)
w
24
ranging from 10.0 km inside to 41.0 km beyond the shelf-break.
Pacific white-sided dolphins were sighted in waters of 10.0°C to 18.9°C.
Fewer than expected sightings occurred in the 9.0°C to 12.9°C range, and more
in the 15.0°C to 17.0°C range (Fig. 6).
Dolphins were observed more often in high relief areas than in low relief
areas (Fig. 7). The Contour Index averaged (±SO) 3.9±1.21 for all sightings.
Dolphins were frequently sighted (50.6%) in small groups of 1 to 50
individuals, only 5.7% of groups contained a thousand or more dolphins (Fig.
8). Mean (±SO) group size was 203±395.4, with one to 4000 dolphins per
group.
Based on dolphin mean group size by quadrat, groups greater than 300
individuals occurred only in the outer Bay waters, especially near Carmel Bay
and the north rim of the' Monterey Canyon (Fig. 9). Quadrats with relatively large
mean group sizes extended from Carmel Canyon across the Bay to the north
rim, surrounded to the inside and outside by quadrats with lower mean group
sizes. Within the Bay, relatively large groups ( 151 to 300) occurred only in
quadrats over the main Canyon's southern edge and Soquel Canyon. Dolphin
group sizes were lower in other quadrats inside the Bay, especially in shallow
shelf waters.
Although there was no significant difference between group size and
season, due to high variability among group sizes, there were some observable
trends. Based on sightings plotted by group size and season, the upwelling
season was characterized by smaller group sizes both inside and outside the
Bay, with only 3.4% of the groups containing 500 or more dolphins. Pacific
45
40 (1.) 0 t:: 35 (1.) .... .... :::J 30 0 0 0 25 -0 ;::... 20 0 t::
~ 15 C" CD .... 10 1.1..
';:/!., 0 5
0
n=469
9.0-10.9
• Expected
bJ Observed
11.0-12.9 13.0-14.9 15.0-16.9 17.0+ Temperature (°C)
Figure 6. Observed and expected percent frequency of occurrence of Pacific white-sided dolphin sightings among five temperature categories.
80
(I) 70 CJ c (I) 60 a.. a.. :s CJ 50 CJ 0 0 40 > CJ c 30 (I) :s tr ~ 20
1.1.. ~ 0 10
0
n=469
1 2 3 Contour Index
• Expected
[ill Observed
4 5
Figure 7. Observed and expected percent frequency of occurrence of Pacific white-sided dolphin sightings among five contour index classes.
250 n=443
U) 200 Cl) 0 c Cl) ..... ..... 150 :I 0 0 0 -0 100 ..... Cl) .c E :I
50 z
0--L--' 1-50 51-100 101-300 301-500 501-1000 1000+
Group Size
Figure 8. Number of occurrences of Pacific white-sided dolphin group sizes.
37"00711
group size
01-50
051-150
~151-300 !11301-500
Ill 501+
Pt. Sur
28
Figure 9. Mean group size of Pacific white-sided dolphins perquadrat, for quadrats with greater than 20 km effort. Quadrats are shaded relative to five group size categories. Numbers in the lower left corner of each quadrat represent actual mean group size per quadrat. Numbers are rounded to the nearest 1.0.
29
white-sided dolphin groups during the oceanic season also predominantly
contained less than 50 individuals, but were more evenly distributed among the
classes, with groups over 500 dolphins comprising 9.1% of the sightings. Most
sightings during the oceanic season were concentrated around the southern
Canyon rim and near Carmel Canyon, with relatively large dolphin groups
observed in the outer Bay waters. The Davidson season was characterized by a
relatively high proportion of large groups; 20.6% contained over 500 dolphins.
Most sightings were in the outer Bay waters except for a few sightings around
Soquel Canyon and the inner southern canyon rim, and were concentrated
north of Pt. Pifios and around Carmel Canyon (Fig. 10, 11 ). The occurrence of
large group sizes (500+) was greatest during the Davidson season ( 14. 7%),
followed by the oceanic season (9%), and least during the upwelling season
(2.9%) (Chi2=6.646,' df=2, P<0.05).
Mean group sizes were greater during the Davidson season, mainly
because the largest groups of the year were observed in February. Smallest
mean group sizes occurred during the upwelling season, especially during
June and July (Fig. 12). Dolphins were nearest to the shelf-break during the
Davidson season (H=15.045, df=2, P<0.001 ).
Dolphin group abundance differed significantly among oceanographic
seasons (H=7.419, df=2, P<0.025) and rnonths (H=22.419, df=11, P<0.025).
More dolphin groups per krn occurred during the oceanic season, especially
during September and October (Fig. 13). Fewer sightings occurred during the
upwelling season, especially during May.
z 0 en <1: w en z 0 en c ~ c
z 0 en ~ en 0 z <1: w 0 0
Q) N
"05 c. ::J e Ol
"0 c C1l ~
c 0 C/)
"0 "> C1l 0 u ·;:: C1l Q) u 0
Ol c
l ::J ~
c 0 C/)
re C/)
>.0 C/)
Ol c
:;:: ;:::: Ol
"05 c :c c. 0 "0 "0 Q)
"0 "05 ' Q) -:c
;;: u
;;::::
-~ a.. 0 .,... w ... ::J Ol u::
30
80
80
Upwelling Season n=B9
1-50 51-100 101-300 301-500 501-1000 1000+
Oceanic Season n=252
1-50 51-100 101-300 301-500 501-1000 1000+
Davidson Season n=102
1-50 51-100 101-300 301-500 501-1000 1000+ Group Size
Figure 11. Percent frequency of occurrence of Pacific white-sided dolphin group sizes for each oceanographic season.
31
32
1000 100
900 IJ Group size
800 E) EHort
Ql 700 "tl
"' N ... iii (')
600 "' 0. ::l ::l -0 500 50 m ~ =I:
C!J 0 1: 400 ::+
"' Ql 300 :::!:
200
100
0 Month I M A M J J A s 0 N D J F I
I I Season
,,
Ql "tl N 100 iii "' ... 0.
(')
"' ::l ::l 0 200 50 -...
C!J m =I:
1: 0
"' 300 ...
Ql -:::!:
400 100
Figure 12. Mean Pacific white-sided dolphin group size (±SE) for each month and oceanographic season. Number of groups per month and season are indicated above error bars. The dashed boxes indicate approximate correspondence between months and seasons. Percent days of effort for all months and seasons during the study period are represented as white bars.
0.01
0.012 E ~ ... 0.01 (!) c. U) g. 0.008 0 ... Cl 0.006 c :E c. 0 0.004 0
"" 0.002
0 Month 1
U) c. :::J 0 ... Cl c .c c. 0 0
""
Season I
33
M A M J J A s 0 N D J
•'
Figure 13. Relative group abundance of Pacific white-sided dolphins, represented as the number of dolphin groups per km each month and oceanographic season. The dashed boxes indicate approximate correspondence between months and seasons.
34
Number of dolphins per km also differed among seasons (H=11.175,
df=2, P<0.005) and months (H=21.377, df=11, P<0.05; Fig. 14). Dolphins were
most abundant during the Davidson season, especially during November and
December. There was a significant difference in number of dolphins sighted per
km among years during the oceanic (H=17.674, df=3, P<0.001) season, and no
significant difference among years during the upwelling (P<0.1 0) and Davidson
seasons (P<0.25).
Behavior Observations
Behavioral observations were collected during 201 sightings (134 days
and 219 hours). Observations were from 30 to 360 min duration. Mill was the
most frequentlY" observed behavior (33.3% of observations), followed by feed
(23.9%), travel (21.9%), social (17.9%), and rest (3.0 %).
Environmental Factors and Behavior
Mean depth (H=18.943, df=3, P<0.001 ), mean distance to the shelf-break
(H=11.817, df=3, P<0.01), and mean Contour Index values (H=13.271, df=3,
P<0.005) for dolphin sightings differed among behaviors (Fig. 15). Feeding
dolphins occurred in shallower depths (mean (±SO), 529.7±477.53 m), closer to
the shelf-break, (mean (±SO), 4.0±5.46 km), and in areas with greater bottom
relief (mean (±SO), 4.2±1.12; Chi2=7.535, df=3, P<0.05) compared to other
behaviors. Traveling dolphins occurred in the deepest waters, further from the
6
5
~4 ... Ill c. '" 3 c: :c c. i5 c
Month I M A M J
I Season
-+---
J A S 0 1 N D J
I
#dol=10,714 km=3043
35
F
Figure 14. Relative abundance of Pacific white-sided dolphins, represented as the number of dolphins per km for all months and seasons during the study period. The dashed boxes indicate approximate correspondence between months and seasons. The number of dolphins counted and number of km under effort for each oceanographic season are indicated below bars.
1000
900
'E ~BOO a " c " 700 .. " ::;
'E :::. "" "' " ~ .c ,.!. iii .:: rn .S! " u
" .s .. c " "' " ::;
~ "C .5 ~
::l 0 E 0 u " "' " ::;
600
500 Mill
Mill
Mill 289
Travel Social
Travel Social
Travel Social 196 149
Behavior
Feed
Feed
Feed 198
Figure 15. Mean depth (m), mean distance to shelf-break (km), and mean contour index for Pacific white-sided dolphins engaged in milling, travelling, socializing, and feeding. Standard error is indicated above bars. Numbers below behaviors indicate number of scan samples.
36
37
shelf-break, and in areas with less bottom relief. Pacific white-sided dolphins
fed significantly more in waters over depths of 1 to 200 m than in deeper water
(Q.b12=10.051, df=2, P<0.01). whereas they traveled more often in waters
greater than 1000 m depth (Chi2=6.544, df=2, P<0.05). There was no difference
among categories lor socializing and milling dolphins.
Behavioral differences with a significance of P<0.20 occurred among 66
of 123 quadrats with observations (Fig. 16). Feeding was the predominant
behavior in nine quadrats (range p=0.0001 to 0.11 ), traveling in 22 quadrats
(range p=0.0004 to 0.16), socializing in 11 quadrats (range p=0.0002 to 0.16),
and milling in 24 quadrats (range p=0.0001 to 0.14). Dolphins frequently fed in
quadrats overlying the shelf-break off Pt. Pinos, over the head of Soquel
Canyon, and over the Canyon rim west of Moss Landing. Dolphins traveled in
outer Bay waters offshore of the shelf-break, except near Pt. Lobos, where they
traveled over Carmel Canyon. The predominant behavior within a quadrat
differed significantly according to depth (H=10.677, df=3, P<0.01 ), distance to
the shell-break (H=7.974, df=3, P<0.025), and Contour Index values (H=10.677,
df=3, P<0.01; Table 1).
For all seasons, Pacific white-sided dolphins fed more often in the
morning (Q.bl2=19.6, df=2, P<0.001) and socialized (Q.bl2=32.137, df=2,
P<0.001) more in the early afternoon. Dolphins were observed milling and
traveling with similar frequency in the morning and early afternoon (Fig. 17).
During the oceanic season, feeding often occurred in the morning, with
socializing observed more in the early afternoon. However, feeding occurred
throughout the day during the upwelling and Davidson seasons.
37"00'N
Behavior
D Mill
osocial
~Feed II Travel
36"20'N
Pt Sur
38
Figure 16. Quadrats where Pacific white-sided dolphins exhibited predominant behaviors (mill, social, feed, travel) with a significance of P=.20 or less.
Table 1. Mean depth (m), mean distance to the shelf-break (km), and mean contour index for quadrats where Pacific white-sided dolphins predominantly fed, milled, travelled, or socialized. Standard deviations, sample sizes (number of quadrats), and level of significance are shown.
FEED MILL Mean Depth (m) 373.5 914.9
so 391.4 570.1 Mean Distance
to Shelfbreak (km) 2.89 6.87 so 4.17 6.58
Mean Contour Index 4.56 3.29 so 1.01 1.16 n 9 24
TRAVEL SOCIAL 1017.7 1068.1 529.7 497.7
7.76 9.26 4.95 7.68
2.96 3.09 1.4 1.22 22 11
SIGNIFICANCE H=10.677
.025<P<.01 H=7.974
. .05<P<.025
H=10.677 .025<P<.01
UJ (lJ
40
Q) 35 (J
5i 30 ... ... :I
8 25 0 0 20 > (J
5i 15 :I 0'" I!! 10
Ll..
~ 0 5
0 Mill 289
Travel 196
Social 149
Feed 198
Ill 0800-1 000
~ 1000-1200
Ill 1200-1400
Figure 17. Percent frequency of occurrence of behaviors (mill, travel, social, feed) exhibited by Pacific white-sided dolphins during morning, mid-morning, and afternoon. Numbers below behaviors indicate number of scan samples.
41
Social Factors and Behavior -
Group size differed among behaviors (H=37.625, df=3, P<0.001; Fig. 18).
Pacific white-sided dolphins traveling "(mean(±SD), 438.0±462.73) and resting
(mean (±SD), 366.7±294.58) were in larger group sizes than dolphins feeding
(mean (±SD), 204.0±268.72), socializing (mean (±SD), 166.5±224.34), and
milling (mean (±SD), 115.6±202.64).
The frequency of dolphin group size categories were different when
feeding (Chi2=12.177, df=3, P<0.01); milling (Chi2=13.391, df=3, P<0.005); and
traveling (C.h.i2=29. 754, df=3, P<0.001) but not socializing (Fig. 19).
The cohesiveness of dolphin groups differed according to behavior
category (Chi2=132.585, df=1, P<0.0001; Fig. 20). Dolphins were in scattered
subgroups while milling, socializing, and feeding; whereas dolphins were tightly
grouped while traveling and resting.
The frequency of occurrence of aerial activity varied among behaviors
(Chi2=110.15, df=1, P<0.0001; Fig. 20). Dolphins were moraaerially active
while traveling and socializing, and less so while milling and feeding, with no
difference during resting. Percussive repetitive aerial behavior occurred most
often while dolphins traveled; single leaps occurred most while dolphins
socialized and fed.
While resting, traveling, and feeding, dolphins were associated with other
species greater than 50% of sightings, and while milling and socializing,
dolphins associated with other species less than 50% of sightings
(C.hl2=23.779, df=1, E.<0.001; Fig. 20).
500
400
.f!j 00 300 c. ::I 0 .... CJ s:: 200 ct1 (I)
~
100
196
Mill Travel Social Feed Rest
Figure 18. Mean Pacific white-sided dolphin group size for each behavior. Standard error is indicated above bars. Numbers above bars indicate number of scan samples.
60
B 5o c !!! ... :I 40 u u 0 0 30 ~ c ~ 20 I:T !!! u. ~ 10 0
0 <50
• Mill
Em Social
• Travel
Ill Feed
55-199 200-499 500+ Group Size
Figure 19. Percent frequency of occurrence of Pacific white-sided dolphin group size categories among behaviors (mill, social, travel, feed). Sample size for each behavior is indicated above bars.
44
D% Tight
Ill %Scattered
Group Cohesiveness
Mill Travel Social Feed Rest
.. EJ %Aerial u
II %No Aerial c: I!! ~
:::1 Aerial Behavior u ...
0 -0
"' u c .. :::1 ... ! IL
;f.
Mill Travel Social Feed Rest 1 [ill %Mix
Ill %No Mix
Mulli·Spe<;ies Groups
Mill Travel Social Feed Rest ""' 289 196 149 198 44
Behavior
Figure 20. Percent frequency of occurrence of group cohesiveness (tight vs. scattered), aerial behavior (no aerial vs. aerial), and multi-species associations (no associations vs. associations) among behaviors exhibited by Pacific whitesided dolphins. Sample size is indicated below behavior.
45
Multi-Species Associations
Thirteen other species of cetaceans and four species of pinnipeds were
sighted in addition to Pacific white-sided dolphins during the study period. In
67% (n=314) of the sightings, Pacific white-sided dolphins were the only
species observed, 33% (n=155) of the time one or more other cetacean species
(6 total) or pinniped species (1 total), also were observed with Pacific white
sided dolphins. Northern right whale dolphins, Risso's dolphins, and California
sea lions (Zalophus californianus) were most frequently sighted in association
with Pacific white-sided dolphins. Although Dall's porpoise {Phocoenoides
dalli) and harbor porpoise (Phocoena phocoena) inhabit Monterey Bay year
round (Jefferson 1991, Dorfman 1990), only 4.8% of the time were Pacific white
sided dolphins seen with Dall's porpoise and they were never seen with harbor
porpoise.
Of all cetacean multi-species combinations, Risso's dolphins and
northern right whale dolphins were associated most frequently (coeff. of
assoc.=0.202), followed by Pacific white-sided dolphins and northern right
whale dolphins (coeff. off assoc.=0.184; Table 2).
Pacific white-sided dolphins occurred significantly more than expected
with northern right whale dolphins (Chi2=9.211, df=2, P<0.01) during the
oceanic season, and less than expected during the upwelling season. Pacific
white-sided dolphins were sighted in significantly larger groups when mixed
with northern right whale dolphins (Z.=-5.121, P<0.001) than when sighted
alone. Group size of Pacific white-sided dolphins when alone was not different
than when mixed with Risso's dolphins (Fig. 21 ). Northern right whale dolphin
Table 2. Coefficient of association values for all cetacean multi-species groups. Lo=Pacific white-sided dolphin, Lb=northern right whale dolphin, Gg=Risso's dolphin, Dd/Dc=common dolphin, Pd=Dall'.s porpoise, Pp=harbor porpoise, Oo=killer whale, Bb=Baird's beaked whale, Bm=blue whale, Mn=humpback whale, Ba=minke whale.
SPECIES Lo Lb Gg ", Dd/Dc Pd P_p Oo Bb Bm Mn Lo Lb 0.184 Gg 0.108 0.202
Dd/Dc 0.01 0.01 0.006 Pd 0.018 0.01 0.013 0 Pp 0 0 0 0 0 Oo 0.002 0 0.005 0 0 0 Bb 0 0 0 0 0 0 0 Bm 0.024 0.005 0 0 0.011 0 0 0 Mn 0.057 0.021 0 0 0 0 0 0 0.013 Ba 0.004 0 0 0 0 0 0 0 0 0
Lb/Gg 0.062 0.01 0.01 0 0 0 0 0
Ba
0
450
400
350
:!! 300 Ci5 g. 250 0 ... " 200 t: Ill ~ 150
100
50
0 Lo Lo/Gg Lo/Lb
324 57 93
B Pacific white-sided dolphin = Lo
B Northern right whale dolphin = Lb
, D Risso's dolphin = Gg
Lb Lb/Lo Lb/Gg Gg Gg/Lo Gg/Lb 20 93 50 165 57 50
Dolphin Species
Figure 21. Mean group size of Pacific white-sided dolphins, northern right whale dolphins, and Risso's dolphins for single species groups (Lo, Lb, Gg) and multi-species groups (Lo/Gg etc.) when associated with one another. Standard error is indicated above bars. Sample sizes are indicated below bars.
48
group sizes tended to be greater when sighted alone than in association with
Pacific white-sided or Risso's dolphins. Northern right whale dolphins were
seen more often in mixed groups (84"/o) than alone. Risso's dolphins were seen
in significantly smaller group sizes when sighted alone than when associated
with Pacific white-sided dolphins (b=-7.101, P<0.001) or northern right whale
dolphins (Z=-7.164, P<0.001). This indicated that northern right whale dolphins
tend to join large Pacific white-sided dolphin groups and that both Pacific white
sided dolphins and northern right whale dolphins tend to join large groups of
Risso's dolphins.
When Pacific white-sided dolphins and northern right whale dolphins
were associated, there was no significant difference among the five behaviors
(Fig. 22). Behaviors of Pacific white-sided dolphins differed when associated
with Risso's ddlphins (Chi2=8.755, df=3, P<0.05); usually Pacific white-sided
dolphins were milling and traveling. In association with blue whales, Pacific
white-sided dolphins predominantly were socializing (Chi2=25.984, df=3,
P<O.OOS); and with humpback whales (Megaptera novaeangliae) and California
sea lions dolphins were predominantly feeding (Chi2=13.32, df=3, P<O.OOS,
Chi2=39.519, df=3, P<0.001, respectively). Although sample sizes were small,
when Pacific white-sided dolphins were mixed with common dolphins or Dall's
porpoise, they were either milling or feeding.
Killer whales (Orcinus orca) were never observed in direct association
with Pacific white-sided dolphins (Table 2). However, on two occasions Pacific
white-sided dolphins exhibited a flight response, swimming rapidly away, when
sighted near killer whales. On one occasion, a group of 400 Pacific white-sided
Q) u
90
80
lii 70 .... .... ~ 60 u 0 50 -0
~40 c: g: 30 C"
3: 20 ;,g 0
10
0
Lo/Lb Lo/Gg 78 32
Ill Mill
~ Travel
Ill Social
l!lll! Feed
Lo/Dc Lo/Pd Lo/Bm Lo/Mn Lo/Zc 4 5 7 12 53
Figure 22. Percent frequency of occurrence for behaviors that Pacific white-sided dolphins (Lo) were engaged in while associated with other species. Sample size is indicated above bars. Species codes: Lb=northern right whale dolphin, Gg=Risso's dolphin, Dc=common dolphin, Pd=Dall's porpoise, Bm=blue whale, Mn=humpback whale, Zc=California sea lion. Sample sizes are indicated below bars.
l 50
dolphins was sighted traveling rapidly south (porpoising), with a large amount
of white water. A group of 10 killer whales was subsequently sighted about 1.5
km from the dolphins. On another occ·asion, a small mixed group of Pacific
white-sided dolphins and Risso's dolphins were observed, traveling rapidly
away from an adult male killer whale that was following them within 0.5 km. No
attack or aggressive behavior was observed.
Calves
Small calves with fetal folds were first observed in mid-June and were
seen throughout August to early September. Calves sighted in October
appeared less muted in color and larger than those seen in early summer. In all
instances when young calves were observed, they were closely associated with
an adult, presumed to be the mother, and often were in subgroups containing
other mother/calf pairs. Northern right whale dolphin calves and Risso's dolphin
calves were sighted from late October through February.
On 22 August 1987, epimeletic behavior in Pacific white-sided dolphins
was observed. A dead calf with fetal folds was found among a subgroup of 1 o
other Pacific white-sided dolphins. There were at least three other dolphin
subgroups within a 0.5 km area. A large school of 1 ,500 white-sided dolphins
was present 18.5 km away (R. Ternullo, pers. comm.). It appeared that the adult
dolphins were taking turns supporting the calf. One dolphin would support the
calf on its beak for up to 1 min, release the calf, and as the calf began to sink,
another dolphin would swim below it and again support it on its beak. At least
five different dolphins in the subgroup exhibited this behavior. This continued
for 1.5 hours, at which time the wind increased and observations ended.
Photo-Identification
51
One hundred sixty-two Pacific white-sided dolphins with distinctive nicks
or markings on their dorsal fins, and fifteen others with an anomalous coloration
pattern were identified by use of photographs. Because the trailing edge of most
dolphin dorsal fins were not nicked, dolphins were frequently sighted in groups
of 100 or more, and there were limited observation periods, it became apparent
that identifying most individuals would be difficult, resulting in few resightings.
However, anomalous colored dolphins were much easier to sight and
photograph even in groups numbering more than 1000 individuals. Although
these dolphins were not always photo-identified, it was common to sight at least
one "white" dolphin within large groups. Thirteen of these anomalous-colored
dolphins were predominantly white with small areas of black-pigmentation on
their sides, heads, and fins (Fig. 23). Dolphin #13 was orangish-tan dorsally
where a normally colored dolphin would be gray. Another dolphin (#12) was
normally colored except for an unusual white stripe extending up from its flank
and widening over each eye, similar in coloration to the dolphin reported by
Brownell (1 965).
These individual dolphins were identified from one to eight times within a
particular season and between two seasons in a year, as well as among similar
seasons in different years. Dolphin #4 was identified on six occasions over a
two-month period during one oceanic season, and several others were sighted
52
Figure 23. Example of anomalous-colored Pacific white-sided dolphin.
53
several times during one oceanic season. Another dolphin {#B) was sighted
during three consecutive years, all during the month of October (oceanic
season). Dolphin #6, identified by D. Ekdahl {Shearwater Journeys) in October
1980 was resighted seven years later during this study, also in October. Fifty
seven percent of initial sightings and resightings of anomalous individuals
occurred during September and October (oceanic season), 27% during
November and December (oceanic and Davidson seasons), only 7% during the
upwelling season, and the remainder during other months within the oceanic
and Davidson seasons {Table 3).
Ten anomalous-colored dolphins were sighted in the same group with up
to three other anomalous colored dolphins, and were found within the same
subgroup in a few instances. One such pair {#8, #11) was resighted together 14
months after they were initially photographed together. One "white" dolphin
{#11) was photographed with a normally colored newborn calf in early August,
again in December of the same year, presumably with the same calf, and was
last sighted with this calf the following year during December. Dolphin #4 was
identified in Monterey Bay several times during the oceanic season,
photographed again in Monterey Bay four years later, and near the Farallon
Islands (P. Pyle, pers. comm.) the following year.
Radio-Track
Pacific white-sided dolphins were tracked (not continuously) a maximum
of 42.7 hr., and were tracked a maximum of 865 min. Mean (±SD) dive duration
for all dolphins was 23.5 ±1.92 sec. Maximum dive duration recorded for D1
54
Table 3. Anomalous-colored Pacifie: white-sided dolphins, including dolphin number, date, group size, associated species and their group size. Lo=Pacific white-sided dolphin, Lb=northern right whale dolphin, Gg=Risso's dolphin, Ze=California sea lion.
9/l;Al7 250 Lb-10
9114187 1000 Ll>-100 wilh Dolphin #5 9124187 50 Wllh Dolphin #5
10113!87 300 Llr10 11/15/87 40 Lb-200
11/S/91 500 Ur30 with Oolpt!in #7 Islands
with DOlphin #11{same subgroup), #12, #13
#13
55
was 372 sec. Mean (±SO) respiration rate for all dolphins was 2.5±.32. Mean
(±SD) speed for all dolphins was 7.6±2.19 km/hr for all dolphins. The minimum
distance traveled was 38.0 km in 5.8 hr. (Table 4).
For each dolphin, seventy percent of dives were less than 20 sec. Mean
dive duration significantly differed among behaviors; slow travel, fast travel,
mill/feed (H=44.326, df=2, P<0.001). Dolphins exhibited the regular dive pattern
90% of the time during slow or fast travel, and the clumped pattern 60% of the
time during mill/feed behavior (Fig. 24).
Environmental Correlations
Interannual variability among eight environmental variables was most
constant during each Davidson season, whereas variables within the upwelling
seasons fluctuated the most among years. Group and individual abundance of
Pacific white-sided dolphins were greatest during the 1987 oceanic season.
Compared to the other oceanic seasons, this period had the lowest upwelling
index value, percentage of days of intense upwelling, and salinity. Also, the
greatest mean sea surtace temperature, nearshore frontal gradient, and the
greatest temperature anomaly for the study period were recorded during the
1987 oceanic period. Dolphin group and individual abundance were low during
the 1991 upwelling season, corresponding to the lowest salinity, low near-shore
frontal gradient, high upwelling index value, percent days of intense upwelling,
and the shallowest mixed layer among upwelling seasons. The lowest mean
temperature and temperature anomaly for the study period occurred during this
season (Fig. 25).
RADIO TRACK SUMMARY DOLPHIN 1
Sex Length (em) Tag Date Track Time (min) Contact Time (hr) Number of Dives Mean Dive Duration (sec) so Max Dive Time (sec}
Respiration Rate Mean Speed (km/hr} Max Speed (km/hr) Travel Speed (km/hr} Mill/Feed Speed (km/hr) Min Distance (km)
F 165 8/10/88
521.4 38.1
1050 25.7 33.7
372 (6.2 min)
2.1 5.0
14.8 5.6 4.7
32.6 (19.2 hr}
DOLPHIN 2
F 172 1/25/90 865.3
42.7 2324
22.0 23.0
208 (3.5 min)
2.7 8.9
20.2 9.9 2.6
38.0 (5.8 hr)
DOLPHIN 3
M 185 1/25/90
595.0 35.3
1515 22.9 26.6
196 (3.3 min)
' 2.6 8.9
20.2 9.9 2.6
38.0
Table 4. Summary of three radio-tagged and tracked Pacific white-sided dolphins. Sex, length, track time, dive variables, and speed of movement are included. Ul
en
Ul1 QJ
-~, o, ..... 0 .... QJ .c E ::J z
u QJ (/) -c 0 -.:: ~ ::J Cl QJ
-~ Cl c ctl QJ
::2:
c .... QJ --ctl
0... ~ 0
DOLPHIN 2
n=2324
a
60 80 1 00 120 140 160 , 80 200
Dive Duration
Fast Trav Slow Trav Mill/Feed
c DOLPHIN 3
Fast Trav Slow Trav Mill/Feed
BEHAVIOR
EJ Dolphin 1 D Dolphin 2
IIlli Dolphin 3
lllll Clumped
1111 1111 Ill
Regular
Ill I II II I II Min
57
Figure 24. a) Frequency histogram of dive duration for 02; b) Mean dive (±SO) duration by behavior for 01, 02, 03; c) Percentage pattern type (clumped or regular) by behavior for 03.
~,::: 14
F 13.5
~ !3
~ 12.5
: 12 ~
5 11.5 A ~ 11L-~0~--------------- 0 •
u,..,
0.9 "' s 45 40
! " I : 1 ~ li 10
E 2 '.~·--"----------------0.4 =------------------ 0-Ocannlt: Oav!d.!lon Ocnanlc 0itllldaan • 0.0170
" ·~------------------Oceanic UpwcU O:aanlc Davida011
~0.7 i o.e s 0.5
)o• ~ 0.3
]02
~0.1 ~ ,_,.c"--------,._--------
upwan Oceanic DaMon
500
450
400 ~350 cg_300 i!.so "200
~150 100
50 o K
0
150
14(1"
J120 .ll,oo loo "so J40
0 p
0
Figure 25. Mean environmental conditions (A) sea surface temperature, (B) sea surface temperature gradient in bay, (C) nearshore frontal gradient, (D) upwell index, (E) salinity, (F) depth mixed layer, (G) sea surface temperature anomaly, (H) percentage days intense upwelling, and (I) number of dolphins per km, (J) number dolphin groups per km, (K) group size, (L) distance to shelf-break, for each oceanographic season (Upwell, Oceanic, Davidson) during each year of the study.
Davidson
59
There was a significant correlation between dolphin group and individual
abundance, group size, sighting distance to the shelf-break, and environmental
variables for seasons. Pacific white-sidEld dolphin group abundance was
positively correlated to temperature (Is=0.622, n=13, P<0.001) and the near
shore frontal gradient (rs=0.472, n=13, P<0.01 ). Relative abundance also was
positively correlated to temperature, and negatively correlated to the upwelling
index and percentage days of intense upwelling per season. Dolphin sighting
distance to the shelf-break was positively correlated to temperature (Is=0.486,
n=13, P<0.01) and the near-shore frontal gradient (Is=0.305, n=13, P<0.05).
When the temperature anomaly was high, dolphins were more abundant
(rs=0.471, n=13, P<0.01 ), and occurred closer to the shelf-break (Is=0.519,
n=13, P<0.01 ).
60
DISCUSSION
Distribution
Line transect methodology was not used because the main purpose of
this study was not to estimate actual abundance but instead to spend time
observing dolphins. Although surveys were "casual", observer effort was
consistent throughout all trips providing data on distribution, and relative
individual and group abundance. Biases may have been incurred in the relative
abundance and distribution because pre-established random or consistent
transects were not used.
Based on other surveys off California, Oregon, and Washington, Pacific
white-sided dolphins are concentrated between 200 m and 2000 m, usually
within 180 km of shore (Doh I et al. 1983, Brueggeman 1992, Green et al. 1993,
Barlow in press). The study area off Monterey encompassed shallow shelf
waters out to just beyond 2000 m depth. Due to the near-shore proximity of the
Monterey Submarine Canyon, the prime habitat of these dolphins was regularly
surveyed and accessed for detailed distributional patterns. However, more effort
in the southern Bay may have resulted in underestimating the importance of the
northwest portion of the study area. Dolphins were not randomly distributed
throughout the Monterey Bay area, but instead frequented, and were relatively
abundant over, inner slope waters in areas of high relief, around the northern
edge of the Monterey Submarine Canyon and especially between Pt. Pinos and
Pl. Lobos. These areas are characterized by complex circulation patterns, and
steep, heterogeneous bathymetry encompassing several mini-canyons
61
overlying the shelf-break and oriented perpendicular to the coastline. Dolphins
infrequently occurred in small groups over Monterey Bay shelf waters. Dahl et
aL (1983) and Leatherwood et aL (1984) al~o found that Pacific white-sided
dolphins were uncommon and usually in groups of less than 10 over shell
waters. As in the Monterey Bay area, especially in Carmel Bay and south of Pt.
Lobos, Pacific white-sided dolphins occurred closest to shore where the shelf
break approached the coastline (Dahl et aL 1983, Bruggeman 1992,, Green et
al. 1993, Barlow in press).
Beca!Jse specific behaviors appeared more frequently in particular
quadrats, it is likely that dolphins differentially use microhabitats within the Bay.
High dolphin abundance extended from near-shore Carmel Canyon and
offshore, crossing the Bay to the northern outer rim of Monterey Canyon. This
relatively deep water area was frequented by traveling dolphins, which tend to
occur in larger groups. This area could represent groups of dolphins bypassing
the Bay while moving up or down the coast, possibly in search of prey within
their preferred inner slope habitat. Hawaiian spinner dolphins (Stene/la
longirostris) use extensive areas of the coast for feeding, and concentrate daily
activities in certain coastal regions, occasionally moving away from these areas
for several days (Norris et aL 1994).
Pacific white-sided dolphins frequently milled in quadrats adjacent and
offshore of feeding areas which were located in shallower waters near the shell
break, especially in high relief quadrats. Pilot whales (Giobicephala
macrorhynchus; Shane 1984), Hawaiian spinner dolphins (Norris and Dahl
1980a), and humpback dolphins (Sousa chinensis; Saayman and Taylor 1979)
62
also exhibited particular behaviors in specific locations. Heimlich-Baran (1988)
found that habitat use patterns of resident killer whales in the Pacific Northwest
were centered around feeding areas. Feeding quadrats of killer whales were
characterized by steep underwater slopes rising to within 10m of the surface.
The main prey of these whales was salmon, which fed on smaller bait fish. Killer
whales traveled between feeding areas over deep water areas with low relief.
Pacific white-sided dolphins near Monterey Bay also traveled across deeper
outer Bay waters with relatively low relief.
Risso's dolphins (Kruse 1989), and Dall's porpoise (Jefferson 1991) in
the Monterey area, other dolphin species (Hui 1979, Evans 1982, Kenney and
Winn 1986, Cipriano 1992) and seabirds (Ainley and Jacobs 1981, Briggs el al.
1987, Heinemann el al. 1989) frequent high relief, heterogeneous, and
shelfedge habitats: .Hui (1979) found that common dolphins were abundant
over complex submarine topographies, similar to the situation with Pacific
white-sided dolphins in Monterey Bay. He suggested that complex bottom
topography often results in modified currents, increased mixing; therefore,
greater food abundance. Evans (1971) suggested that common dolphins use
passive listening, differences in currents, and thermal structure to locate specific
bottom features, such as escarpments and seamounts, associated with
abundant prey. Alternatively, orientation to these areas may involve learning
and past experiences (Kenney and Winn 1986, Wi.lrsig 1986). High relief areas,
and likely abundant food sources, occur throughout the Monterey Bay area,
probably accounting for the frequent presence and abundance of Pacific white
sided dolphins.
63
Although dolphins in Monterey Bay were in groups of 1-50 individuals,
similar to the findings of Dahl et al. (1983), mean group size was greater in
Monterey Bay than in most other locations (Dahl et al. 1 983, Leatherwood et al.
1984, Brueggeman 1992, Hill and Barlow 1992, Barlow 1993b, Buckland et al.
1993, Green et al. 1993). Wells et al. (1980) suggested that group size may vary
greatly among regions, and was related to season and habitat type.
Leatherwood et al. (1984} summarized all Pacific white-sided dolphin sightings
from 1949 to 1979 in the eastern North Pacific by season and mean group size,
and found that herds were significantly larger in southern (<30°N) and northern
(>55°N) areas than in their central (30-55 °N) range.
The difference between mean group size reported by Leatherwood et al.
(1984), and that reported by Dahl et al. (1983) and in this study may represent
less effort obtained for this area before 1979. The greater mean group size
reported here may be due to a higher proportion of larger dolphin groups near
Monterey. Large group sizes in bottlenose dolphins (Tursiops truncatus) are
considered advantageous for feeding on abundant and patchy prey (WOrsig
1979, Shane et al. 1986, Scott and Chivers 1 990). When dusky dolphins
(Lagenorhynchus obscurus) and common dolphins fed on anchovies, they
occurred in large groups consisting of 300 or more dolphins, compared to
smaller groups when feeding on other prey (Hui 1979, WOrsig and WOrsig
1980}. Similarly, where Pacific white-sided dolphins predominantly led on prey
that congregate into large schools, such as anchovy, Pacific whiting, and Loligo,
in southern and central California, dolphin group sizes were relatively large
(Stroud et al. 1981, Leatherwood et al. 1984, Walker et al. 1986, Ch. 2). In
64
contrast, off Washington and in the northern North Pacific where these dolphins
fed mainly on cephalopods and mesopelagic fishes, prey that may not
congregate into dense schools, group si.zes were relatively small (Stroud et al.
1981, WOrsig et al. 1989, Buckland et al. 1993, Walker and Jones 1993). Given
Monterey Bay's complex physiography and oceanography, and great
abundance and diversity of seabirds and marine mammals, the Bay must
contain an extremely abundant and aggregate food source.
Throughout the year, most large Pacific white-sided dolphin groups
(>150) were found in outer bay waters, whereas groups inside the bay were
predominantly small (<150). Larger groups of Pacific white-sided dolphins,
however, occurred inside the Bay over the edge of Soquel Canyon and near
the southern rim of Monterey Canyon, areas of high relief and jagged
bathymetric lines:· Large dolphin schools of several species frequent offshore
waters with smaller groups found coastally in Japan (Kasuya 1971). Group size
of bottlenose dolphins increased with water depth, distance to shore, and
openness of habitat (WOrsig and WOrsig 1979, Wells et al.1980, Shane et al.
1986, Scott et al. 1990). The relatively limited deep water between shallow
shelves in the north and south of Monterey Bay may result in smaller Pacific
white-sided dolphin group sizes, presumably reflected in prey type or
abundance here compared to outer bay waters.
Seasonality
Although group sizes of Pacific white-sided dolphins were not
significantly different among oceanographic seasons due to high variability,
' '
65
small groups frequently occurred during the upwelling season and relatively
larger groups occurred during the Davidson season, Leatherwood et al. (1984)
found seasonal differences in group size of Pacific white-sided dolphin among
five latitudinal belts along the eastern North Pacific. In some cases, differences
by season were extreme, particularly off Baja, California where the largest
mean group sizes occurred from July to September and the smallest from April
to June; and from Oregon to British Columbia where mean group size was
greatest from April to June and smallest from January to March. Seasonal
differences in group size are common in other dolphin species, and in most
cases represent seasonality of particular prey species (WOrsig 1978, Hui 1979.
WOrsig and WOrsig 1980, Ballance 1990, Cipriano 1992).
In the Monterey Bay area, Pacific white-sided dolphins differed
significantly in relative individual and group abundance among the three
oceanographic seasons. Although Dohl et al. (1983) used the four solar
seasons in comparing dolphin abundance off central and northern California,
their results were similar to this study despite greater effort during the oceanic
season. During the upwelling season, Pacific white-sided dolphins were least
abundant and occurred less often, usually in small groups. Dahl et al. (1983)
counted the fewest number of Pacific while-sided dolphins during spring and
summer months off central and northern California. During spring, Pacific white
sided dolphins appeared equally distributed off central California. relatively
close to shore (<15 nm), and in small groups, with half their sightings consisting
of groups less than nine dolphins. For three survey years, counts during May
were consistently low, even though Dahl et al. (1983) reported good sighting
'~f'2 '
66
conditions. This corresponds to peak upwelling in the area, possibly suggesting
a reduced abundance or availability of food at this time. By summer, Dohl et al.
(1983) found that 90% of dolphin sighting~ occurred in central California, still in
relatively small groups.
In Monterey Bay, Pacific white-sided dolphins occurred most frequently
during the oceanic season. Their frequent presence during this season,
especially from September through October, indicated that they were exploiting
abundant food sources. Dahl et al. {1983) also found that Pacific white-sided
dolphins were most abundant during the fall, with counts about three times
greater than at other seasons. Although dolphins were distributed along the
central and northern California coastline, they were most concentrated off
central California, within 10-30 km of shore in relatively large groups.
More reproductive behavior and more young of the year were observed
during fall than at other times (Doh! et al. 1983). Norris and Prescott (1961)
reported small calves from May to September in southern California. During this
study, young Pacific white-sided dolphin calves were seen from mid June to
early September. These nursery groups, which could represent the
congregation of breeding and calving dolphin groups, may frequent Monterey
Bay at this time, for predictable prey during an increased need for food by
lactating females. Breeding appears seasonal, as indicated by the extreme
enlargement in male testis size from mid to late summer (Ridgway and Green
1967); with a gestation period of 10 to 12 months (Leatherwood and Reeves
1983).
~·
!
67
During the oceanic season, Pacific white-sided dolphins fed
predominantly in morning hours and socialized more in the afternoon. No
similar pattern occurred during the other two seasons. Brager (1993) found that
bottlenose dolphins exhibited diurnal behavior patterns in summer only,
possibly because of differences in prey seasonality. In contrast, Norris et al.
(1994) suggested that spinner dolphins in Hawaii exhibited distinct diurnal
behaviors because of their year-round, abundant prey in the deep scattering
layer.
When present, Pacific white-sided dolphins were frequently found in
large cohesive groups during the Davidson season. Dohl et al. (1983) also
found that group sizes were greater during winter than during spring and
summer. During the Davidson season, large but separated groups of Pacific
white-sided dolphins may reflect the presence of abundant but patchy prey.
Norris and Dohl (1980b) suggested that some species of dolphins travel in
large schools that are broader than long, allowing them to search a wide area
for prey. Locating a large, single prey school could provide food for hundreds of
dolphins. Also, because dolphins were observed only during daytime, it is
possible that they remain together during daytime and disperse later to feed at
night. Two Pacific white-sided dolphins radio-tagged from the same school in
the Monterey Bay area during this season, remained together during the day
and through the late night, but separated during the early morning hours before
dawn, and joined again later that day.
Pacific white-sided dolphins, although present year-round, probably
were not daily residents of the Monterey Bay area, as suggested by
68
Leatherwood and Reeves (1983). Photo-identification evidence gathered
during this study indicated that certain dolphins frequent the Monterey area,
particularly during the oceanic season, rat~er than new groups of dolphins
continually passing through. This idea fits with frequent sightings of scattered
and milling subgroups in the area during the oceanic season, and contrasts to
the Davidson season, where larger groups of traveling dolphins were frequently
sighted. Identified dolphins usually were seen only once during this season.
One Pacific white-sided dolphin opportunistically photographed near the
Farallon Islands, 130 km north of Monterey Bay, was identified several times in
the Monterey Bay area during this study. This indicated, as Evans (1982) found
for common dolphins, that white-sided dolphins may frequent areas tor variable
periods, then move to other productive feeding areas. A radio-tagged common
dolphin released off southern California was resighted ott Baja, California, 500
km from the release site 10 days later (Evans 1982). Also, a radio-tagged dusky
dolphin was released off north-central South Island, New Zealand, and tracked
over 222 km to the North Island in a period of three days (B. WOrsig, pers.
comm.) Two Pacific white-sided dolphins, radio-tagged near Monterey Bay
during this study, traveled 38 km in 5.8 hours, indicating that they are capable,
just as common dolphins and dusky dolphins, of traveling great distances.
Behavior
The behaviors of Pacific white-sided dolphins were quite diverse and
appear highly variable, as Wilrsig and Wilrsig (1980) suggested tor dusky
dolphins, a species closely related and similar in appearance to white-sided
dolphins. Pacific white-sided dolphin behavioral states differed among group
sizes, degree of school cohesiveness, frequency and types of aerial activity,
and multi-species associations.
69
Evans (1 971) suggested that estimates of delphinid group size could be
affected by the tendency of some species to fluctuate between dispersed
subgroups and cohesive large groups. Subgroups that appear independent
and separated by several kilometers or more, can be actually part of a larger
school. During aerial surveys of common dolphins, Dohl et al. (1986) found two
subcategories of large schools; dispersed schools with many distinct
subgroups, and tightly grouped, rapidly moving, compact schools. These
temporary dispersals into smaller subgroups may represent the basic social unit
with long term integrity (Norris and Dohl1980b, Wiirsig and Wiirsig 1980).
Similar to Wiirsig and Bastida's (1 986) finding of long-term affiliation between a
pair of dusky dolphins, a pair of Pacific white-sided dolphins found within the
same subgroup in Monterey Bay were sighted again together after a year.
Perrin (1 972) suggested that these large groups may be isolated from other
large groups of the same species, forming primary breeding herds.
Pacific white-sided dolphins tended to form scattered subgroups during
periods when they occurred for several consecutive days in the Monterey Bay
area, and frequently led in this dispersed pattern. Although prey type was often
unknown, fishes occasionally were identified as dolphins led near the surface.
During the oceanic season, dolphins occasionally led in scattered subgroups
on Pacific saury (Cololabis saira) 5+ km offshore. Individual dolphins chased,
70
abruptly blocked, and disoriented fish by a quick turn at the surface with <
considerable whitewater. Also during the oceanic period, large groups in
excess of 500 individuals were observed feeding on anchovies. Also feeding
there were pelicans, gulls, shearwaters, California sea lions, and humpback
whales. As suggested by Wursig (1986), dolphin feeding strategy is probably
flexible according to the amount and type of prey available, often involving the
coordination of many individuals. He suggested that where prey is relatively
constant, such as in the deep scattering layer preyed upon by Hawaiian spinner
dolphins and New Zealand dusky dolphins. foraging strategy and group
structure is fairly constant year-round. Where prey type is more variable, the
dolphins' strategy must also change (Wursig et al. 1989). Killer whales fed on
Dall's porpoise and pinnipeds in relatively small groups by rapid "surprise"
attacks, compared to larger groups involved in the attack and pursuit of baleen
whales during periods of several hours (Ternullo et al., unpubl. ms).
Pacific white-sided dolphins frequently exhibit aerial activity, as is
common in many delphinid species (Pilleri and Knuckey 1969, Norris and Doh!
1980b, Wursig and Wursig 1980, Wursig 1986, Cipriano 1992). Heimlich-Baran
(1988) suggested that percussive behaviors may serve a social signaling
function in dispersed traveling groups of killer whales. Leaps may produce
short-range omni-directional sounds important for communicating among
dolphins that are visually isolated. Leaping often occurs at night and in
dispersed schools of Hawaiian spinner dolphins (Norris and Doh! 1980a).
Percussive repetitive leaps can be heard up to 0.5 km away (WOrsig and Wursig
1980).
71
While engaged in this type of aerial activity, Pacific while-sided dolphins
were frequently found in large traveling groups, as opposed to widely dispersed
groups engaged in other activities. If these aerial behaviors function to facilitate
communication or structuring of the school, this activity would probably be most
advantageous when dolphins are in these large tight groups, as individual
vocalizations may be "drowned out", making more distinct signals necessary.
While engaged in this activity, an individual Pacific white-sided dolphin exhibits
one of about 13 different types of leaps, often repeating the same type up to 20
times in quick repetition. However, single noiseless leaps occurred most when
Pacific white-sided dolphins were socializing, probably resulting from
excitement and exuberance rather than communication. In this case, Pacific
white-sided dolphins performed single leaps, often in unison with one or more
dolphins, entered· nose first with little splashing, probably to surface and gain
more momentum while diving back down again. This behavior also was
exhibited by dusky dolphins (Wursig and WOrsig 1980).
Many multi-species associations involving Pacific white-sided dolphins
probably are prey related. Food-based associations are common among widely
diverse species types, such as dolphins, whales, pinnipeds, birds, and fishes
(Perrin et al. 1973, Au and Perryman 1985, Martin 1986, WO rsig 1986, Katona
and Whitehead 1988, Norris and Schilt 1988, Scott and Chivers 1990). Norris
and Dahl (1980b) suggested that mixed schools occur in areas with a
predominant prey source. In southern California waters, anchovy and squid are
dominant prey and mixed species groups are common. Evans (1982)
suggested that the large opportunistic multi-species aggregations of seabirds,
72
dolphins, and tunas were related to high biological productivity. Spotted
dolphins and yellowfin tuna (Thunnus albacares) in the eastern tropical Pacific
have similar diets and are closely associated (Perrin et al. 1973). California sea
lions, humpback whales, and birds associated with Pacific white-sided dolphins
in Monterey Bay mainly during feeding episodes. Antonelis et al. {1984)
observed sea lions feeding with Pacific white-sided dolphins in southern
California when anchovies were abundant. It is possible that Pacific white-sided
dolphins search for large groups of Risso's dolphins possibly for enhanced
cephalopod foraging, just as bottlenose dolphins appear to join pilot whales
(Shane 1984).
The reasons for species associations that are not primarily food based
are less clear. Scott and Chivers (1990) suggested that in pelagic waters, mixed
species herds ctluld provide a similar protective function as large single
species herds, and provide more options in feeding strategies. The eastern
tropical Pacific association among spinner dolphins, spotted dolphins, and
yellowfin tuna is possibly due to the fact that spotted dolphins and tuna are
diurnal feeders, preying on epipelagic species, whereas spinner dolphins feed
nocturnally on mesopelagic species. Spinner dolphins join spotted schools in
the morning and spend the day resting within spotted dolphin schools (Fitch
and Brownell 1968. Perrin et al. 1973). Similarly, northern right whale dolphins
off California fed predominantly on mesopelagic fishes and cephalopods
(Leatherwood and Walker 1979) probably at night, whereas Pacific white-sided
dolphins fed on epipelagic fishes and cephalopods (Stroud et al. 1981, Ch. 2),
often during the day. Although in the northern North Pacific, these two species
''~J"'' "~1
73
fed on very similar prey, it is not clear to what extent they associate (Walker and
Jones 1993).
Photo-identification
Anomalous-colored Pacific white-sided dolphins have been reported
elsewhere (Brown and Norris 1956, Brownell 1965, Hain and Leatherwood
1982, Leatherwood and Reeves 1983, Walker et al. 1986, Stacey and Baird
1991), although no one has previously photo-identified such individuals.
Several anomalous-colored (white) dusky dolphins exist in New Zealand and
have been tracked for at least five years (B WOrsig, pers. comm.). Photo
identification in Monterey Bay of "white" Pacific white-sided dolphins provided
valuable information on movements, associations, and residency, that
otherwise might' be difficult to obtain without actually tagging dolphins. At least
one of these dolphins had a calf that survived over a year, and several "white"
dolphins were sighted throughout three years. It is possible that these dolphins
could be used as "herd" markers to determine residency patterns, movements,
and seasonal shifts of large groups of dolphins. However, considerably more
effort is needed in the Monterey Bay area and other areas frequented by Pacific
white-sided dolphins.
Radio-Track
The maximum dive recorded for 01, 372 sec, was more than twice the
maximum dive duration previously published for this species. This was the first
dive recorded after the dolphin was released, and may represent an unusually
74
long dive time due to the stressful situation. The remainder of dives were less
than 196 sec. Hall (1970) recorded a maximum dive duration of 134 sec for a
female Pacific white-sided dolphin trained for open ocean release. This
occurred when the dolphin dove to 214m and activated a sound device. After a
3-min dive, a trained bottlenose dolphin had a low percentage of lung oxygen,
and was virtually anaerobic after a 6 min dive and spent 4 to 5 min recovering at
the surface. This dolphin could dive continually if dives were less than 2 min
duration. Kanwisher and Ridgway (1983), therefore, suggested bottlenose
dolphins limit dive duration to avoid oxygen debt, with long dive durations the
exception. Radio-tagged dusky dolphins, very similar in appearance and
slightly smaller than Pacific white-sided dolphins. dove a maximum of 182 sec
duration (Wursig et al. 1985, Cipriano 1992). Because Pacific white-sided
dolphins exhibited relatively short dive durations (70% less than 20 sec) and
dive durations greater than 90 sec were rare, indicated that dives greater than
this may create an oxygen debt They are probably not deep divers, and
presumably feed on prey at the surface or prey that vertically migrate at night,
matching the habits of most of their major prey items (Ch. 2).
The two distinct dive patterns observed in radio-tagged Pacific white
sided dolphins were similar to those of radio-tagged spotted dolphins in the
eastern tropical Pacific (Leatherwood and ljunblad 1979, Scott and Wussow
1983}. Spotted (Stenel/a attenuata) and common dolphins observed feeding,
spotted dolphins milling over a sharp dropoff in depth, and foraging bottlenose
dolphins exhibited the clumped pattern of dive durations. Regular short dives
were associated with traveling spotted and bottlenose dolphins, and mass
>'if''' ! '
75
movements of common dolphins (Leatherwood and Ljunblad 1979, Evans
1971, Scott and Wussow 1983, Lockyer and Morris 1987). Similarly, Pacific
white-sided dolphins exhibited the clumped pattern most often while they milled
or fed, and the regular pattern while they traveled.
Environmental Conditions
Pacific white-sided dolphins varied in relative group and individual
abundance, and behavior among the three oceanographic seasons in the
Monterey Bay area. Each oceanographic season in Monterey Bay can
generally be characterized by specific environmental features (Table 5; Bolin
and Abbott 1963, Breaker and Broenkow 1994, Chavez et al. 1991 ). The mean
of eight environmental variables for each season during this study (Table 6)
appeared consistent with •normal" oceanographic conditions, although, dolphin
occurrence and abundance was not consistent among similar seasons in
different years. Interannual variability occurs among these seasons in both
onset and intensity of characteristic features (Ainley and Boekelheide 1990),
which may affect variability in dolphin ecology among years. An extreme
example of this occurred during an El Nino period corresponding to
anomalously high temperatures when some bottlenose dolphins moved from
southern California into Monterey Bay (Wells et al. 1990).
Some seasonal differences in Pacific white-sided dolphin occurrence,
relative abundance, group size, and distance to the shelf-break may be
explained by sea surface temperature. This environmental variable is
commonly used to explain biological effects because of its distinct physiological
Table 5. General oceanographic characteristics (months, sea surface temperature, salinity, coastal fronts, mixed layer, upwell index, surface currents, winds, temperature gradient in bay) of the three seasons (upwelling, oceanic, Davidson) as related to Pacific white-sided dolphin relative individual and group abundance. The designations of high, med, and low are relative comparisons among the seasons.
OCEANOGRAPHIC
GROUP
strong temp gradient
(surface-50m),
steep rise in isotherms,
low
deeper isotherms,
increased stratification,
strong gradient, distinct
onshore CA Current
deepest mixed layer, temp
uniform to considerable
·depths
Davidson Current
Table 6. Mean ±SD of eight environmental conditions (sea surface temperature, salinity, nearshore front intensity, relative front intensity, depth mixed layer (m), upwell index, percent days of intense upwelling, temperature gradient in bay) as related to Pacific white-sided dolphin relative individual and group abundance among the three oceanographic seasons (upwelling, oceanic, Davidson) during the study period.
ENVIRONMENTAL
RELATIVE FRONT INTENSITY
DEPTH MIXED
PERCENT DAYS INTENSE UPWELL
TEMPERATURE
GROUP
ABUNDANCE
INDIVIDUAL
shore
.62±.10 .34±.15 .12±.03
1.42
78
effects (Bakun and Parrish 1980). The occurrence and movements of several
dolphin species were related to temperature (Gaskin 1968, Leatherwood et al.
1980, WOrsig and WOrsig 1980, Kasuya -and Jones 1984, Au and Perryman
1985, Selzer and Payne 1988, Breese and Tershy 1993). Dohl et al. (1986)
thought the movements of common dolphins in and out of the Southern
California Bight were temperature related, with peak numbers of dolphins f
occurring 3 to 5 weeks after the intrusion of warm water. When the water cooled,
dolphins shifted their distribution south into warmer waters. In Monterey Bay,
Pacific white-sided dolphins were more abundant, occurred more frequently,
and were sighted closer to the shell-break as the temperature increased, with
larger group sizes occurring most often in warmer temperatures. Dahl et al.
(1983) also found that Pacific white-sided dolphins were most abundant during
warmer temperature periods, although not observed in water greater than
18.3°C.
In the Monterey Bay area, the temperature is warmest during the oceanic
period, especially when the wind relaxes and warmer offshore water is
advected onshore. When this occurs, frontal gradients can intensify in near
shore waters. Fronts can be located further from shore during persistent
upwelling events. Pacific white-sided dolphin occurrence was high when the
intensity of near-shore thermal gradients increased. Along-shelf fronts form
between warm, low salinity (>13°C, <33.5psu) well stratified offshore water, and
cool, high salinity, vertically homogeneous upwelled water (Kinder et al. 1983,
Schwing et al. 1991 ). Mobile zooplankton are concentrated at fronts, providing
T
79
increased food for fishes, cephalopods and seabirds (Brown 1980, Briggs et al.
1984, Haney 1985; ).
Dolphins of the genus Stene//a"migrate near Sagami Bay, Japan, in
years when the Kuroshio current is near-shore, with dolphins frequenting the
frontal area where the colder Kuroshio Current meets warm water (Miyazaki et
al. 1974). Sightings of Stene/la spp. in the eastern tropical Pacific are
concentrated in warm water near the equatorial front, with movements
correlated to changing oceanographic conditions (Au and Perryman 1985).
Smith et al. (1986) found an abrupt change in birds and mammals near these
fronts off California, and suggested that the tilting and shallowing of the mixed
layer near fronts could increase food availability. In outer waters of Monterey
Bay, these fronts may approach the shelf-break, especially if winds have been
light for many days. Frontal convergences near shelf-breaks often intensify
aggregations of prey and predators (Fournier et al. 1979, Herman and Denman
1979, Ainley and Jacobs 1981 ), possibly explaining why dolphins were sighted
closer to the shelf-break in warmer temperatures when thermal gradients
increased.
Fronts also may be topographically induced near headlands, causing
eddies in the lee of the headland to congregate fishes, seabirds, and mammals
(Dykstra et al. 1984, Wolanski and Hamner 1988). It is possible that this
phenomenon could occur on a small scale near Pt. Pinos, Cypress Pt., or Pt.
Lobos, contributing to the high abundance of dolphins frequently sighted there.
Strong frontal gradients in Monterey Bay occur most often during the upwelling
and oceanic seasons, although the upwelling season generally has fewer
80
periods of wind relaxation events compared to the oceanic season. Although
frontal gradients occur during the upwelling season, the lower abundance of
dolphins at this time may be the result of increased and intensely turbulent
periods. Prey may disperse and there may not be adequate time for an
abundant prey base to stabilize (Reilly 1990). The fall of 1987 was
characterized by calm winds for relatively long periods, with a strong near-shore
frontal gradient and a high temperature anomaly possibly resulting in the
concentration or stabilization of prey. Dolphins were highly abundant, frequently
sighted, and occurred closest to the shelf-break during this season compared to
all other seasons during the study period.
The presence, strength and depth of a thermocline also influence marine
organisms. Some species of dolphins aggregate in areas of the eastern tropical
Pacific where the thermocline shoals under tropical surface water (Au and
Perryman 1985). The shallow thermocline in this area may act as a vertical
aggregating mechanism for squids and fishes (Green 1967, Reilly 1990). The
prey are concentrated near the surface instead of spread through the water
column as in other tropical areas (Pryor and Shallenberger 1991 ). In the
California Current area, the thermocline slopes upward toward the coast (Hicky
1979, Mclain and Thomas 1983) and varies seasonally (Chavez et al. 1991 ).
The thermocline is used as a boundary for some species of zooplankton,
affecting the distribution of their fish and cephalopod predators. Areas or
seasons characterized by a deeper thermocline or none at all may result in
lower prey concentrations due to dispersal throughout the water column (Briggs
et al. 1987).
T '
81
The thermocline is least stable during the upwelling period in Monterey
Bay, corresponding with lower occurrence and smaller group sizes of Pacific
white-sided dolphins. This contrasts with the oceanic season, in which surface
heating during periods of light wind can produce strong stratification and a
shallow mixed layer (Husby and Nelson 1 982). Increased occurrence of Pacific
white-sided dolphins during this season may reflect a stable and concentrated
prey base. In certain years, more frequent upwelling events during the oceanic
season could destroy stratification, possibly disrupting prey and therefore
influencing dolphins. This may have occurred during the 1 989 oceanic season,
corresponding to the greatest percentage of days with intense upwelling and
the lowest dolphin occurrence for any oceanic season. The greatest mean
group sizes also occurred during this season, indicating that dolphins were
probably just traveling through the area.
The thermocline during the Davidson Current period is persistent and
relatively deep, but strong winter storms can disrupt this, destroying stratification
and further deepening the mixed layer (Nelson 1 977). The higher abundance of
dolphins during this period was due to relatively larger group sizes, but a lower
occurrence, indicating dolphins may exploit abundant food patches during
periods with a strong and relatively shallow thermocline, then move as local
conditions change.
Biological gradients at eddies also influence phytoplankton, zooplankton,
cephalopods, and fishes (Haney 1 986). Eddies occur frequently in the
California Current (Fiedler 1 986), and often approach the coast. A warm, low
salinity eddylike feature consistently appears west of Monterey Bay (Schwing et
82
al. 1991, Breaker and Broenkow 1994). The close approach of this eddy near
shore appears unique to the area, as no similar features are found off central or
northern California (Schwing et al. 1991). This phenomenon may contribute to
higher dolphin abundance around Monterey compared to other coastal areas.
Each oceanographic feature considered separately may not explain
dolphin occurrence patterns, but the combination of these features with
physiography may provide ideal conditions for cetaceans (Brown and Winn
1989, Bruggeman 1992). Surface eddies and convergences caused by
complex topographies (Neumann 1960) may concentrate dolphin food (Au and
Perryman 1985). Within the study area, the shelf-break from Pt. Pinos to Pt.
Lobos is relatively steep, with interspersed mini-canyons running perpendicular
to the break, a region of high dolphin concentration. The importance of the
shelf-break habitat in Monterey Bay has been demonstrated for organisms
found concentrated there, including euphausiids, shortbelly rockfish (Sebastes
jordam) and blue whales (Chess et al. 1988, Schoenherr 1991).
Strong upwelling events and increased nutrient availability are seasonal,
creating a temporal lag in productivity. Phytoplankton production in Monterey
Bay peaks from March to August (Schrader 1981). The abundance and
distribution of higher trophic level organisms are spatially separated from
physical indicators of primary production (Croll1990). This is exemplified by the
seasonality of northern anchovies (Engraulis mordax) and common dolphin
distribution in the Southern California Bight (Hui 1979). The seasonal
abundance of common murres, (Uria aa/ge) in Monterey Bay was associated
with peak juvenile rockfish abundance (Croll 1990). Some of the main prey
species of Pacific white-sided dolphins also tend to be seasonally abundant,
including northern anchovies (Engraulis mordax), Pacific whiting (Merluccius
productus), and Loligo opalescens. -
83
Anchovies are one of the most abundant pelagic schooling fishes in the
northeast Pacific and are a common prey item for fishes, birds, and mammals in
Monterey Bay (Morejohn et al. 1978). High concentrations occur over
submarine canyons and escarpments in upwelling regions (Mais 1974, Baxter
1966). Anchovies differed in abundance, fish school size, and distribution
among seasons (Baxter 1966, Mais 1974, Smith 1981, see Ch. 2). Patterns of
these fishes were related to Pacific white-sided dolphin distribution, grouping
patterns, behaviors, and seasonal abundance in southern and central
California (Dohl et al. 1983, Leatherwood et al. 1984, this study, see Ch.2).
Horizontal movement of anchovies may be blocked by sharp discontinuities at
fronts (Biaxter and Hunter 1982), possibly contributing to the greater occurrence
of dolphins in the study area when frontal gradients are great near-shore. This
information strongly indicates that Pacific white-sided dolphins are influenced
by anchovies.
Pacific whiting is another important prey item for Pacific white-sided
dolphins. The high biomass of juvenile fishes concentrated near the shelf-break
(Alverson and Larkins 1969, Bailey et al. 1982) likely contributes to the high
abundance of dolphins found off central California compared to other areas of
their range (see Ch. 2). Productive fishing grounds for whiting are associated
with prominent geographical sites such as banks and sharp curves in the
r !
continental slope and canyons (Bailey et al. 1982), features prominent in
Monterey Bay.
84
Cephalopods also comprise a major portion of the dolphins' diet, but
patterns of their abundance and distribution are poorly known, except for Loligo
opalescens. Lo!igo moves inshore to spawn in shallow waters (<100m) year
round, with a peak in Monterey Bay from May-July; otherwise, schools occur
widely in coastal and offshore waters (Frey 1971, Cailliet et al. 1979, Hardwick
and Spratt 1979, Fiscus 1982). During the spawning season, squid are inshore
and therefore unavailable to dolphins, corresponding to a time of low dolphin
abundance in Monterey Bay, and central California in general (Dohl et al.
1983).
Pacific white-sided dolphin abundance, distribution, and behavior are
reflected by the behavior and abundance of certain prey. Dolphin presence may
be explained by a few predominant prey species. Also, the relatively large
groups of dolphins observed in the Bay compared to other locations within their
range could be influenced by the infrequent but year-round occurrence of killer
whales. Killer whales in this area are the transient type, often occur over or near
the shelf-break, and frequently feed on several species of marine mammals in
the Bay (Black et al. 1993, unpub. ms, Ternullo et al. 1993, unpub. ms.).
Although killer whales have not attacked Pacific white-sided dolphins in
Monterey Bay, dolphins have exhibited distinct flight responses when in close
proximity to these whales, and killer whales are known predators of white-sided
dolphins (Dahlheim and Towell 1994). The deep water but near-shore habitat in
Monterey Bay may provide some degree of protection from predators.
T I
85
Monterey Bay appears to be important for Pacific white-sided dolphins,
but they are not abundant year-round. Therefore, the dolphins move elsewhere
or change their grouping patterns, probably reflecting seasonality and behavior
of prey (Norris and Prescott 1961, Shane 1984, Kenney and Winn 1986).
Although Pacific white-sided dolphins are less abundant during the
cooler spring upwelling season in the Monterey Bay area, it is not known if they
tend to move further offshore, north, or south. Because peak upwelling periods
progress seasonally northward from Baja, California to Washington, dolphins
may move into more favorable, less intense upwelled areas. Upwelling off
Oregon and Washington occurred during summer and fall, peaking during July
and August (Bakun 1973), and encounter rates for Pacific white-sided dolphins
were relatively low at this time (Green et al. 1993). Upwelling was most intense
off Baja during April and May (Bakun 1973). and Pacific white-sided dolphin
abundance peaked from July-September (Leatherwood et al. 1984).
There are four possible explanations for low spring dolphin abundance
in the Monterey area. Dolphin distribution shifts north, as suggested by
Bruggeman (1992), but dolphins do not appear to be abundant enough off
Oregon and Washington (Bruggeman 1992) to account for a shift from
California (Barlow 1993, in press). Secondly, dolphins could shift offshore, as
suggested by Dahl et al. (1983). However, Dahl et al. (1983) conducted
extensive aerial surveys in central and northern California and found dolphins
mostly within 110 km of shore, even though survey effort extended to 278 km
offshore. Barlow (in press) surveyed California waters out to 556 km during
T !
86
summer/fall and found white-sided dolphins within 185 km of shore. There was
no evidence of a major shift offshore during spring/summer.
Pacific white-sided dolphins co-uld shift into southern California.
Leatherwood et al. (1984) reported Pacific white-sided dolphins to be most
abundant in southern California from November to May. He suggested that
dolphins may shift south into Baja, California during summer and fall. Pacific
white-sided dolphins are abundant in central California from September to
February, overlapping their abundance in part off southern California. Because
peak densities off southern California are greater than reported for Baja
California (Leatherwood et al. 1984), it is possible that some of the southern
California dolphins also may shift north into central California during summer
and fall. Adding to the complexity, the two forms (large-southern, small
northern) of Pacific white-sided dolphins, appear to overlap in southern
California (Walker et al. 1986). It is possible that the southern form shifts south
into Baja, and the northern form shifts into central California. Both forms may
occur in abundance off southern California when anchovies are most
concentrated there.
Lastly, in addition to a partial shift south, dolphins may change their
grouping patterns and disperse along the coast. This idea is supported by the
observation of Dohl et al. (1983) that Pacific white-sided dolphins were
distributed along the entire central and northern California coastline during
spring. In other seasons, dolphin distribution was more localized.
Although the complex linkages between environmental processes and
biological consequences are difficult to observe (Bakun and Parrish 1980), an
87
investigation of environmental variables, prey abundance and distribution, and
dolphin patterns of abundance and behavior can elucidate apparent trends.
The behavior, movements, and occurrenc"e of Pacific white-sided dolphins must
be largely prey related, but also influenced by habitat type and predators.
Because Pacific white-sided dolphins are present year-round, and often in
groups in excess of 500 individuals, with frequent feeding activity, certain
locations in the Monterey Bay are clearly important for dolphins, providing a
predictable and abundant food source.
CHAPTER 2
FOOD HABITS OF PACIFIC WHITE-SIDED DOLPHINS, OFF CENTRAL CALIFORNIA
88
89
INTRODUCTION
Pacific white-sided dolphins (Lagenorhynchus obliquidens) occur
throughout temperate waters of the North Pacific Ocean, 23°N to 61 aN
(Leatherwood et al. 1984) and are the most abundant cetacean off central and
northern California (Doh! et al. 1983). Pacific white-sided dolphins are
commonly found in groups of several hundred to more than a thousand
individuals, often in the company of northern right whale dolphins (Lissodelphis
borealis} and Risso's dolphins (Grampus griseus; Dohl et al. 1983,
Leatherwood and Reeves 1983). Pacific white-sided dolphins reach 2.3 m in
length and 150 kg in weight, have a relatively short beak, and an average of 30
teeth in each jaw (Walker et al. 1986).
Pacific white•sided dolphins are known to eat 24 taxa of fish and 10 taxa
of cephalopods, mainly schooling epipelagic species in California and
Washington (Jones 1981, Stroud et al. 1981, Walker et al. 1986). Walker and
.···Jones (1993) described 36 taxa of fishes and 12 taxa of cephalopods in dolphin
stomachs, dominated by mesopelagic species, especially myctophids, in the
offshore waters of the northern North Pacific. In the western Pacific, Wilke et al.
(1953) found that white-sided dolphins ate predominantly myctophids. Because
these dolphins live in a variety of habitats, encompassing a wide range of
oceanographic conditions, prey types probably vary according to region and
season.
Although several studies of food habits exist for this species, there is little
information on the estimated length, weight, and relative importance of
T
90
particular prey species. Further knowledge of these variables and the
seasonality, abundance, and food habits of prey, should increase our
understanding of the role of Pacific white:sided dolphins in the food web.
Therefore, the objectives of this study were to: (1) identify prey types, (2)
estimate standard fish lengths and weights, (3) estimate cephalopod mantle
lengths and weights, and (4) compare prey types from central California to other
coastal locations.
91
METHODS
Stomach contents were examirled from 18 Pacific white-sided dolphins
that stranded on beaches of Sonoma Co. (1 ), Marin Co. (1 ), San Francisco Co.
(2), San Mateo Co. (3), Santa Cruz Co. (3), and Monterey Co. (8). Sixteen
dolphins were collected as dead strandings and two dolphins stranded alive but
later died. Dolphins were all adults, from 172 to 225 em standard length, and
were collected from March through October, 197 4-1992. Dolphin stomach
contents and stranded dolphins were obtained from Moss Landing Marine
Laboratories, Long Marine Laboratory (U.C. Santa Cruz), Robert Jones (U.C.
Berkeley), and California Academy of Sciences.
After measurements and dissections were performed on these dolphins,
both ends of the stomachs were tied off, the stomachs removed, and frozen for
later analysis. Stomachs were thawed and contents strained through a 0.01 mm
sieve, and otoliths and beaks removed. Otoliths were washed, counted (right
and left), measured with vernier calipers to the nearest 0.1 mm length, and
stored dry in glass vials. Upper and lower cephalopod beaks were separated,
counted, measured (lower rostral length for squids, lower hood length for
octopus), and placed in 50% isopropyl alcohol for storage.
Otoliths were identified by the author using Fitch {1964, 1968, 1969) and
the reference collection of J.T. Harvey at Moss Landing Marine Laboratories.
Lower beaks of cephalopods were identified by the author using Wolff (1984),
Clarke (1986a), and a reference collection of J.T. Harvey at Moss Landing
Marine Laboratories, with assistance and confirmation to voucher specimens
from E. Hochberg, Santa Barbara Museum of Natural History and W. Walker,
National Marine Mammal Laboratory, Seattle.
92
Prey items were identified to the lowest possible taxon. Number of each
prey species/genus for each stomach was determined from the number of lower
cephalopod beaks and the most numerous left or right otolith. The estimated
length and weight of each prey item were calculated using regressions of fish
standard length (SL) and weight to otolith length (Harvey et al. in press) and
cephalopod dorsal mantle length (DML) and weight to lower beak rostral length
(LRL; Wolff 1984, Clarke 1986a, W. Walker, pers. comm.) for species in which
values have been determined. For those species with no regressions, the
genus/family or closest similar species' length/weight equations were used. The
index of relative importance (IRI= (%number+% reconstituted mass) x%
frequency of occurrence) for each prey item was calculated following Pinkas et '
al. (1971) with the replacement of mass for volume (Hyslop 1980, Finley and
Gibb 1982, Cockcroft and Ross 1990, Dorfman 1990).
93
RESULTS
Sixteen stomachs of Pacific wh1te-sided dolphins contained prey with
one to 12 different prey species. A total of 1,048 otoliths and 503 lower
cephalopod beaks was identified to species or genus. Seven fish species and
one fish taxon (67.6% number, 87.5% occurrence, 80.2% mass), and 10
cephalopod species and three cephalopod taxa (32.4% number, 68.8 %
occurrence, 19.8% mass) were identified. Pacific whiting, Merluccius productus,
was the most frequently occurring fish species in all stomachs (69%) followed
by northern anchovy, Engraulis mordax, and plainfin midshipman, Porichthys
notatus. Loligo opalescens and Onychoteuthis borealijaponicus were the most
frequently occurring cephalopod species. Based on IRI values, Pacific whiting,
plainfin midshipman, northern anchovy, Sebastes sp., Gonatidae, L.
opalescens, and 0. borealijaponicus were the most most important prey
species, respectively (Table 7). Fish species were an average 15.9 ± 7.53 em
Sl and 111.8±118.56 g weight (Fig. 26}. Cephalopod species averaged
8.6±5.11 em DMl and 75. 7±127.65 g (Fig. 27}.
Stomachs contained two to 66 otoliths of Pacific whiting, one to 118
otoliths of northern anchovy, and three to 192 otoliths of plainfin midshipmen.
Although Sebastes sp. were low in occurrence, one stomach contained 178
otoliths of this species. The remaining fish species, Pacific sanddab
(Citharichthys sordidus), jack mackerel (Trachurus symmetricus), white croaker
(Genyonemus lineatus}, and Pacific sardines (Sardinops sagax) were low in
Table 7. Prey of 16 Pacific white-sided dolphins collected off central California.
Preyftam Tolai
Fish
Batracholdki.:IS
Pvrichlhys flola!us 9olhldaa
Cifherichlhys sordidus Cnmngldae
Tmchutm S}'Ttlfl'll!tricUS Clupllitii\G
Snrd:'fWPS ~ ... Etlgraultd!dan
Engmulis f1JOfiW: Mnriucclldaa
Marfucdus pnxtuctus Sclasnldnn
Benjltl\1omus linusrus Scorpaanldna
StJbaslas sp"
Cephalopods
Ghlrateulhldaa Ghim;ou/1115 sp.
Ctanch~dao G,;tli/aurhfs sp.
Enoplo!O\llhldan Abroliopsls fells
Gona«dna GevtllM$i.tA Goostusbef'/}'1 Goootopsis borealis
Hls!iotoulhlda!! Histlotrmth!.f ha/oropsis
ln!!glnlda-e Lo!iDO cplllll$CC!rl5
Octopolnulhlduo Octopolnvlhls deltJttOn
Oclopodldoa Octopus rutmsc<tns
Ocythcldna Ocylho6' h.Jbsrr:ulala
Onychotnulhldoo Onycholoulhls
boroafiiapcnlcus Seplolldan Rossfn ci5ea
Number N "k
Occurrence N % 15 1000
1040 6Hi 1o1 1175
J63 23.7 0
lG 1.0 12.5
5 02 2 125
0.1 OJ
291 10.6 ' 553
"" lUI 11 GaB
0.3 53
160 1U.i '2 12.5
sro 32.4 1' oo.e
11 0.7
42 .2.7
00 ,. 159' 10.3
63 4-1 .Bl 5r2 15 1.0
O.J
143 92
4 0.1
"' 05
5 0.1
38 2.5
0.1
., 5 313
' 250 G ::175
' 250 4 25.0 J HV3
63
7 ..,. ., ,;o
2 12,5
7 ""~
63
Mass (g} Total % 03,709.3 tOM
75, Hi£1.4 nn.2
62(L0 0.7
1,167.1 1.9
4.t 0.0
2,961.5 32
21,\?25.2 2.2.7
559.1 0.6
34,SJ1.5 37.2
18.539.5 19.8
11:10 O.t
597.2 0.6
14111 0.2 5,514.9 5.9 1,421.6 1.5
781.1 o.o 267.0 J.S
133 OJ}
J,o;m.e 3.3
159.2 0.2
Sil7.t Q.9
2,260.0 2.3
5,e:J4.7 6 . .2
13.6 0.0
IRI
1,990
21
<1
1,239
2.374
0
010
5
'"' "" 600 ... Hill
"'
2
" , ...
<1
IRI O!oilth/Beak rnnk Length{mm)
maon(lSO}mnge
2 4.9±1.0 2.3-9.5
15 3.8±1.2
I 4 5Jrl0_J 5.D-5Jl
20 2.5
3 J.atoJt
10.7±3.7
JB O.Rl:L3
Est Length (em)
mMn{iSO) mnga
Est. Weight (g)
msen(±SO) rnnga
12At2.5 G.2·~H.I 35Jl±2U .t.2·1SO
13.7±3.7 10.3·20.2 4l.!H29.!i 19.1·121
29.2!:0.1 29.0-29.3 353.4.±3.6 347-357
4.1
9.0±1.3 5.2·13.7 10. H:JA 2.0·26.9
4~·13.2 l5J>±J.9 9.3·2lMJ 193.0H59 31·1115
17 2.4:«UI
9 2.0llil.B
10 1.lli:0.2
g 3.1:t0.J 7 2.4.Hl.l
11 4.3±1.7
19 LG
5 1.2±0.2
18 1.0±0.5
12 1.6±0.2
~3 6.7±1.7
6 :lru.1A
21 L3
1.4·3.5
1.0-3.5
0.7·1.'1
2.3-4.1 1.643 2.Ht3
0,7·19
1.2·2..2
1.1·2.0
4.0-8.1
1.1-S.ll
7.0±1.9
liJ.ru2.fl:
3JUR6
ti.BftA S.!liL5
l6.7i5.B
3.5
11J.7t1A
4.!l.tl.4
J.!I-HI.S
H!.Zi4,7
14..1:1:.8.7
3.9
~-0· 8.7
S.J.t5.S 14.2:UUl .2.1·311.1'1
2.6-5.4
.5.5·13.2 2.:S.9.B 0.()-.JOJ:I
2.5±1,0
22.6±7.8 9.frt4.5
228.0±224
1~l3
LQ-.54
0.3·57.0 2.5·.25.0 34·061
7.5·14J} 21.7±5.5 9,3·39.5
J9.Bt.2t.O B.7·52.4
21.7:16.7 6.6·39.6
10.7·22.3 440±134
2.9·30.0 154.!::188
13.6
200·500
10·609
95
45 ~
183
E 40 ,£.
:535 "' 1: 5
"' 30 ~ 180 ..J J:
"' 25 u:: 368 4 'D 20 16
EE "'
EB -.. E 15 291
:;:; + <II w 10 1: 1 .. --"' 5 ::;:
0 1115
500
~450
"' -E 400 "' ~ 350
-:ii 300
u:: 250 'D
"' a; 2oo E
"iii 150
~ w
Eb 1: 100 ., "' 50 ::;:
0 .,J;,
<II <II <II ~ X Ul Ul ci. :::J :::J :::J "' :::J :::J Ul
iii "C u Ol "C u iii E ·.:: "' 0 Ul 0 Qj :::J <lJ <lJ
0 Ul
E "C :§ 1ii c: E Ul E! Ul "' Ul E c. .!!l c. Ul .0 >. Ul 0 .c >. >. c: "S Ul :::J <lJ
:E .c Ul i5 "' :::J E CfJ u :E Ul iii 0, ·c:; <lJ
·:s u 2 u c: ·~ CfJ c:
:::J 0 a. :::J w
~ >.
£ .c c: u ., C3 ~ ::;: CJ
1- Fish Species
Figure 26. Mean estimated standard lengths (em) and estimated weights (g) for eight fish species. Standard deviations are represented by boxes and ranges by vertical lines. Sample sizes are indicated above vertical lines.
96
40 'E ..,
35 -'J. ,. 3!1
1: 30 !l ! 25 5 c:
~ .. 20 ::;: '!!I 42
~ 15
ai$$~$ 143
.E 10 $ ;;
UJ 4 c: 5 lil
39 1 .. 1 $ " ::;;
0
509
+ ~ <¥ <i <i "' < "' "' ~ g. ~ .!!!
.!!l :; '§ "' "' c. :>
" -~ .!2 1ii 2 1ii 1ii t: 0 :E (!) D (!) u <
~ "' .!!l "' c: ~ "' "' "' ~ "' c: E 1ii :> ,!,! c. "' a " 'l:i "' !:! e " 1ii :; '2 D 0 "' " " !:! 0 "' Q) .!!! "' n "' D
" '0 .0 " 0..
2 .!!! " 2 .0 t~ "' .. .t:; n .!!! .a 05 "' 0 1ii <: 0..
~ .t:;
!!l "' 0 0 0 'S .. :!! 0 (!) 1ii .!2' "' c. 0 0 0::
15 0 .g .<: D <: "' ~ 0 ~ -' c.
~ f.!l 0 0 0 .!!! u :X: 0 "' Cephalopod Species 0
.t:; u
"" c: 0
Figure 27. Mean estimated mantle lengths (em) and estimated weights (g) for 13 cephalopod genus/species. Standard deviations are represented by boxes and ranges by vertical lines. Sample sizes are indicated above vertical lines.
97
occurrence and few in numbers. Estimated standard fish length was 3.3 to 41.8
em (Fig 28), and estimated weight was 2.6 to 1,114.8 g (Fig. 29).
Stomachs contained one to 92 lower beaks of L. opalescens. one to 17
lower beaks of 0. borealijaponicus, and one to 59 lower beaks of Gonatus sp.
A, Gonatus berryi, Galiteuthis sp., Abraliopsis felis, and Octopus rubescens. The
remaining cephalopod species, Gonatopsis borealis, Ocythoidae tuberculata,
Chiroteuthis sp., Octopoteuthis deletron, Histioteuthis heteropsis , and Rossia
pacifica were low in occurrence and few in numbers. Cephalopod estimated
mantle length was 2.5 to 30.6 em (Fig. 30), and estimated weight was 1.0 to
861.4 g (Fig. 31 ). The largest cephalopods were G. borealis (30.6 em DML and
861.4 g weight) and 0. borealijaponicus (30.0 em DML and 608.9 g).
> u c .. ::J .,. l!! ... -r: .. 1:! .,
Q.
60
60
40
30
20
10
0 1 3 5 7
60
60
40
30
20
10
Engrau/ls mordax
""""'
9 111315171921232527 29313335373941
Porlchthys notatus n...:368
60
60
40
30
20
10
0
60
60
40
30
20
10
Merlucc/us productus ~11}3
Sebastes sp. n=1S()
0 Qi------L-------------=~~~---------1 3 57 911131517192123252729313335373941 1 3 57 911131517192123252729313335373941
Estimated Standard Length (em)
Figure 28. Frequency histograms of estimated standard lengths (em) for four fish species found in Pacific white-sided dolphin stomachs.
10
Engraulls mon:lax
~·
0~----------------------------------
,. 12
"' 25
5
ol---~=£~--------------------------
16
99
Figure 29. Frequency histograms of estimated weights (g) for four fish species found in Pacific white-sided dolphin stomachs.
50
45
4{)
g'35 • t: ~20 u
~ ~5 10
5
Chlroteuthls sp. ~"
100
60
50 Gomrtus berryi
~· 40
30
20
10
0~--~----J----------------------'O~J-______ J_ ____________________ _
1 3 5 7 9 11 13 15 17 19 21 23 25 'Z1 29 31 1 3 5 7 9 11 13 15 17 19 21 23 25 27 29 31 4{) ~
30
25
20
15
10
5 5
o~~L_ ________ _L _____________ , 0
1 3 5 7 9 11 13 15 17 19 21 23 25 27 29 31 70 50
60 45
Abnlllopsi!J fells 4{) - ~
30
25
20
15
10 10
5
0~~--J_-------------------------0
70 1 3 5 7 • 11 13 15 17 19 21 23 25 27 29
Gonstus sp. A
"""'
31
1 3 5 7 9 11 13 15 17 19 21 23 25 27 29 31
20
16
16'
14
12
10 ., ., 4
2
0
1 3 5 7
1 3 5 7
r-
-
~
1 3 5 7
Estimated Mantle Length (em)
9 11 13 15
9 11 13 15
' -
'-
-
Gonstopsis borealis ~"
17 19 21 23 25 27 29
Loligo opalescens
~"'
31
17 19 21 23 25 'Z1 29 31
Onychotcuthls boreslijsponlcus ~
-
~ '---
9 11 13 15 17 19 21 23 25 'Z1 29 31
Figure 30. Frequency histograms of estimated mantle lengths (em) for eight cephalopod genus/species found in Pacific white-sided dolphin stomachs.
1
60
50
~ u s 40 , 0" • 0::30 c • 220 • ...
10
0
50
45
40
~35 c
~30 • 0::25
~20 u ~ 15
10
5
0
100
90
BO ~
70 u c • 60 0 0"
:r. 50 c 40 • ~ 30 ...
20
10
70
Chlroteuthls sp. 60
50
40
30
20
10
Gonatus berry/
~·
101
l----L---------,~~~~~~Ol---~----------~----------
•••••••••••------- ··········--------Gallteuthls sp. -
60
50
40
30
20
10
Gonatopsls borenlls
~·
~----~~~~~~L------------Ol---~-L-LJ-L-L-~L_--l_~L--L~ ··········-------- ··········--------60
40
30
10
Lollgo opalescens
~"'
0 ~L-----------,L-----------0~----L--------r----------············------- ········----------60
50 Gonatus sp. A ....,
50
40
30
20
Estimated Weight (g)
Onychoteuthls boresfl}aponlcus
'""'
Figure 31. Frequency histograms of estimated weights (g) for eight cephalopod genus/species found in Pacific white-sided dolphin stomachs.
102
DISCUSSION
In evaluating the food habits of·marine mammals based on stomach
contents, inherent biases must be considered. First, cephalopod beaks are
resistant to digestion and may be retained in stomachs longer than otoliths,
which are more easily eroded by stomach acids (Clarke and Kristensen 1980,
Finley and Gibb 1982, Harvey 1987). The importance of cephalopods,
therefore, may be overestimated compared to fishes (Ross 1984, Bigg and
Perez i 985). Because otoliths are susceptible to dissolution and small otoliths
may be digested faster than larger ones, the estimated length of fish prey based
on otolith length may be considerably smaller than the actual fish length,
although this appears most significant in samples collected from scats (Jobling
and Breiby 1986, Harvey 1989a). This may be especially true for anchovy, with
small and numerous otoliths in stomachs, which may be easily dissolved or
quickly passed through dolphin stomachs.
Secondary prey, species in the stomachs of dolphin prey, may occur in
stomach samples. Walker and Jones (1993) found some intact squid in northern
fur seal (Caflorhinus ursinus) stomachs that contained myctophid otoliths and
other cephalopod beaks. They suggested that isolated otoliths and beaks of
small prey may be considered secondary prey. Fiscus et al. (i 989) suggested
that single L. opalescens and A. felis beaks found in the stomach of sperm
whales (Physeter macrocephalus) are probably secondary, because both are
schooling species that when present usually comprise a large proportion of
stomach contents of marine mammals.
103
Another problem that could result from analysis of stranded dolphin
stomachs is the occurrence of prey species that are not normally part of their
diet (Clarke 1986b). However, Leatheiwood et al. (1978) and Barros and Odell
(1990) found no difference in food habits between stranded and healthy
(fisheries caught) dolphins. Perhaps species found in low numbers in white
sided dolphin stomachs could be anomalous prey of sick individuals.
Lastly, the sample size in this study may not be adequate for completely
describing dolphin prey. Pacific white-sided dolphins rarely strand, and since
1965 there were only 41 stomach samples previously analyzed from dolphins
found in central and northern California (Fiscus and Niggol 1965, Morejohn et
al. 1978, Jones 1981, Stroud et al. 1981). Despite the low sample number, all
four studies also reported northern anchovy, Pacific whiting, L. opalescens, and
a few other oceanic cephalopod species as common prey for these dolphins.
Pacific white-sided dolphins feed opportunistically on abundant species
of fishes and cephalopods, and based on prey habits, feed diurnally and
nocturnally. The predominant prey of these dolphins are pelagic schooling
species, particularly those less than 30 em in length. Stomachs have been
examined from 101 Pacific white-sided dolphins in coastal waters of the eastern
North Pacific. Northern anchovy was the most frequently occurring species,
followed by L. opa/escens, Gonatus sp., Pacific whiting, and O.borea/ijaponicus.
A division of the samples into three areas; south of Point Conception, Monterey
Bay, north of Point Conception, and Washington State, revealed some
differences among regions (Table 8). Off southern California, samples were
dominated by a diversity of fish species and few cephalopods.
Table B. Percent frequency of occurrence of Pacific white-sided dolphin prey from southern California, Monterey Bay, northern California, and Washington State.
FISH SpECIES sc MB NQ ws All FISH SpECIES Icon! l sc Ml NQ ws AIL Numbor ol samplos 3J 26 57 11 101 Salmonldae
Oncclfi}'T'Chus kola 9.1 1.0 Anop!oporn.aUdno Oncorl1ynchusldsurrh 18.2 2.0
Anopfopoma fimbria 30 1.0 Onyrorllynctlus spp. 72.7 7.9 Athorinldao Sclmm!daa Bothy1agldao '· Gon)OOomus /fnastus 9.1 3.0 1.8 48
LourogiO:SSus sfffblus 38 1.0 Sariphus polirus 6.1 , Ba\rnchoididao Scomberasocldoo
Porlchthys notarus 15.2 "19.2 15.8 13.9 Co/oiEbls saim 11.5 17.5 9.9 Porldl!hys myrill.SI!Jr 12.1 4.8 ScombridBB
Bothldao Pnoumntophol'lr.J japoll!CU!l 3.8 1.8 Citllarichthys sordldus 6.1 7.7 5.3 5.0 ScombDr japook:us 3.0 1.0
Calilngidao TrnO'Iurus syrrmolrla.J$ 30.3 7.7 7.0 13.9
ScorpaanldBo Sobrulos sp. 15.2 "7.7 7.0 8.9
Controloph!dao Sorrnnldeo /c:ictrltlys loddngtonl 1.8 1.0 Pnra.fabrax dalhrotus 3.0 1.0
C!upoldi!O Pamlabmx nobuffor 6.1 2.0
""""""""" So!eldao Sarrtioops sngnx 9.1 " 1.8 4.0 S)mphurus slrlaum 3.0 1.0
Emb!otoddao 3.0 1.0 Sphymon!da11 Engroulldaa Sphyrnanllnrgflnloo 3.0 1.0
Engrnlfisl'1'10fdal' 69.7 "69.2 54.4 53.5 Stromatoldon ExocooUdao Poprllus sJmllus B.! ~0
Cypsofurus c.a!ifomlcus 3.0 1.0 Tracfl!plolldao Gobndoo 30 10 lrm;!JII2Wo/S. a/Jb:Illi$. ao 10 Kyphocldao 3.0 1.0 li!bridno CEPHAlOpoD SpECIES sc MB m ws ALL
Oxylui!LlS caBiomlcll 3.0 1.0 Merluo:::lklao Chlrotouthldao
Marluccius productus 48.5 '38.5 43.9 40.6 Chlrcteuthls sp. 3.0 7.0 27.3 5.9 Myctophldao Crnnchlldna 15.4 14.0 27.3 10.9
Dlophus rhatn 3.0 1.0 Enoplotoulhldoo S~opiKJrtJS c.afifomlans/s 1.8 1.0 Abra/1opsls foils 3.0 '30.8 35.1 100.0 31.7 T rlpha/IJnJS moldcanus 6.1 2.0 G0110lldae 6.1 38.5 28.1 100.0 28.9
Ophldlldao Galai'!Jssp. 12.1 '5o.o 50.1 100.0 45.5 Gilamln)bl 12.1 4.0 Hlstlo!outh!dao
Osmeridao Hislloleulhls hoteropsJs 3.B 1.8 1.0 0/oph/di!XTI scrlppsJ 3.0 1.0 loflg!n!doo 5pirlnch~n starks/ 1.8 1.0 Lol/go opa/oScm!s 54.5 '61.5 47.4 27.3 47.5
Plouronocllform Octopodldao Plouronoclldna Octopus sp. 21.2 '23.1 15.8 27.3 18.8
Hypsop!lotfa gulfala 3.0 1.0 OctopotoulhldaB Microstomus paci(ICUS 3.0 1.0 Dctopofoutflfs sp. 9.1 23.1 24.8 81.0 2"-ll Pfauronactos 11allllus 3.0 1.0 Onychotauthldno
Pomncantr1dao Onyr:hoteurhls Chromis punctipinnls 6.1 2.0 boroaWjoponlcus 12.1 '34.6 38.6 100.0 36.6
Soplolldno _B~fa..m1t;;i • from Mora]ohn ol Dl. 1978, no data on lnd!vlduDI samples allailnblo
~
0 .,.
105
Northern anchovy was most frequent, followed by Pacific whiting, L.
opalescens, and jack mackerel. A mixture of fish and squid predominated off
central and northern California. Gonatus sp. was most frequent followed by
northern anchovy, Loligo opa/escens, Pacific whiting, and 0. borea/ijaponicus.
Squid species predominated off Washington State; 0. borealijaponicus, A felis,
Gonatus sp., and Gonatidae occurred with equal frequency, followed by
Octopoteuthis sp. and Oncorhynchus sp. Specifically for Monterey Bay,
northern anchovy and L. opalescens dominated the samples followed by
Gonatus sp., Gonatidae, and Pacific whiting (Table 8; Scheffer 1953, Brown and
Norris 1956, Houck 1961, Fiscus and Niggol 1965, Fitch and Brownell 1968,
Morejohn et al. 1978, Stroud et al. 1981, Jones 1981, Walker et al. 1986,
Schwartz et aL 1992, this study).
Walker and Jones (1993) found that 0. borealijaponicus was the most
frequently occurring prey item in Pacific white-sided dolphins found in offshore
waters of the northern North Pacific, followed by Myctophidae, A fells,
Bathylagidae, Argentinidae, Pacific saury (Colo/abis saira), Gonatus sp., and G.
borealis. Myctophidae were predominant in this area, similar to Pacific white
sided dolphin prey off Japan (Wilke et al. 1953), compared to the virtual non
occurrence of this lam ily in dolphin stomachs from coastal waters of the eastern
North Pacific.
Most of the fishes occurring in Pacific white-sided dolphin stomachs are
abundant within the California Current system, which extends from 23°N to
50°N (Bailey et al. 1982). In CaiCOFI collections, northern anchovy and Pacific
whiting comprised the majority of larval fish (Ahlstrom 1969). Deep-sea pelagic
106
fishes also were abundant, such as myctophid lanternfishes, gonostomatid
lightfishes, and deep-sea smelts, but are generally small (a few em), more
abundant in offshore waters than in the California Current region, and usually
do not form dense schools (Ahlstrom 1869). Therefore, these fish are probably
not important in the diet of white-sided dolphins inhabiting coastal waters. Fish
diversity is greatest in southern California and declines northward (Horn and
Allen 1978), which may explain why Pacific white-sided dolphins consumed the
greatest variety of fishes in southern California.
Pacific white-sided dolphin distribution, relative abundance, habitat, and
behavior off California appear most influenced by their primary prey (based on
frequency of occurrence), the northern anchovy. Anchovy is one of the most
abundant fishes in the northeast Pacific (Baxter 1966). Anchovy are most
abundant off sot:Jthern California during the spawning season from February to
May, corresponding to a time when white-sided dolphins are most abundant
(Leatherwood et al. 1984). Anchovy migrate north after spawning and become
important to seabirds in central California during late summer (Ainley and
Boekelheide 1980). Anchovies were one of the most abundant fishes collected
in summer midwater trawls in Monterey Bay (Cailliet et al. 1979). Northern
anchovy tagged off southern California were recovered in Monterey Bay
primarily from September to January (Haugen et al. 1969), a time when white
sided dolphins were most abundant in the bay. In Monterey Bay, dolphins
frequented waters overlying the shelf-break, particularly along steep canyon
edges (Ch. 1).
107
High densities of anchovy similarly occur over submarine canyons
(Baxter 1966, Mais 1974). Dolphins were most abundant and found in the
largest group sizes during fall and winler, whereas during spring, dolphins were
least abundant and found in small groups. Similarly, the number of anchovy
schools per km and anchovy school size were greatest in fall, and the fewest
anchovy schools per km and the smallest schools occurred during spring
{Smith 1981). Dusky dolphins (Lagenorhynchus obscurus} off Argentina fed in
relatively large groups (>300) when southern anchovy were most abundant,
and fed in smaller groups when anchovy were not present (WOrsig and WOrsig
1980). Pacific white-sided dolphins were observed feeding on anchovy during
the daytime in Monterey Bay and off southern California (Norris and Prescott
1961, Ch. 1 ), probably due to the dispersal of anchovy at dusk and their
reformation into dense schools just before dawn (Mais 1974). Anchovy reach a
maximum size of 23 em length and 60 g weight (Eschmeyer et al. 1983),
juveniles are 2.5 to 14 em in length and become mature at 7.8 to 14 em
between 1 and 2 years of age (Frey 1971, Hart 1973). Pacific white-sided
dolphins fed on the juvenile and young adult size classes (peak at 10.4 em).
which probably reflected a higher abundance of juvenile fish available rather
than a selection for size. Similarly, Harvey (1989b) found anchovies with an
average size of 10.5 em in the stomachs of blue sharks collected in Monterey
Bay.
Pacific whiling, which had the greatest IRI and was the second most
frequent prey item for Pacific white-sided dolphins, have an estimated biomass
off California second only to the northern anchovy (Alverson and Larkens
108
1969). In Monterey Bay, whiting were the second most abundant fish in trawls
(Cailliet et al. 1979). Adults undergo seasonal migrations from summer feeding
grounds in the Pacific Northwest to spawn·off southern California and Baja,
California, during winter. Mature whiting off California averaged 47 em length
and greatest length was 90 em. The greatest biomass of whiting consisted of
juveniles (1 to 4 years) off central California, which concentrate near the shelf
break (Bailey et al. 1982), similar to white-sided dolphin distribution. Pacific
white-sided dolphins in central California fed mainly on juvenile whiting (mean
size 23 em, 0-4 year old fish). Occurrence of whiting in dolphin stomachs was
low off Washington where adult fish predominate (Best 1963), but was common
in stomachs from California. This indicated that white-sided dolphins select
juvenile whiting. High juvenile fish biomass in central California likely
contributes to the high abundance of dolphins found off this area compared to '
other areas of their range.
Plainfin midshipman was next in importance after Pacific whiting in
Pacific white-sided dolphin stomachs from central California. The occurrence of
this fish in dolphin stomachs indicates that Pacific white-sided dolphins also
feed nocturnally. Midshipmen bury in the bottom during the day and feed at
night, forming schools that may rise to 150 m above the bottom (Lavernberg and
Fitch 1966). Midshipmen mature at 14 em length and reach 38 em (Love 1991 ).
Pacific white-sided dolphins fed on the juvenile and young adult size classes.
Jack mackerel occurred in nearly one third of dolphin stomachs off
southern California (Brown and Norris 1956, Fitch and Brownell 1968, Scheffer
1953, Stroud et al. 1981, Walker et al. 1986, Schwartz et al. 1992), and was
109
rare in northern California samples. This probably reflects the opportunistic
feeding nature of white-sided dolphins, because maximum densities of juvenile
and young adult fishes occur from Point Conception to central Baja, California
(Blunt 1969).
Although the various species of Sebastes are difficult to distinguish from
otoliths, Pacific white-sided dolphins probably fed on shortbelly rockfish
(Sebastes jordani) the most predominate rockfish prey of many other marine
vertebrates (Chess et al. 1988, Morejohn et al. 1978). Shortbelly rockfish occur
from northern Baja, California to Vancouver Island, British Columbia, but are
most abundant off central California (Miller and Lea 1972) particularly near
submarine canyons (Chess et al. 1988). They attain lengths of 32 em and are
found in depths from 90 to 280 m (Miller and Lea 1972), overlapping the depths
where white-sided dolphins frequently occur. Cailliet et al. (1979) found large
numbers of juvenile rockfish, 7.6 to 15.3 em length, in Monterey Bay during
summer. Juvenile rockfish are abundant seasonally; therefore, their importance
to white-sided dolphins may be underestimated because of the few dolphin
stomachs collected during peak juvenile fish biomass off central California. This
may be exemplified by one dolphin collected in Monterey Bay during June that
contained 328 rockfish otoliths, representing a minimum of 178 fishes.
Pacific white-sided dolphins collected during this study consumed 13
cephalopod genus/species, of which six families occurred in 25% or more
stomachs. Except for L opa/escens, cephalopod occurrence in stomachs was
rare off southern California and greatest off Washington (Stroud et al. 1981,
Walker et al. 1986). Lo!igo are most prominent in samples from Monterey Bay.
110
Loligo is possibly one of the most abundant cephalopods off California (Young
1972), and are most common from southern California to Monterey Bay (Fields
1 965). Lo/igo was the most frequently occurring species in summer midwater
trawls in Monterey Bay (Cailliet et al. 1979). This squid moves inshore to spawn
in shallow waters (<100m) year-round, with a peak in Monterey Bay from May
to July; otherwise schools occur widely in coastal and offshore waters (Frey
1971, Hardwick and Spratt 1979, Fiscus 1982). During the spawning season,
squid are inshore, and therefore unavailable to Pacific white-sided dolphins,
corresponding to a time of low dolphin abundance in Monterey Bay. Loligo
mature between 1 and 2 years of age as they grow from 0.2 to 14 em DML. At
maturity, males are 7 to 13 em length and females 8 to 14 em length, and can
reach 23 em during their two-year life span (Fields 1965). Commercial catches
of spawning squid in· Monterey Bay are dominated by squid with mantle lengths
of 14 to 15 em length (Fiscus et al. 1989), larger than the average size of 10.7
em found in Pacific white-sided dolphin stomachs. Similarly, ·Harvey (1989b)
found L. opalescens averaging 9.6 em DML in blue shark stomachs from
Monterey Bay.
Little is known about the other frequently occurring cephalopods in the
diet of Pacific white-sided dolphins. Although none are commercially important,
many are commonly found in the stomachs of other marine mammals and
seabirds (Baltz and Morejohn 1977, Fiscus 1982, Antonelis et al. 1987, Fiscus
et al. 1989, Lowry et al. 1990). Most are epipelagic to mesopelagic species,
undergo vertical migrations, and at least one, A felis, is a schooling species
(Roper and Young 1975, Jefferis 1983). The family Gonatidae is the most
111
abundant cephalopod group in the epipelagic and mesopelagic subarctic
Pacific waters (Fiscus 1991 ). There are at least three genera and 12 species
found in the Pacific (Young 1972), although most are difficult to identify by their
beak. Pacific white-sided dolphins are probably not deep divers and most likely
feed at night on many of these vertically migrating cephalopods. Histioteuthis
heteropsis and R. pacifica both occurred in only one white-sided dolphin
stomach, and have not been previously reported as prey for these dolphins.
They could, however, represent secondary prey.
Many of the cephalopods found in Pacific white-sided dolphin stomachs
also occurred in the stomachs of sperm whales from California. However,
several species frequently found in dolphins, such as L opalescens, Gonatus
spp., 0. borea/ijaponicus, and A. felis were infrequent in sperm whale
stomachs. Similarly, species prominent in the diet of sperm whales, Moroteuthis
robusta, G. borealis, Histioteuthidae, Cranchiidae, and Octopoteuthis were rare
or absent in the diet of Pacific white-sided dolphins. Where there was overlap,
sperm whales fed on cephalopods with larger lower rostral lengths than those
found in dolphins. This indicated that dolphins fed predominantly on juvenile
cephalopods of certain species, while sperm whales may select the larger
adults that occur deeper in the water column (Fiscus et al. 1989). The
distribution of the two species reflects their differences in diet. Sperm whales in
California occur in offshore waters, usually beyond 1000 m, whereas Pacific
white-sided dolphins frequent shelf and slope waters, primarily from 200 to
1000 m (Doh I et al. 1983).
112
In contrast to the sperm whale, several other odontocetes frequent shelf
and slope waters off California, sharing a distribution similar to Pacific white
sided dolphins. Northern right whale dolphins and Risso's dolphins do not
completely overlap the diet of white-sided dolphins. Northern right whale
dolphins fed primarily on L opalescens, myctophids, bathylagids, and
cephalopods from the families Gonatidae, Enoploteuthidae, Histioteuthidae,
and Onychoteuthidae (Leatherwood and Walker 1979, Sullivan and Houck
1979, Clarke 1986b). These cephalopods also occurred in white-sided
dolphins, whereas the deep-sea fishes were rare or absent in white-sided
dolphin stomachs off California. Risso's dolphins fed only on cephalopods,
many of which occurred in white-sided dolphin stomachs (Stroud 1968, Fiscus
1993, Black, unpubl. data), however, the predominant species and size classes
may differ.
Dall's porpoise off central California primarily fed on Pacific whiting,
northern anchovy, Pacific saury, L. opalescens and 0. borea./ijaponicus (Loeb
1972, Morejohn 1979, Stroud et al. 1981); overlapping the primary prey of
white-sided dolphins. In contrast to northern right whale dolphins and Risso's
dolphins, Dall's porpoise rarely associate with Pacific white-sided dolphins in
Monterey Bay and occur only in small groups (<20). Similar to white-sided
dolphins, however, they frequent waters overlying canyon edges in the bay
(Jefferson 1991 ). Northern fur seals off central California also fed on similar
fishes and cephalopods as white-sided dolphins (Stroud et al. 1981 ), and like
Dall's porpoise occurred singly or in small groups.
113
Before 1991, common dolphins primarily overlapped the range of Pacific
white-sided dolphins only in southern California. Similar to white-sided
dolphins, common dolphins fed predominantly on anchovies and L. opalescens
(Evans 1975). Numbers of Delphinus peaked in January, June, September, and
October (Evans 1975), and Pacific white-sided dolphins were most abundant off
southern California from November to April. Therefore, Pacific white-sided
dolphins appear to occupy a distinct niche; associating with abundant species
(northern right whale dolphins, Risso's dolphins) that do not completely overlap
their diet, and they may alternate their abundance with common dolphins that
feed on similar prey. Those species that consume similar prey and overlap
Pacific white-sided dolphin distribution, occur in small groups (Dall's porpoise,
northern fur seals) compared to the relatively large groups of Pacific white-sided
dolphins.
Pacific white-sided dolphins are opportunistic predators, feeding
primarily on abundant prey within the various habitat types they occupy. This is
reflected by variations in primary prey of dolphins found off California,
Washington, offshore northern Pacific waters, and the western Pacific. Similarly,
other members of the genus Lagenorhynchus also fed opportunistically on
abundant fishes and cephalopods. Atlantic white-sided dolphins,
Lagenorhynchus acutus, co-occurred with peak abundance of sand lance in the
Gulf of Maine and also fed on short-finned squid, smelt, herring, and hake
(Sergeant et al. 1980, Seizer and Payne 1988), all abundant schooling species.
Dusky dolphins in New Zealand fed on lanternfishes, hoki, and cephalopods
associated with the deep scattering layer around the Kaikoura Submarine
114
Canyon (Cipriano 1992). In a contrasting habitat off Argentina, dusky dolphins
fed on anchovies during summer in a relatively shallow, low relief area (WOrsig
and WOrsig 1980).
In Monterey Bay, Pacific white-sided dolphins fit into a complex food web,
centering around euphausiids, particularly Thysanoessa spinifera and
Euphausia pacifica (Morejohn et al. 1978, Cailliet et al. 1979). Distinct
assemblages of organisms occurred in pelagic waters of Monterey Bay. Loligo
and anchovy were prominent in these assemblages, which also included
plainfin midshipman, Pacific whiting, juvenile rockfish, Pacific herring, and
Pacific sanddab. Two species of Gonatus comprised an assemblage, but were
not abundant (Cailliet et al. 1979). All species except herring occurred in
dolphin stomachs. Cailliet et al. (1979) suggested that the euphausiid "link" may
be the reason for these recurrent groups. Although dolphins do not directly feed
on krill, krill is consumed by the primary prey of dolphins off central California
found within these assemblages. Little is known about the prey of cephalopods
other than L. opalescens. although their predators are diverse. It seems
important that the ecology of pelagic cephalopods be examined off central
California to fully understand the ecological patterns of Pacific white-sided
dolphins.
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