Download - POPULATION DYNAMICS, MOVEMENTS, AND SPAWNING
POPULATION DYNAMICS, MOVEMENTS, AND SPAWNING HABITAT OF THE
SHORTNOSE STURGEON, Acipenser brevirostrum, IN THE ALTAMAHA RIVER
SYSTEM, GEORGIA
by
ROBERT JASON DEVRIES
(Under the Direction of Douglas L. Peterson)
ABSTRACT
In Georgia, the Altamaha River supports what is believed to be one of the largest
remaining shortnose sturgeon populations south of Chesapeake Bay, however the
current status and recent population trends of this population are unknown. A
population estimate for shortnose sturgeon in the Altamaha River was generated using
POPAN within Program MARK. Annuli on pectoral fin rays were used to determine age.
Mortality estimates were calculated from catch curves. Radio telemetry was used to
monitor movements and habitat use of 12 adult shortnose sturgeon. The results of this
study indicate a population size of 6320 (95% C.I. 4387-9249) with a disproportionate
number of juveniles. Ages ranged from 4 to 14 yr. Estimated annual mortality ranged
between 29-34%. Spawning runs initiated in late December and were long, single-step
migrations. Juveniles and adults coinhabited summer habitat in deep riverine stretches
and appeared confined to freshwater when water temperatures exceeded 27 oC.
INDEX WORDS: Shortnose sturgeon, Acipenser brevirostrum, Abundance estimate,
Population dynamics, Migration, Habitat, Radio telemetry, Altamaha River
POPULATION DYNAMICS, MOVEMENTS, AND SPAWNING HABITAT OF THE
SHORTNOSE STURGEON, Acipenser brevirostrum, IN THE ALTAMAHA RIVER
SYSTEM, GEORGIA
by
ROBERT JASON DEVRIES
B.S., College of Charleston, 1998
A Thesis Submitted to the Graduate Faculty of The University of Georgia in Partial
Fulfillment of the Requirements for the Degree
MASTER OF SCIENCE
ATHENS, GEORGIA
2006
© 2006
Robert Jason DeVries
All Rights Reserved
POPULATION DYNAMICS, MOVEMENTS, AND SPAWNING HABITAT OF THE
SHORTNOSE STURGEON, Acipenser brevirostrum, IN THE ALTAMAHA RIVER
SYSTEM, GEORGIA
by
ROBERT JASON DEVRIES
Major Professor: Douglas L. Peterson
Committee: Cecil A. Jennings Steven B. Castleberry
Electronic Version Approved: Maureen Grasso Dean of the Graduate School The University of Georgia August 2006
ACKNOWLEDGEMENTS
I would like to thank my advisor, Dr. Douglas Peterson for his guidance and support.
I would also like to thank the other members of my committee, Drs. Cecil Jennings and
Steven Castleberry.
Special thanks also go to David Higginbotham, for his help with logistics and
statistics; Drs. James Peterson and Michael Conroy, for their help and input for the
population estimation; to Paul Schueller and Jason Meador for their help with GIS; and
to Brian White, Clay Fordham, and the host of technicians who assisted me during the
course of this study.
I would also like to extend my gratitude to the Georgia Department of Natural
Resources and Gordon Rogers for their input and expertise throughout this project.
Funding was provided by the National Marine Fisheries Service.
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TABLE OF CONTENTS
Page
ACKNOWLEDGEMENTS ...............................................................................................iv
LIST OF TABLES...........................................................................................................vii
LIST OF FIGURES..........................................................................................................ix
CHAPTER
1 INTRODUCTION AND LITERATURE REVIEW............................................. 1
Taxonomy and Systematics....................................................................... 1
Morphology ................................................................................................ 3
Genetics..................................................................................................... 6
Life History................................................................................................. 8
Habitat and Movements ........................................................................... 14
Distribution and Legal Status ................................................................... 16
Mechanisms Leading to Declines ............................................................ 16
Management Approaches........................................................................ 20
Research Objectives and Justification ..................................................... 21
2 STUDY AREA AND METHODS .................................................................. 24
Study Area ............................................................................................... 24
Materials and Methods............................................................................. 26
Abundance Estimates ............................................................................. 28
Population Age Structure ......................................................................... 29
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Identification of Critical Habitat for Shortnose Sturgeons......................... 33
3 RESULTS.................................................................................................... 37
Abundance Estimates.............................................................................. 37
Population Age Structure ......................................................................... 42
Habitat and Seasonal Movements ........................................................... 56
4 DISCUSSION .............................................................................................. 62
Abundance Estimates.............................................................................. 62
Population Age Structure ......................................................................... 63
Habitat and Seasonal Movements ........................................................... 71
Management Implications ........................................................................ 75
Summary ................................................................................................. 76
LITERATURE CITED .................................................................................................... 78
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LIST OF TABLES
Page
Table 3.1. Trammel net and gill net capture comparison in 2004 and
2005. Trammel nets were constructed from an inner panel of
7.6 cm and 2 outer panels of 30.5 cm stretched mesh. Gill nets
were constructed from 10.2 and 15.2 cm stretched mesh.
Experimental gill nets were constructed from 7.6, 10.2, and
15.2 cm stretched mesh ............................................................................. 39
Table 3.2. Estimates of shortnose sturgeon abundance in the Altamaha
River, Georgia ............................................................................................ 41
Table 3.3. Age-frequency key for shortnose sturgeon captured in the Altamaha
River system in 2004. Ages were determined from the first pectoral
fin ray. The ages from each individual subsample were applied
proportionally across the same length-group. Note: age-2
or age-13 individuals were not obtained during this study .......................... 48
Table 3.4. Age-frequency key for shortnose sturgeon captured in the Altamaha
River system in 2005. Ages were determined from the first pectoral
fin ray. The ages from each individual subsample were applied
proportionally across the same length-group. Note: age-2
or age-13 individuals were not obtained during this study .......................... 49
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Table 3.5. Radio tagged shortnose sturgeon release dates and number of
relocations in the Altamaha River, Georgia. The * denotes an
individual tag that did not change locations after an initial ~100 km
upstream migration after tag implantation. ** indicates a censored
individual after x days at large .................................................................... 59
Table 4.1 Coefficient of variance (CV) of age estimates for between-observer
precision for reported long-lived species .................................................... 65
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LIST OF FIGURES
Page
Figure 1.1. Lateral view illustrations of two adult shortnose
sturgeon, Acipenser brevirostrum (Illustrations by Paul Vecsei)................... 2
Figure 1.2. Head and mouth variations of adult shortnose sturgeon,
Acipenser brevirostrum. (Illustrations by Paul Vecsei).................................. 5
Figure 2.1. The Altamaha River watershed (gray) including the headwater
tributaries, the Ocmulgee and Oconee Rivers with impoundments ............ 25
Figure 2.2. Shortnose sturgeon capture sites in the tidally influenced
lower 40 rkm of the Altamaha River (sampling sites are denoted by o)...... 27
Figure 2.3. Altamaha River basin covered on a weekly basis by boat equipped
with a scanning receiver and loop antenna ................................................ 36
Figure 3.1. Size distributions of captured shortnose sturgeon from the
Altamaha River, Georgia in 2004 (top) and 2005 (bottom). In 2004,
individuals were caught in 10.2 and 15.2 cm gill nets and trammel
nets. In 2005, gill nets were replaced with experimental gill nets
constructed from 7.6, 10.2, and 15.2 cm panels......................................... 38
Figure 3.2. Monthly catch per unit effort (CPUE) of shortnose sturgeon
in the Altamaha River in 2004 and 2005. (Sampling not conducted
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during months denoted with an asterisk.) ................................................... 40
Figure 3.3. Nominal age frequency of shortnose sturgeon captured in the
Altamaha River, Georgia in 2005. Ages of individual fish were
determined from pectoral fin ray sections and interpreted by 3
independent observers period 1 ................................................................. 44
Figure 3.4. Age-bias graph for 2 interpreters of sampled fin rays from Altamaha
River shortnose sturgeon. The dashed line designates the 1:1
ratio line. Age estimation bias is present ................................................... 46
Figure 3.5. Growth of shortnose sturgeon (both sexes) in the Altamaha River,
Ogeechee River, and Hudson River ........................................................... 47
Figure 3.6. Weight – length relationship of shortnose sturgeon (both sexes)
from the Altamaha River, Georgia .............................................................. 48
Figure 3.7. 2004 uncorrected catch curve for ages 4 – 12 of Altamaha River
shortnose sturgeon..................................................................................... 51
Figure 3.8. 2005 uncorrected catch curve for ages 4 – 12 of Altamaha River
shortnose sturgeon..................................................................................... 52
Figure 3.9. Size composition of shortnose sturgeon captured in trammel
nets and gill nets from the Altamaha River, Georgia in 2004.
Catch per unit effort is the number of shortnose sturgeon captured
per net per slack tide .................................................................................. 52
x
Figure 3.10. Size composition of shortnose sturgeon captured in trammel nets
and experimental gill nets (constructed from 7.6, 10.2, and 15.2 cm
stretched mesh) from the Altamaha River, Georgia in 2004. Catch
per unit effort is the number of shortnose sturgeon captured per net
per slack tide .............................................................................................. 53
Figure 3.11. Indirect selectivity of trammel nets, 10.2 cm, and 15.2 cm
stretched mesh gill nets from the Altamaha River, Georgia in 2004........... 54
Figure 3.12. Indirect selectivity of trammel nets, and experimental gill nets
composed of 7.6, 10.2, and 15.2 stretched mesh panels from the
Altamaha River, Georgia in 2005) .............................................................. 55
Figure 3.13. Confirmed and suspected spawning areas of shortnose sturgeon in
the Altamaha River, GA (2005)................................................................... 60
Figure 3.14. Non-spawning habitat of shortnose sturgeon in the Altamaha River, GA
(April-December 2005) ............................................................................... 61
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CHAPTER 1
INTRODUCTION AND LITERATURE REVIEW
The family Acipenseridae is composed of 27 species dispersed among 4 genera with
a life history characterized by a long life span, delayed maturity, and protracted
spawning periodicity (Artyukhin 1995, Bemis and Kynard 1997, Billard and Lecointre
2001). The shortnose sturgeon (Acipenser brevirostrum) (Figure 1.1) is the smallest
species within this family. Although no historically large populations have been
described, shortnose sturgeon were nevertheless exploited along with the sympatric
Atlantic sturgeon (Acipenser oxyrinchus) (LeSueur 1818, Smith et al. 1984). Shortnose
sturgeon became sufficiently scarce that they were declared an endangered species in
the United States in 1967, and are considered a species of special concern in Canada
(Dadswell et al. 1984, COSEWIC 2005). Today, few healthy populations exist and
many anthropogenic factors continue to impede restoration efforts (Kynard 1997).
Hydroelectric dams are perhaps the most significant limiting factor because they
obstruct access to spawning grounds and alter many critical habitats (Auer 1996,
Kynard 1997).
Taxonomy and Systematics
The earliest known appearance of acipenserids within the fossil record dates to the
Upper Cretaceous with possible ancestors originating during the Lower Jurassic, some
Figure 1.1. Lateral view illustrations of two adult shortnose sturgeon, Acipenser
brevirostrum. (Illustrations by Paul Vecsei)
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200 million years ago (Gardiner 1984, Findeis 1997, Bemis and Kynard 1997,
Choudhury and Dick 1998). Though many aspects of sturgeon phylogeny remain
uncertain (Artyukhin 1995), most taxonomists have categorized the genus Acipenser as
monophyletic (Bemis et al. 1997), although recent studies have disputed the validity of
this classification (Birstein et al. 1997, Choudhury and Dick 1998, Birstein et al. 2002).
More recently, genetic analyses have been used to further refine phylogeny as well as
taxonomy within the Acipenseriformes. The shortnose sturgeon has been of particular
interest because of its unique 16N ploidy, which has given rise to conflicting theories
regarding the relationships and phylogeny of the order. Contrary to Birstein et al.
(1997), who considered shortnose sturgeon a young species, Choudhury and Dick
(1998) regarded the shortnose sturgeon as ancestral to all interior eastern North
American acipenserids resulting from glacially induced isolation. Further complicating
these phylogenetic arguments, Artyukhin (1995), Choudhury and Dick (1998), and
Krieger et al. (2000) suggested that the shortnose sturgeon and the lake sturgeon (A.
fulvescens) are sister species. Regardless of which of these arguments are correct,
researchers agree that acipenserids are an ancient group and that surviving lineages
are best explained by more recent geological and climatic changes (Choudhury and
Dick 1998). However, determining actual relationships among species will continue to
be problematic for years to come.
Morphology
Acipenserids are distinctive in morphology with characteristics such as trunk
armoring scutes, a ventral mouth, rostral chemosensory barbels, spiral valve intestine,
3
and a swim bladder that has retained some of the lung-like characteristics of early
actinopterigeans (Dadswell et al. 1984). The shortnose sturgeon is distinguished from
other North American sturgeons by a wide mouth, absence of a fontanelle, nearly
complete absence of postdorsal scutes, and preanal scutes often arranged in a single
row (Scott and Crossman 1973, Dadswell et al. 1984). Dorsal, lateral, and ventral scute
counts vary from 7-13, 21-33, and 6-11, respectively. Dorsal fin rays number 38-42 and
anal fin rays number 19-22 (Scott and Crossman 1973, Dadswell et al. 1984). The
rostrum is disproportionately large in juveniles, exceeding postorbital distance in
individuals <39 cm FL, gradually decreasing as individuals age (Vladykov and Greeley
1963). Shortnose sturgeon have a large, transverse mouth averaging 74% of
interorbital width (Vladykov and Greeley 1963). Mouth shape and size is similar to that
of lake sturgeon (A. fulvescens) and is a prominent characteristic used to distinguish the
shortnose sturgeon from the sympatric Atlantic sturgeon (A. oxyrinchus) (Figure 1.2)
(see Vladykov and Greeley 1963, Dadswell et al. 1984). Barbels are situated closer to
the tip of the snout than to the mouth. Gill rakers are long and triangular, usually
numbering 22-29 on the first branchial arch (Vladykov and Greeley 1963, Scott and
Crossman 1973).
Body armoring is extensive in shortnose sturgeon, although weakly developed in
adults compared to that of other North American sturgeons (Vladykov and Greeley
1963, Dadswell et al. 1984). Sharp, apical hooks are prominent on scutes of juveniles
but are gradually lost as adults. The body lacks scales, and adults are generally
smooth-skinned. Body coloration is typically dark brown along the dorsal surface,
4
Figure 1.2. Head and mouth variations of adult shortnose sturgeon, Acipenser
brevirostrum. (Illustrations by Paul Vecsei)
5
becoming a yellowish-brown laterally, and white or cream colored ventrally. Scutes are
uniformly colored and are much lighter than the surrounding dorsal and lateral tissue.
Scott and Crossman (1973) and Dadswell et al. (1984) noted that juveniles exhibited
black blotches over much their body, although this has not been observed in juveniles of
similar size observed from the Altamaha River.
Genetics
Polyploidy is currently thought to have had an important role in the evolution of the
Acipenseriformes (Blacklidge and Bidwell 1993, Ludwig et al. 2001). All acipenserids
can be genetically categorized into one of two groups: those with 112-120
chromosomes, those with 240-250, and those with more than 250 (Blacklidge and
Bidwell 1993, Birstein et al. 1997, Fontana et al. 1999, Kim 2004). Within the
acipenserids, the shortnose sturgeon and the Sakhalin sturgeon (A. mikadoi) are the
only two known allopolyploids, having derived from 2 different ancestral species
(Birstein et al. 1997). There currently is much debate on the specific ploidy number of
shortnose sturgeons. Blacklidge and Bidwell (1993) categorized shortnose sturgeons
as dodecaploid (12N), and proposed that this species originally derived from the
hybridization between 4N and 8N species. Birstein et al. (1997) however, proposed that
shortnose sturgeons were instead 16N because allopolyploidy was unknown in other
sturgeon species. Ludwig et al. (2001) and Kim et al. (2005) disputed both these
theories, with the former proposing that shortnose sturgeons were instead octoploids.
Kim et al. (2005) however agreed with Blacklidge and Bidwell (1993) that the number of
chromosomes possessed by shortnose sturgeon (362-372) was actually far less than
6
the 500 proposed by Birstein et al. (1997). Blacklidge and Bidwell (1993) also argued
that the number of chromosomes was more indicative of dodecaploidy. Regardless of
whether the chromosomal number is 500 or the more probable ~360, the shortnose and
Sakhalin sturgeons have among the highest number of chromosomes yet reported for
any vertebrate (Birstein et al. 1997).
Recent genetic studies also have attempted to explain shortnose sturgeon
population stock structures. Wirgin et al. (2005) found that southern populations
displayed genetic differences among rivers and argued that this could be explained by
relatively small population sizes and their inherent susceptibility to stochastic drift. The
large degree of genetic variation among southern rivers also may be explained by lack
of influence of Pleistocene glaciation. Recolonization of northern rivers would have
occurred over the past 10,000 years and genetic differentiation would not be as
pronounced as in southern rivers (Wirgin et al. 2005). This pronounced difference in
genetic stock structure among populations, particularly in southern rivers, indicates
minimal gene flow among populations and genetic similarities may actually be indicative
of environmental similarities (Quattro et al. 2002, Waldman et al. 2002). Gene flow,
however, may be a function of population size rather than adjacent river proximity. For
example, Kynard (1997) and Walsh et al. (2001) argued that the Hudson River was a
possible source of individuals to other regional rivers because of its large population
size.
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Life History
The first studies focusing on shortnose sturgeon were conducted until the late
nineteenth-early twentieth century. These studies were initiated primarily to investigate
aquacultural techniques (Ryder 1890, Meehan 1910). More recent studies have
focused primarily on life history, population dynamics, and habitat utilization. These
studies have primarily originated in the northern extent of the range, where several large
populations now exist as a result of conservation and recovery efforts (Dadswell 1979,
Pekovitch 1979, Taubert and Dadswell 1980, Hastings et al. 1987, Hoff et al. 1988, Bain
et al. 1995, Kynard et al. 2000, Secor and Woodland 2005). Studies of southern stocks
have revealed a slightly different life history pattern in terms of spawning periodicity and
migrational patterns (Heidt and Gilbert 1978, Marchette and Smiley 1982, Hall et al.
1991, Collins and Smith 1993).
Eggs
Shortnose sturgeon eggs are darkly colored, usually dark brown, black, or olive
gray (Meehan 1910, Dadswell 1979, Hoff, 1988, Kynard 1997). Egg diameter is
typically 3.00-3.20 mm (Dadswell 1979) and does not change after fertilization or
immersion in water (Buckley and Kynard 1981, Dadswell et al. 1984). Fecundity
estimates range from 76,000 to 95,200, but averages 11,600 eggs per kg of fish (Heidt
and Gilbert 1978, Dadswell 1979, COSEWIC 2005). Eggs are not adhesive when first
spawned. Special protuberances on the egg membrane that maximize surface area
available for attachment develop within a few minutes after water exposure (Meehan
1910, Markov 1978, Dadswell et al. 1984). Development of fertilized eggs is directly
8
related to water temperature (Wang et al. 1985, Hardy and Litvak 2004). Meehan
(1910) found that eggs in water temperatures of 8-12 oC hatched 13 days after
fertilization—however incubation time was reduced to 8 days at 17 oC (Buckley and
Kynard 1981).
Yolk-sac larvae
At hatching, yolk-sac larvae are between 7-11 mm TL. Larvae are dark gray, with a
large, slightly pigmented yolk-sac, unpigmented eyes, undeveloped fins and an unopen
mouth (Meehan 1910, Taubert 1980a, Taubert and Dadswell 1980, Buckley and Kynard
1981, Dadswell et al. 1984, Bain 1997). They are benthic and photonegative, often
forming dense aggregations with other larvae when hiding (Buckley and Kynard 1981,
Dadswell et al. 1984, Richmond and Kynard 1995, Bain 1997). Dorsal and lateral
pigmentation is well developed by 15 mm TL and is most pronounced in the caudal
region (Taubert and Dadswell 1980).
The spiral valve is discernable in individuals ≤11.6 mm and rudimentary gill
structures are visible on specimens ≤9.5 mm TL (Bath et al.1981). At hatching, the
mouth is merely a small indentation, but begins to develop at 8.4 mm TL and small,
unicuspate teeth are present by 15 mm TL (Bath et al.1981, Taubert and Dadswell
1980, Richmond and Kynard 1995). Barbels first appear as tiny buds at 9.7 mm TL, but
are well developed by 15 mm TL (Bath et al.1981, Taubert and Dadswell 1980). The
eyes, fin buds, gills, and lateral line are either undeveloped or poorly developed in newly
hatched yolk-sac larvae, but are well defined by 15 mm TL (Bath et al.1981, Taubert
and Dadswell 1980, Richmond and Kynard 1995).
9
Post yolk-sac larvae
The larval stage begins with initiation of active feeding. Active feeding of larval
shortnose sturgeon begins with the depletion of the yolk, which typically occurs after 12
days, or at 15 mm TL at 15-17 oC (Taubert and Dadswell 1980, Bath et al.1981, Buckley
and Kynard 1981, Kynard 1997). The finfold increases with size, gradually separating
into the median fins, dorsal and anal fin ray basal supports, and dorsal and anal scutes
(Bath et al.1981). Spiracles, dermal ossification and scutes as well as fin rays, which
were absent earlier, begin to develop at 18-19 mm TL (Bath et al.1981, Gilbert 1989).
Larvae become photopositive after yolk sac depletion and can become lighter or darker
in response to changes in light intensity (Buckley and Kynard 1981, Richmond and
Kynard 1995, Kynard and Horgan 2002). The duration of downstream drift by larval
shortnose sturgeon is brief, usually about 2 days, and typically occurs at about 20 mm
TL (Richmond and Kynard 1995, Bain 1997). Richmond and Kynard (1995) postulated
that downstream movements of larvae serve to help disperse larvae from spawning
grounds and to help them find suitable cover.
Larvae larger than 15 mm TL are active swimmers, but are only capable of short
bursts of movement. At this stage most individuals are associated with the deepest
water available (Taubert and Dadswell 1980, Bath et al. 1981, Hoff et al. 1988, Dovel et
al 1989, Richmond and Kynard 1995, Kynard and Horgan 2002). Swimming activity
increases after 9-14 days (14-17 mm TL), possibly signifying that individuals would be
migrating from the spawning areas (Richmond and Kynard 1995). This movement is
most likely nocturnal to take advantage of low light levels and the increased cryptic
coloration of the larvae (Buckley and Kynard 1981, Richmond and Kynard 1995).
10
Juveniles
Juveniles are morphologically similar to adults, possessing a full complement of
rayed fins. At the onset of the juvenile stage, the tooth-filled mouth becomes toothless
and protrusible (Richmond and Kynard 1995). The scutes, which are sharply tipped as
juveniles, are maintained throughout the life of the sturgeon, demonstrating the
relationship between the need for body armoring and the presence of large predators.
As juveniles, the only confirmed predator of shortnose sturgeons is the yellow perch,
Perca flavescens (Dadswell et al. 1984). In southern populations alligators (Alligator
mississippiensis), sharks, or other fishes (e.g. catfish) also may be potential predators;
however, predation on adult shortnose sturgeon has not been documented (Scott and
Crossman 1973, Gilbert 1989).
Metamorphosis into the juvenile stage is complete by 31.5 mm TL. The juvenile
stage continues until the fish matures after 3-10 years (Gilbert 1989, Richmond and
Kynard 1995). Immature individuals remain primarily within river channels and
commonly feed on aquatic insects, isopods, and amphipods (Dadswell 1979, Carlson
and Simpson 1987, Bain 1997). Downstream migration continues throughout the first
year and individuals age 1 and older utilize the same habitats as adults (Dovel et al.
1989, Kynard 1997). Juveniles older than 1 year also make seasonal migrations,
moving upriver during warmer months where they shelter in deep holes, before
returning to the fresh/salt water interface when temperatures cool (Flournoy et al. 1992,
Collins et al. 2002a).
11
Adults
Although adult shortnose sturgeon may attain maximum lengths of 143 cm TL and
weights greater than 20 kg, they rarely exceed 122 cm TL (Dadswell 1979, Gilbert
1989). However, maximum size varies with latitude with southern individuals attaining
smaller maximum sizes (Dadswell 1979, Gilbert 1989). Growth rates and maximum
ages also are thought to vary with latitude. Dadswell et al. (1984) and Heidt and Gilbert
(1978) both assert that southern stocks grow faster and reach maturity earlier than do
their northern counterparts. Contrary to the 67-year old shortnose sturgeon captured in
the Saint John River, Canada by Dadswell (1979), the oldest reported individual from a
southern population was 14 years old and measured 98.5 cm FL in the Ogeechee
River, Georgia (Fleming et al. 2003).
Adult shortnose sturgeon also exhibit a latitudinal gradient relative to age of
maturation, spawning time, and migratory behavior (Dadswell et al. 1984, Gilbert 1989,
Rogers and Weber 1994). In Georgia waters, male sturgeon are thought to mature at 2-
3 years while females mature by age-6 (Dadswell et al. 1984, Kynard 1997). In more
northerly populations, such as the St. John River population, males may mature at 10-
11 years of age and females at age 12-18 (Dadswell et al. 1984, Bain 1997, Kynard
1997).
Feeding Behavior
The shortnose sturgeon is a benthic invertivore that finds prey by using its barbels as
tactile receptors and vacuuming either the substrate or plant surfaces with its
protuberant mouth (Dadswell et al. 1984, Gilbert 1989). Juveniles feed indiscriminately,
12
often ingesting large amounts of mud, stones, and plant material along with prey items
(Curran and Ried 1937, Dadswell 1979, Carlson and Simpson 1987). Several studies
have documented juveniles with up to 90% of the gut content containing non-food
material (Curran and Ries 1937, Dadswell 1979, Marchette and Smiley 1982). The diet
of juveniles includes small insects and cladocerans (Dadswell 1979, Marchette and
Smiley 1982, Dadswell et al. 1984, Carlson and Simpson 1987, Gilbert 1989). Adults
also appear to feed indiscriminately, but may be more capable of separating food and
non-food material prior to ingesting, depending on substrate (Dadswell 1979, Dadswell
et al. 1984, Gilbert 1989). Studies of gut contents show that the diet of adult shortnose
sturgeon typically consists of small bivalves, gastropods, polychaetes, and even small
benthic fish (McCleave et al. 1977, Dadswell 1979, Marchette and Smiley 1982,
Dadswell et al. 1984, Gilbert 1989, Moser and Ross 1995, Kynard et al. 2000).
Adult shortnose sturgeon feed throughout the year; however, Dadswell (1979) found
that females ceased feeding nearly eight months before spawning. Conversely, males
continue to feed throughout the fall and winter as long as they are located in saline
waters (Dadswell et al. 1984). Dadswell (1979) also documented individuals of both
sexes actively feeding immediately after spawning. Feeding occurs primarily at night in
water 1-5 m deep, although depth is directly related to temperature with feeding
occurring deeper during warmer months (Dadswell et al. 1984, Gilbert 1989).
13
Habitat and Movements
Habitat
Shortnose sturgeon use several distinct habitats throughout their life cycle with each
stage of development requiring a specific habitat. Adult and juvenile habitats differ
depending on seasonal and stochastic variables. Seasonal changes in habitat have
been well documented, but vary with latitude (Kynard et al. 2000). In northern
populations, both juveniles and non-spawning adults use deep segments of rivers, often
deeper than 10 m, during fall and winter months. In warmer months, they forage widely
throughout the river and estuary (Dadswell 1979, Hastings et al. 1987, Geoghegan et al.
1989, O’Herron et al. 1993, Kynard et al. 2000, Welsh et al. 2001). In southern
populations, individuals take refuge in deep, freshwater habitat during the summer,
rarely moving from them, and forage widely throughout the estuary during winter
months (Collins and Smith 1993, Rogers and Weber 1994, Weber et al. 1998). Small
juveniles, however, are much less tolerant of salinity and grow more slowly in a saline
environment and therefore remain primarily within freshwater habitats (Jenkins et al.
1993, Jarvis et al. 2001).
Shortnose sturgeon can be found over a variety of substrates, but probably select
areas based on food availability and thermal tolerances (Crance 1986). Riverine
habitats often are characterized by sandy-mud substrates, although shortnose sturgeon
do not avoid vegetated areas as evidenced by the presence of ingested plant material
likely consumed while foraging for gastropods (Dadswell 1979, Marchette and Smiley
1982). Specific data on the thermal preferences of shortnose sturgeon is limited, the
best data relating only to spawning adults and small juveniles.
14
Migrational Behavior
Shortnose sturgeon display both random and non-random movements (Dadswell et
al. 1984, Buckley and Kynard 1985). Although some populations are known to remain
within the tidally influenced portions of their home rivers, only Taubert (1980b)
described a population home range. Populations with access to the sea roam freely
throughout estuarine and riverine environments although these movements are often
dictated by temperature (Gilbert 1989). As water temperatures increase during
summer, individuals typically move upstream and remain near the fresh-salt water
interface until temperatures begin to fall (Dadswell et al. 1984, Buckley and Kynard
1985, Rogers and Weber 1994, Weber 1996, Collins and Smith 1997, Palmer 2001).
All sturgeon species undertake some type of spawning migration (either in the form
of anadromy or potomodromy), and all spawn in freshwater (Auer 1996, Fontana et al.
2001). Upstream migrations usually are associated with spawning behavior, while
downstream migrations normally are associated with feeding behavior (Bemis and
Kynard 1997). Gerbilskiy (1957) and Bemis and Kynard (1997) described spawning
migrations as either “one-step” or “two-step” movement patterns; the former typified by
a long, uninterrupted spring migration followed immediately by spawning; the latter
characterized by an abbreviated fall migration followed by an overwintering period near
the spring spawning site. This distinction also can be used to differentiate sympatric
subpopulations in many river systems (Bemis and Kynard 1997). Throughout the
southern portion of the range of the shortnose sturgeon, single-step migrations are
typical. Single-step migrations have been well documented in the Cape Fear (Moser
and Ross 1995), Cooper (Palmer 2001), Savannah (Hall et al. 1991, Collins and Smith
15
1993), and Altamaha (Heidt and Gilbert 1978, Flournoy et al. 1992, Rogers and Weber
1994) rivers. Among northern coastal rivers, two-step migrations have been
documented for shortnose sturgeon in the Hudson (Pekovitch 1979, Dovel et al. 1989),
Connecticut (Buckley and Kynard 1985), and Delaware (O’Herron et al. 1993) Rivers,
however, Kieffer and Kynard (1993) documented both migration types in the Merrimack
River. Regardless of which pattern is used, each of these studies has shown that
dispersal from the spawning site occurs immediately after spawning.
Distribution and Legal Status
Shortnose sturgeon inhabit large coastal rivers of the Atlantic coast of North America
from the St. John River in Canada to the St. John’s River in northeast Florida (Vladykov
and Greeley 1963, Moser and Ross 1995, Waldman et al. 2002, Collins et al. 2003). At
present, most stocks are depleted or extirpated over much of this range (Collins et al.
2002). Within the United States, shortnose sturgeon currently are protected under the
Endangered Species Act of 1973, having been listed by the U.S. Fish and Wildlife
Service as endangered in 1967. In 1980, Canada listed the shortnose sturgeon as a
species of special concern. The World Conservation Union (IUCN) has given the
shortnose sturgeon Red Book status as a vulnerable species throughout the range.
Mechanisms Leading to Declines
Overharvest
Sturgeon have long been harvested for both their flesh and roe, which is typically
processed for caviar. Klyszejko et al. (2004) found evidence that sturgeons were
16
commercially harvested in Europe dating back at least 1,000 years. Archeological
evidence from North America indicates that indigenous Americans also used sturgeons
as food for many centuries (Ritchie 1969). Beginning in the 1600’s, North American
sturgeon were commercially harvested for export to Europe (Murawski and Pacheco
1977). During the 1800’s, sturgeon were harvested not only for their roe and flesh, but
also used to make other products such as isinglass and paint additives (Scott and
Crossman 1973, Smith et al. 1984). In 1870, demand for sturgeon increased
dramatically and coastal stocks suffered serious declines. Landings fell from 2.9 million
kilograms 1892 in the Delaware Bay area to only 0.11 million by 1901 (Cobb 1900,
Gilbert 1989). After 1901, southern stocks became the major source of sturgeon in the
United States with harvest peaking in 1969 with 78,471 kg landed throughout the South
Atlantic Bight before declining once again (Murawski and Pacheco 1977, Smith et al.
1984, van den Avyle 1984, Gilbert 1989).
The importance of shortnose sturgeon to total landings prior to 1973 cannot be
evaluated because no distinction between shortnose sturgeon and Atlantic sturgeon
was made (Murawski and Pacheco 1977, Smith et al. 1984). LeSueur (1818)
commented that fishermen valued shortnose sturgeon more highly than the sympatric
Atlantic sturgeon because they had a higher market price. However, by 1890 the
shortnose sturgeon had virtually no economic importance because of its small size and
scarcity relative to the larger, more profitable Atlantic sturgeon (Ryder 1890). Today,
Atlantic sturgeon continue to be more sought after than shortnose sturgeon in Canadian
waters, where harvest continues on a limited basis (Trencia et al. 2002).
17
Since the shortnose sturgeon was listed as an endangered species in 1967, concern
over the frequency of incidental bycatch of shortnose sturgeon by commercial fisheries
has increased. Spawning migrations of shortnose sturgeon and American shad overlap
in time and space and commercial and recreational shad fishermen often catch the
migrating shortnose sturgeon in their nets (Dahlberg and Scott 1971, Scott and
Crossman 1973, Heidt and Gilbert 1978, Boreman et al. 1984, Collins et al. 1996,
Weber 1996, Collins and Smith 1997, Collins et al. 2000). Moser and Ross (1995) and
Weber (1996) reported that female shortnose sturgeon aborted eggs and returned
downstream when captured more than once. Collins et al. (1996) found that shrimp
trawlers, while routinely capturing juvenile Atlantic sturgeon, rarely encountered
shortnose sturgeon, however the commercial shad fishery had much higher encounter
rates (52 and 83% respectively) for both species.
Habitat Degradation
Shortnose sturgeon require specific habitats during various life stages and hence are
sensitive to habitat alterations. Construction of dams restricts movements and often
cuts off access to historic spawning grounds. Impoundments may also alter natural
river flow and temperature regimes needed for successful spawning (Kieffer and Kynard
1996, NMFS 1998, Cooke and Leach 2004). Though shortnose sturgeon apparently
are incapable of using modern fish ladders, fish lifts have been employed with limited
success in some rivers. For example, the fish lift located at Holyoke Dam on the
Connecticut River passed less than 5 shortnose sturgeon annually over a period of 21
years (Kynard 1998). With the loss of historic migratory routes, individuals downstream
18
of impoundments utilize spawning habitat similar to what had been historically utilized
and may spawn in close proximity to the impoundment (Kynard 1997, Cooke and Leach
2004, Duncan et al. 2004). Additionally, reproduction and recruitment within the
downstream segment of these impoundments is often poor, possibly prohibiting
recovery of these populations (Kynard 1997, Cooke and Leach 2004, Cooke et al.
2004).
Shortnose sturgeon also are thought to be sensitive to contaminants, which are
associated with impaired reproduction (Cameron et al. 1992, Longwell et al. 1992), early
survival (Dwyer et al. 2000) and susceptibility to disease (Sindermann 1979).
However, few studies have been conducted that examine these impacts. As long-lived,
benthic feeders, shortnose sturgeon are susceptible to bioaccumulation of heavy metals
and other contaminants (Dadswell 1979, Ruelle and Keenlyne 1993).
Other threats affecting recovery of shortnose sturgeon populations include dredging
and poor water quality. Dredging of navigation channels can destroy feeding habitats,
and there is documentation of mortalities from hydraulic pipeline operations (NMFS
1998). Industrial and agricultural discharge often contains contaminants that promote
high biological demand that results in lowered dissolved oxygen levels (NMFS 1998).
Low dissolved oxygen levels are of particular concern during early development of
shortnose sturgeon. Jenkins et al. (1993) found high mortality of juveniles less than 100
days old when dissolved oxygen concentrations were less than 2.5 mg/L. Although
shortnose sturgeon tolerance of low dissolved oxygen appears to increase with age,
Flournoy et al. (1992) reported that adults were stressed during periods of high
temperature and low dissolved oxygen levels. Over these periods, sturgeon often
19
congregate in deeper, cooler river regions that help alleviate the physiological stress
associated with high temperatures (Flournoy et al. 1992, Mason and Clugston 1993).
Absence of habitat that provide such refugia, especially in southern populations, has
been attributed to high juvenile mortality and extirpation of some populations (Collins
and Smith 1993, Rogers et al. 1994, Rogers and Weber 1995, Collins et al. 2000).
Management Approaches
Stocking and Reintroductions
Over the past two decades, increasing interest in shortnose sturgeon restoration has
spurred debate over whether supplemental stocking is an effective method for
recovering populations. From 1984-1992, the South Carolina Department of Natural
Resources supplemented the Savannah River shortnose population with nearly 100,000
mostly juvenile individuals in response to low juvenile recruitment (Smith and Collins
1996, Smith et al. 2002). Subsequent captures of released hatchery-reared shortnose
sturgeon in neighboring rivers revealed that many individuals did not remain within the
Savannah River system. Those stocked individuals may now constitute a portion of the
total population within those rivers in addition to the Savannah (Smith et al. 2002).
Despite the augmentation of the Savannah River population of shortnose sturgeon, no
increase in juvenile recruitment has been documented in either the Savannah or the
adjacent Ogeechee River suggesting a recruitment bottleneck as juveniles mature
(Collins et al. 2002a, Fleming et al. 2003). Based on the results of this stocking event,
the National Marine Fisheries Service (NMFS) developed guidelines to determine if
augmentation was feasible. Their effort focused on the population status, habitat quality
20
and availability, and availability of natal population broodstock (NMFS 1998). However,
these guidelines have not yet been applied because many rivers with populations in
imminent danger of extirpation continue to have significant anthropogenic factors
prohibiting natural recovery (NMFS 1998). Because of the susceptibility of small
populations to anthropogenic factors, there have been no approved stocking events
since the Savannah River was augmented.
Habitat Protection and Restoration
Habitat loss and degradation threaten the continued survival of shortnose sturgeon
in many rivers. To date, restoration efforts that failed to address the issue of habitat
degradation have generally been unsuccessful (Beamesderfer and Farr 1997). These
efforts have included supplemental stocking and habitat restoration. Because
shortnose sturgeon are more susceptible than many other fish species to
overexploitation because of their longevity, delayed maturation, and spawning
periodicity (Beamesderfer and Farr 1997), they may serve as indicators for system
degradation.
Research Objectives and Justification
Information about the shortnose sturgeon population in the Altamaha River system is
meager. Heidt and Gilbert (1978), Flournoy et al. (1992) and Rogers and Weber (1994)
provide some qualitative information about the status of the Altamaha River population
prior to implementation of the National Marine Fisheries Service (NMFS) recovery plan.
Because there are no current data, the effectiveness of the recovery plan for increasing
21
recruitment and protecting adults has not been evaluated. Additional quantitative data
on population size and structure are needed to allow regulatory agencies, such as
NMFS, to evaluate the effectiveness of the current recovery plan for the Altamaha
shortnose sturgeon population. To date, implementation of this plan has been limited.
Areas of interest have focused primarily on field research (mark-recapture, telemetry,
essential habitat quantification, bycatch assessment, stock augmentation assessment,
etc.). More recent studies have examined the genetic relationships among rivers.
However, because not all populations are currently being investigated, information on
many smaller populations may be outdated or absent.
Protection of spawning habitat is also a crucial component of the NMFS plan for
recovering shortnose sturgeons. Although spawning habitats of shortnose sturgeon
have not been identified in the Altamaha River, previous studies have shown that
spawning shortnose sturgeon select habitats with suitable flow and substrate
characteristics. For spawning to occur, current velocity of 37-125 cm/s is needed
(Pekovitch 1979, Taubert and Dadswell 1980, Buckley and Kynard 1985). Slower flow
allows eggs to clump together while higher velocity currents may prevent the eggs from
adhering to the substrate (Dadswell 1979, Buckley and Kynard 1985, Kynard 1997).
Substrates typically used by spawning shortnose sturgeon consist of sand, gravel,
cobble, or rubble bottoms (Dadswell et al. 1984, Gilbert 1989, Hall 1991, Kynard 1997)
Given the general lack of knowledge regarding the current status of Altamaha River
shortnose sturgeon, this study had three primary objectives: 1) to estimate the number
of shortnose sturgeon, 2) to evaluate the current population age structure, and, 3)
identify spawning sites of shortnose sturgeon in the Altamaha River. This population
22
assessment will help quantify what is thought to be a critically endangered population of
shortnose sturgeon while providing insight into possible trends in abundance.
Identification of spawning habitat will allow local management agencies to protect these
areas in the Altamaha as specified in the NMFS recovery plan.
23
CHAPTER 2
STUDY AREA AND METHODS
Study Site Description
Located entirely within Georgia, the Altamaha River and its main tributaries, the
Oconee and Ocmulgee Rivers, flow over 800 km from the headwaters near Atlanta,
Georgia to the Atlantic Ocean near Darien and drains nearly one-third of the state
(Figure 2.1). Encompassing approximately 36,000 km2, this river system is one of the
largest watersheds on the east coast of the United States and was recently listed as the
seventh most endangered river in the United States by the advocacy group American
Rivers. The Altamaha River is also second only to the Pascagoula River in Mississippi
in length of unimpounded stretch of river from the ocean east of the Mississippi River.
Dynesius and Nilsson (1994) attributed the more than 600 km of unimpounded river to a
lower rate of exploitation by a smaller regional population. The Altamaha is formed at
the convergence of the Ocmulgee and Oconee Rivers 215 river kilometers (rkm) inland.
It averages 50-70 m in width and 2-3 m in depth with a maximum depth of 18 m (Heidt
and Gilbert 1978). The Oconee and Ocmulgee Rivers contain the only impoundments
within the watershed; however, none are found farther downstream than 361 rkm, which
is well upstream of the known habitat of shortnose sturgeon in this system (Rogers and
Weber 1994). Vegetation along the lower reaches of the watershed progresses from
mixed hardwood to cypress swamp to salt marsh (Spartina spp.) near the estuary.
Lloyd Shoals Dam
rkm 604 Sinclair Dam rkm 444
Juliette Dam rkm 573 Oconee
River
Ocmulgee River
Altamaha River
Figure 2.1 The Altamaha River watershed (gray) including the headwater tributaries,
the Ocmulgee and Oconee Rivers with impoundments.
25
26
The average gradient over the lower 200 rkm is 0.13 m per km (GEPD 2003). Average
annual discharge is 381 m3s-1, or 18% of the entire freshwater input to the South
Atlantic shelf (Rogers and Weber 1994).
The sampling area of this study included the entire tidally influenced portion of the
lower river from Altamaha Sound upstream to rkm 40 when (Fig 2.2). I chose net
sampling locations based on the amount of woody debris on the river bottom. Input
from individuals with prior experience in capturing sturgeon also was an important factor
in site selection. To identify potential sampling sites, I first surveyed the river bottom
with a Furuno LS-6100 depth finder prior to gear deployment to ensure that the river
bottom was clear of debris or structure that might otherwise damage sampling gear.
Sites where sampling was not possible either through loss of gear or having extensive
bottom structure were eliminated from sampling. Water quality variables such as
dissolved oxygen, temperature, conductivity, and salinity were measured with a YSI-85
® meter.
Materials and Methods
I sampled the Altamaha River tidal zone from 14 November 2003 to 18 December
2005. I captured shortnose sturgeon were captured with 94.4 m long, 3 m deep
monofilament gill nets and trammel nets hung in a 2:1 ratio. Gill nets were constructed
from single 10.2 cm or 15.2 cm stretched mesh panels and experimental gill nets from
3-30.5m panels of 7.6, 10.2, and 15.2 cm stretched mesh hung in varying orders of
arrangement. Trammel nets were constructed from a single inner panel of 7.6 cm
stretched mesh surrounded by 2-30.5 cm stretched mesh panels. Nets were deployed
Figure 2.2 Shortnose sturgeon capture sites in the tidally influenced lower 40 rkm of the Altamaha River (sampling sites
27
Study Area
are denoted by o).
only during slack tides—typically 45-90 minutes—due to the large tidal amplitudes and
resulting currents. Nets were deployed perpendicular to the current, with net ends
anchored on the river bottom by a 6.8 kg navy anchor attached to the net end by a 0.9
m bridle. Soak time was reduced to 25-30 minutes during summer months when
temperatures exceeded 27 oC to minimize stress and reduce the chance of heat-related
mortalities of shortnose sturgeon (Moser et al. 2000). During net retrieval, all shortnose
sturgeon captured were placed in floating net pens until processing. Each fish was
measured—total length (TL) and fork length (FL) to the nearest mm, weighed to the
nearest 2 g (20 g for individuals heavier than 1.2 kg), and checked for tags. If a tag was
not present, fish were injected with a passive integrated transponder (PIT) tag under the
forth dorsal scute (125 kHz, Biomark). One centimeter segments also were removed
from the leading left pectoral fin ray of a subsample of shortnose sturgeon to estimate
age.
Abundance Estimates
I estimated population size using POPAN-5 within Program MARK (White and
Burnham, Colorado State University). The POPAN-5 program used to estimate
population size in this study is a robust parameterization of the Jolly-Seber model.
Model selection in POPAN is determined by Akaike’s Information Criteria (AIC).
Maximum Likelihood Estimation (MLE) addressed heterogeneity within the models
(Williams et al. 2002). Assumptions with the models vary with the parameterization;
however, the assumptions used in my study were 1) the population was closed to gains
and losses during the each sampling period, 2) tags were not lost or overlooked and are
28
correctly recorded, 3) all fish had equal capture probabilities during all sampling periods,
and 4) survival of each individual was independent of capture probability.
To compare this model with previous estimates for the Altamaha River shortnose
sturgeon population, I conducted a Schnabel estimate for the summers of 2004 and
2005. The assumptions of the Schnabel estimator were: 1) the population was closed
to gains and losses during the sampling periods, 2) marked and unmarked fish were
equally vulnerable to sampling gear, 3) marked fish mix randomly with unmarked fish,
and 4) tags were not lost and marks were not overlooked.
Data used both for the POPAN and the Schnabel estimates were for June-August of
2004 and 2005 only. This was because capture and recapture rates for the remainder
of the year was very low comparatively and because Program MARK can not account
for continuous sampling.
Population Age Structure
Age Determination
I evaluated population age structure using cross sections of pectoral fin spine
samples collected from a stratified subsample of 68 shortnose sturgeon. Age of each
shortnose sturgeon in the subsample was determined from a 1-2 cm segment of the left
leading pectoral fin ray which was removed using a small coping saw (Fleming et al.
2003). This procedure was performed only during cooler months when water
temperature was below 27 oC to minimize the risk of mortality associated with additional
handling. Once removed from the fish, each segment was either allowed to air dry for
45 days or dried in an oven at 65.5o C for 4 hours. Once dry, a Buehler low-speed saw
29
was used to cut the segments to a thickness of 0.50-0.75 mm. Each cross section of
each fin ray segment was mounted on an individual glass slide and viewed by two
independent readers without prior knowledge of fish size, date, or capture location at
30x magnification on a Leica MZ6 dissecting microscope. All disagreements in
assigned ages were reconciled by mutual agreement between observers.
I evaluated the precision of age estimates by coefficient of variation (CV) as given in
the equation:
CV = 100 x XjR
XjXijR
i∑= −
−
1
2
1)(
where R is the number of reads per sample, Xij is the i th determination of the j th fish,
and Xj is the mean age of the j th fish (Chang 1982). This formula gives a single value
for each sample and reader which were averaged across of all spines to give a mean
estimate of precision. The index values were used to evaluate whether error increased
with older fish and to compare the error rate with reported values from other studies on
long-lived fish. Differences in age estimates among readers were plotted and compared
to a line where the slope is equal to 1 (complete agreement) to detect reader bias
(Campana et al. 1995).
An age-frequency histogram was constructed to illustrate the age structure of the
population. Annual mortality was calculated from the catch curve constructed from
these data by linear regression.
I also used the von Bertalanffy growth equation to estimate both the growth
coefficient and the maximum size potential of shortnose sturgeon in the Altamaha River.
The von Bertalanffy growth model is described by the equation:
30
⎥⎥⎦
⎤
⎢⎢⎣
⎡ −−−∞=
⎟⎟⎠
⎞⎜⎜⎝
⎛
0ttKexp1TLtTL
where TLt is the mean predicted length (mm) at age t, TL∞ is the average asymptotic
length of shortnose sturgeon, t0 is the x-intercept associated with the predicted age at
which size is 0 mm, and K is the Brody growth coefficient (Ricker 1975). Parameters
were estimated using the program FiSAT II © (FAO-ICLARM Fish Stock Assessment
Tools 2000), which uses regression to estimate the parameters TL∞, t0, and K.
Predicted weight-at-age from the von Bertalanffy growth model was obtained using the
equation:
Wt = a(TL)b
where the growth parameter b was fitted by regression. The von Bertalanffy growth
model for weight is:
⎥⎥⎥
⎦
⎤
⎢⎢⎢
⎣
⎡ −−−∞=
⎟⎟⎠
⎞⎜⎜⎝
⎛ b0ttK
exp1WtWt
where Wt is the mean predicted weight (g) at age t, W∞ is the average asymptotic
weight (g) of shortnose sturgeon, and t0 and K are parameteritized as per the length-at
age model above. I used this equation to evaluate the growth rates of shortnose
sturgeon in the Altamaha River over time.
Gear Selectivity
Because age data can be biased by gear selectivity, I also evaluated the effect that
the sampling gear may have had on the size of shortnose sturgeon captured. I
estimated mesh selectivity values through manipulation of the equation:
Cm,l = ql,m Nl Pm Sm,l
31
where Cm,l is catch by mesh size m for a given length class l; ql,m is catchability of
length class l in mesh size m; Nl is the abundance of fish of length class l; Pm is the
fishing power or efficiency of mesh size m; and Sm,l is the selection of mesh size m for
length class l (Hovgård and Lassen 2000).
For gear selectivity bias analysis, I used the Baranov model, which assumes that
each mesh size will be the most efficient at capturing a fish of length, lo. I also used the
NORMSEP process within the program FiSAT II © (FAO-ICLARM Fish Stock
Assessment Tools 2000) to generate selectivity curves under the assumptions of the
Baranov model. This procedure uses maximum likelihood estimates to divide length-
frequency classes for each mesh size into normally distributed selectivity curves. All
curves were scaled to a maximum value of 1 (Hamley 1975).
Data Analysis
I evaluated gear selectivity separately for 2004 and 2005. Based on the results from
2004, evaluating gear selectivity became a secondary objective to this study. In 2004, I
compared lengths of shortnose sturgeon captured in trammel nets with those captured
in 10.2 cm and 15.2 cm gill nets. In 2005 I substituted the 10.2 cm and 15.2 cm gill nets
for two experimental nets of 7.6, 10.2, and 15.2 cm stretched mesh hung in different
arrangements and compared lengths of shortnose sturgeon to those captured in
trammel nets only in 2005. To compare sampling efficiency among net types in the
2004, I used a two-way ANOVA with a factorial arrangement of treatments to compare
the effects on catch per unit of effort (CPUE) of net type, tidal stage, and possible
interaction of these variables. CPUE was calculated by determining the number of
32
sturgeon caught per net set. To evaluate potential size selectivity of different net types,
I used Tukey’s honestly significant difference (hsd), which is a conservative means
comparison test, to compare the mean size captured for each net type. Juveniles were
considered to be all fish less than 600 mm TL. This number was derived using linear
regression stipulating the size at maturity as reported by Rogers and Weber (1994). In
2005, I used a two-way ANOVA with a factorial arrangement of treatments to compare
the effects of net types, tides, and possible interactions on CPUE of each net type and
tidal stage for both shortnose sturgeon and other non-target species. To contrast size
selectivity of the net types in 2005, I used t-tests to compare the mean sizes of
shortnose sturgeon captured for each net type for juvenile and adult shortnose
sturgeon. Juveniles and adults were analyzed separately to evaluate susceptibility of
both life stages to the sampling gear. All statistical tests were conducted with an alpha
of 0.05.
Identification of Critical Habitat
To identify potentially critical habitat locations of shortnose sturgeon, I used radio
telemetry to monitor seasonal movements of 12 adult shortnose sturgeon. The radio
tags, manufactured by Advanced Telemetry Systems (Isanti, Minnesota) measured 68
mm in length, 32 mm in width, and weighed 25 g in air. Each tag was programmed to
operate on a 12-hour duty cycle at 48-49 MHz. The battery life of the tags was
approximately 5 years. Tags were surgically implanted into adult shortnose sturgeon
after each tag was coated with an inert elastomer to reduce rejection potential. Adult
shortnose sturgeon selected for radio tracking weighed a minimum of 1,250 g to ensure
33
that weight was <2% of the fish’s body weight as recommended by Winter (1983). I
performed surgeries on shortnose sturgeon in the fall and early winter after water
temperature had fallen to 10.9 o - 21.4 oC, to minimize handling stress (Moser and Ross
1995, Moser et al. 2000). Individuals implanted with tags were first anesthetized in a
250 mg/L solution of tricaine methanesulfonate (MS-222). The fish were then placed on
a portable surgical table equipped with a recirculating pump that maintained a constant
flow of anesthetic (85 mg/L MS-222 and 170 mg/L NaHCO3 buffer in 60 L of water) over
the gills during the procedure. Sex and reproductive condition was determined using a
Karl Storz model 26006 AA laparoscope to examine the gonads of each fish as
described by Hernandez-Divers et al. (2004). Only those fish observed to be in
spawning condition were selected for insertion of radio tags.
This surgical procedure was performed by first making an incision about 4.5 cm long
to allow for insertion of a transmitter. A cordless drill was used to drill a 2 mm hole into
a ventral-lateral scute (located slightly posterior to the incision). A metal catheter was
then inserted through this hole until it reached through the body wall and into the body
cavity. The antennae was inserted through the catheter, the catheter removed, and the
rest of the tag inserted into the abdomen. The incision was closed with a 3/0 cutting
needle attached to a non-absorbable braided nylon suture. The entire procedure
typically required less than 3 minutes. Prior to release, each fish was allowed to
recover by holding the fish in non-treated river water and slowly moving the fish around.
The fish was released once it had regained its balance and could swim without
assistance.
34
35
Tracking of radio-tagged fish was conducted on a weekly basis from a small boat
equipped with a scanning receiver and loop antenna. The area surveyed extended from
10 rkm above the confluence of both the Oconee and Ocmulgee rivers to the coast
(Figure 2.3). Locations of radio-tagged fish were marked to the nearest meter possible
using a handheld GPS receiver. GIS mapping of these data was used to identify
potential spawning sites based on 1) the relative number of fish using an area and 2),
the relative duration of each fish’s stay. I also used a ponar grab to determine bottom
composition where each fish was located.
To confirm spawning at suspected spawning sites, I deployed artificial egg-sampling
mats during February and March 2005. The egg mats were constructed from 40 cm x
20 cm x 10 cm cinder blocks wrapped with hog-hair filter material. Pairs of egg mats
were placed 1 m apart and marked by a single buoy. The number of egg mats
deployed at each site ranged from 5-15 and varied with current velocity and bottom
composition. The egg mats were retrieved and visually examined every three days.
Recovered eggs were preserved in 70% ethanol.
Figure 2.3. Altamaha River basin covered on a weekly basis by boat equipped with a scanning receiver and loop
Head of tide rkm 40
Doctortown rkm ~97
Confluence
36
rkm 215
Baxley rkm 180
Study Area
antennae.
CHAPTER 3
RESULTS
Abundance Estimates
In late 2003 and throughout 2004, I set a total of 612 nets and captured 644
shortnose sturgeon, 49 of which were later recaptured. In 2005, effort was reduced to
276 net sets, yet 283 shortnose sturgeon were captured with an additional 40
recaptured. In total, 929 nets were deployed and 927 captured shortnose sturgeon and
89 subsequently recaptured. Sizes of captured shortnose sturgeon ranged from 290 to
1166 mm TL (Figure 3.1). CPUE averaged 1.16 fish per net set (274.2 m of
entanglement gear fished for 1 slack tide), but varied with season and gear type (Table
3.1). CPUE during July 2004 was greater than at any other time during the study, with a
range of 1.0—23.9 fish/net set (Figure 3.2).
Population estimates ranged between 3400-8233 with the Schnabel method and was
6320 (95% C.I. 4387-9249) using POPAN (Table 3.2). The POPAN estimate assumed
that apparent survival and recapture probability varied temporally and entry probability
was constant. Initial population size did not affect the model through either temporal
variation or constancy.
2004
0.00
0.05
0.10
0.15
0.20
0.25
200
250
300
350
400
450
500
550
600
650
700
750
800
850
900
950
1000
1050
1100
1150
1200
CPU
E
2005
0.00
0.05
0.10
0.15
0.20
0.25
200
250
300
350
400
450
500
550
600
650
700
750
800
850
900
950
1000
1050
1100
1150
1200
TL (mm)
CPU
E
n = 323
Figure 3.1. Size distributions of captured shortnose sturgeon from the Altamaha River, Georgia in 2004 (top)
and 2005 (bottom). In 2004, individuals were caught in 10.2 and 15.2 cm gill nets and trammel nets. In 2005,
n = 685
gill nets were replaced with experimental gill nets constructed from 7.6, 10.2, and 15.2 cm panels.
38
Year Net No. Net Sets
No. Shortnose Captured CPUE Relative
Efficiency
2004 Trammel Net 144 252 1.75 100.0
2004 10.2 cm Gill Net 144 52 0.36 20.6
2004 15.2 cm Gill Net 144 104 0.72 41.3
2005 Trammel Net 48 112 2.33 100.0
2005 Experimental Gill Net 96 136 1.42 60.7
39
Table 3.1. Trammel net and gill net capture comparison in 2004 and 2005. Trammel
nets were constructed from an inner panel of 7.6 cm and 2 outer panels of 30.5 cm
stretched mesh. Gill nets were constructed from 10.2 and 15.2 cm stretched mesh.
Experimental gill nets were constructed from 7.6, 10.2, and 15.2 cm stretched mesh.
CPUE is defined in terms of number of shortnose sturgeon captured per net set.
Figure 3.2. Monthly catch per unit effort (CPUE) of shortnose sturgeon in the Altamaha River for 2004 and 2005.
(Sampling not conducted during months denoted with an asterisk.)
2004
0.000
2.000
4.000
6.000
Jan Feb Mar Apr May June July Aug Sept Oct Nov DecMonth
CPU
E
2005
0.000
2.000
4.000
6.000
Jan Feb Mar Apr May June July Aug Sept Oct Nov Dec
Month
CPU
E
* *
40
Table 3.2. Estimates of shortnose sturgeon abundance in the Altamaha River, Georgia.
Population Estimator (Model)
Model Type
N 95%
Confidence Interval
Sample Period
Schnabel closed 8233 5083 – 21656 1 June 2004 – 31 August 2004
Schnabel closed 3400 2008 – 11074 1 June 2005 – 31 August 2005
POPAN open 6320 4387 – 9249 1 June 2004 – 31 August 2004,
1 June 2005 – 31 August 2005
^
41
Population Age Structure
Age and Growth
Nominal ages determined for 68 fish ranged between 1 - 14, with a modal age of 4
and a mean age of 5.9 (Figure 3.3). Interpretation of annuli was difficult and agreement
among observers was only 55%. However, disagreements of one year (37%)
constituted the majority of the remaining fin ray samples. The greatest age discrepancy
encountered was 2 years, which occurred in 5 individuals. Precision among observers
was variable with a coefficient of variance of 4.58 across both observers and all age
groups. A visual assessment of age-bias plots between observers (Figure 3.4)
indicated that age estimates were relatively precise and that error rates did not increase
with older fish.
Using the von Bertalanffy growth equation, I developed a single growth curve (Figure
3.5) for the Altamaha River population of shortnose sturgeon. The weight to length
relation was
Wt = 775( )
⎥⎦⎤
⎢⎣⎡ −−−
3.401.58t0.09exp1
(Figure 3.6).
Total annual mortality based on uncorrected catch curves (Figures 3.7 and 3.8) was
estimated to be 29% and 37%, respectively. This rate was estimated for fish between
ages 4-12 only because fish from older age classes were rarely captured and because
of the presence of a large cohort of juvenile fish (Tables 3.3 and 3.4). In 2005, this
cohort began to affect the slope of the catch curve, indicating a higher mortality rate
than expected. When the mortality rate was recalculated without this cohort, the
42
43
estimate fell to 34%. Maximum average life span, based on the uncorrected catch
curves was estimated to be 14-15 years.
Gear Selectivity
Each net type and mesh size had a unique lo that corresponded with mesh size
(Figures 3.9 and 3.10). The trammel net, with a 7.6 cm inner panel, had a lower lo in
both sampling periods and lo increased with mesh size. Experimental gill nets and
trammel nets had similar lo’s and selectivity curves. Trammel net lo’s increased from
2004 to 2005 as growth occurred over time.
Trammel nets caught a wider size range of shortnose sturgeon (Figure 3.11) than
either the 10.2 cm or 15.2 cm gill nets in 2004. The results of the ANOVA show that
trammel nets catch a significantly smaller mean size for both juvenile and adult
shortnose sturgeon than did either the 10.2 cm or 15.2 cm gillnets (juveniles:
F2,178=16.00, p<0.0001; adults: F2,166=6.87, p=0.0014). The 10.2 cm or 15.2 cm gill nets
were also found to capture significantly different sizes of both juvenile and adult
shortnose sturgeon (juveniles: mean difference=118.72, 95% confidence limit=15.65-
221.787; adults: mean difference=75.77, 95% confidence limit=20.11-131.42).
Trammel nets caught significantly smaller individuals of juvenile shortnose sturgeon
(Figure 3.12) than did the experimental gill nets (t191=2.59, p=0.0102) in 2005. Trammel
nets and experimental gill nets caught similar size ranges of shortnose sturgeon
individuals, but experimental gill nets caught a higher number of individuals larger than
600 mm TL.
0
2
4
6
8
10
12
14
16
1 2 3 4 5 6 7 8 9 10 11 12 13 14
Age (years)
Freq
uenc
y
Figure 3.3. Nominal age frequency of shortnose sturgeon captured in the Altamaha River, Georgia in 2005. Ages of
individual fish were determined from pectoral fin ray sections and interpreted by 3 independent observers.
44
0
2
4
6
8
10
12
14
16
0 2 4 6 8 10 12 14 16
First interpreter estimates
Seco
nd in
terp
rete
r est
imat
es
CV = 4.58%
Figure 3.4. Age-bias graph for 2 interpreters of sampled fin rays from Altamaha River shortnose sturgeon. The
dashed line designates the 1:1 ratio line.
45
0
200
400
600
800
1000
1200
0 4 8 12 16 20
Age
Tota
l Len
gth
(mm
)
AltamahaHudsonOgeechee
TL = 1389 (1-exp -0.09 ( t-1.58 ))
Figure 3.5. Growth of shortnose sturgeon (both sexes) in the Altamaha River, Ogeechee River, and Hudson River.
46
0
2000
4000
6000
8000
10000
12000
0 200 400 600 800 1000 1200 1400
TL (mm)
Wei
ght (
g)
R2 = 0.9847n = 1012
Figure 3.6. Weight – length relationship of shortnose sturgeon (both sexes) from the Altamaha River, Georgia.
47
Table 3.3. Age-frequency key for shortnose sturgeon captured in the Altamaha River system in 2004. Ages were
determined from the first pectoral fin ray. Final ages from each individual subsample were applied proportionally across
the same length-group. Note: age-2 or age-13 individuals were not obtained during this study.
Age 1 Age 3 Age 4 Age 5 Age 6 Age 7 Age 8 Age 9 Age 10 Age 11 Age 12 Age 14
≤ 300 1 1(1) 1301 - 400 277 5(1) 277401 - 500 39 2(3), 8(4) 8 31501 - 600 29 8(4), 8(5), 3(6) 12 12 5601 - 700 37 2(5), 3(6), 4(7) 8 12 16701 - 800 21 3(6), 2(7), 1(8) 11 7 4
801 - 900 24 4(7), 2(9), 2(11) 12 6 6
901 - 1000 14 1(7), 2(8), 1(9), 1(10), 1(12) 2 5 2 2 2
1001 - 1100 2 1(8), 1(10), 1(12), 1(14) 1 1 1 1
1101 - 1200 2> 1200
Total 446 278 8 43 20 28 37 10 8 3 6 3 1
Length group (mm)
Number in
sample
Number (age) in subsample
Sample allocation per age - group
48
Table 3.4. Age-frequency key for shortnose sturgeon captured in the Altamaha River system in 2005. Ages were
determined from the first pectoral fin ray. Final ages from each individual subsample were applied proportionally across
the same length-group. Note: age-2 or age-13 individuals were not obtained during this study.
Age 1 Age 3 Age 4 Age 5 Age 6 Age 7 Age 8 Age 9 Age 10 Age 11 Age 12 Age 14
≤ 300 1(1) 4301 - 400 4 5(1) 23 92401 - 500 115 2(3), 8(4) 43 43 16501 - 600 103 8(4), 8(5), 3(6) 5 7 10601 - 700 22 2(5), 3(6), 4(7) 7 4 2701 - 800 13 3(6), 2(7), 1(8) 10 5 5
801 - 900 20 4(7), 2(9), 2(11) 1 3 1 1 1
901 - 1000 8 1(7), 2(8), 1(9), 1(10), 1(12) 2 2 2 2
1001 - 1100 6 1(8), 1(10), 1(12), 1(14)
1101 - 1200> 1200
Total 291 4 23 135 48 30 25 6 6 3 5 3 2
Length group (mm)
Number in
sample
Number (age) in subsample
Sample allocation per age - group
49
y = -0.3441x + 5.1842R2 = 0.8076
n = 158
0.0
1.0
2.0
3.0
4.0
5.0
6.0
3 4 5 6 7 8 9 10 11 12 13
Age
Ln N
umbe
r Cau
ght
M = 0.291Maximum Age = 15
Figure 3.7. 2004 uncorrected catch curve for ages 4 – 12 of Altamaha River shortnose sturgeon.
50
y = -0.4731x + 6.3189R2 = 0.8984
n = 291
0.0
1.0
2.0
3.0
4.0
5.0
6.0
3 4 5 6 7 8 9 10 11 12 13
Age
Ln N
umbe
r Cau
ght
M = 0.377Maximum Age = 13
Figure 3.8. 2005 uncorrected catch curve for ages 4 – 12 of Altamaha River shortnose sturgeon.
51
Trammel nets
0.00
0.20
0.40
0.60
200
250
300
350
400
450
500
550
600
650
700
750
800
850
900
950
1000
1050
1100
1150
1200
CPU
E
Gill nets
0.00
0.20
0.40
0.60
200
250
300
350
400
450
500
550
600
650
700
750
800
850
900
950
1000
1050
1100
1150
1200
TL (mm)
CPU
E
10.2 cm15.2 cm
Figure 3.9. Size composition of shortnose sturgeon captured in trammel nets and gill nets from the Altamaha River,
Georgia in 2004. Catch per unit effort is the number of shortnose sturgeon captured per net per slack tide.
52
Trammel nets
0.00
0.20
0.40
0.60
200
250
300
350
400
450
500
550
600
650
700
750
800
850
900
950
1000
1050
1100
1150
1200
CPU
E
Experimental gill nets
0.00
0.20
0.40
0.60
200
250
300
350
400
450
500
550
600
650
700
750
800
850
900
950
1000
1050
1100
1150
1200
TL (mm)
CPU
E
Figure 3.10. Size composition of shortnose sturgeon captured in trammel nets and experimental gill nets (constructed
from 7.6, 10.2, and 15.2 cm stretched mesh) from the Altamaha River, Georgia in 2004. Catch per unit effort is the
number of shortnose sturgeon captured per net per slack tide.
53
lo = 501
lo = 673 lo = 835
0
0.2
0.4
0.6
0.8
1
200 300 400 500 600 700 800 900 1000 1100
TL (mm)
Sele
ctiv
ity
Trammel net ( ) n = 252 10.2 cm gill net ( o ) n = 52 15.2 cm gill net ( ) n = 104
Figure 3.11. Indirect selectivity of trammel nets, 10.2 cm, and 15.2 cm stretched mesh gill nets from the Altamaha
River, Georgia in 2004.
54
Figure 3.12. Indirect selectivity of trammel nets, and experimental gill nets composed of 7.6, 10.2, and 15.2 stretched
55
0
0.2
0.4
0.6
0.8
1
200 300 400 500 600 700 800 900 1000 1100
TL (mm)
Sele
ctiv
itylo = 537 lo = 597
Trammel net ( ) n = 112 Experimental gill net ( o ) n = 136
mesh panels from the Altamaha River, Georgia in 2005.
Habitat and Seasonal Movements
When water temperatures exceeded 27 oC, all radio tagged shortnose sturgeon were
located above the salt-fresh water interface. In 2004, the interface was located as far
inland as rkm 22. River bottom temperatures exceeded 31 oC and dissolved oxygen
(DO) levels fell as low 4.53 mg/L throughout the tidally influenced portion of the river.
Most captures (as many as 23.9 fish per net set) during this time period were in a single
river segment at rkm 22.5 referred to as Ebenezer Bend. This stretch of river reaches a
depth of 16 m at mean low tide and has large woody debris along much of the mostly
sandy bottom.
From mid-August to September of 2004, multiple tropical storm systems moved
through the area and caused discharge to increase and temperatures to decrease.
When sampling resumed in mid-October, water temperature had fallen to 20.2 oC, DO
increased to 5.82 mg/L and capture rates were lower above rkm 20 while increasing
throughout the estuary. During winter, when water temperatures fell below 14 oC,
adults were captured in Altamaha Sound in salinities greater than 18 ‰.
Eight male and 4 female (≥ 768 mm TL) shortnose sturgeon were radio-tagged
between 14 November 2004 and 14 January 2005 (Table 3.5). Nine of these were
tracked until the end of 2005. The remaining individuals were censored after movement
was not detected, or they were not relocated, after a period of 4 months. Periodic
checks for an additional 2 months also showed no movement. There were no known
mortalities directly attributable to the implantation procedure; however, the status of
individuals that were not relocated is unknown.
56
Upstream movement of radio tagged fish, possibly associated with spawning runs,
commenced in mid-December when water temperature was 10.2o C. I observed
ripening females as early as mid-November. Of the 4 females I radio-tagged, at least 3
of these made upstream migrations. The first radio tagged female left the estuary 4
December 2004 and was located near Doctortown (rkm 89). Additional movement was
only 7 rkm upstream 5 days after it was initially located. Further movement was not
observed. Two other females completed runs into the Ocmulgee and Oconee rivers
(individually) and remained there for a period of one week prior to moving downstream
at a rate of 40 km/day. One of these females traveled a distance of at least 340 rkm
over 6 weeks and moved at a rate of nearly 5 km per hour (as determined by monitoring
for a period of 1.5 hours) during downstream migration.
When tracking both of these females, contact was lost at the confluence of the
Ocmulgee and Oconee rivers. This area is characterized by a sandy substrate with a
deep hole excavated by the fast, converging currents. Eggs mats were deployed
throughout this area and 2 eggs were recovered on 20 March, 2005, when the water
temperature was 12 oC. I located two other individual shortnose sturgeon between rkm
159.6 – 178.4 prior to them returning downstream. A single male moved upstream to
rkm 133.2 in late spring of 2006, but returned downstream to rkm 92-94, where it had
resided for the previous year. Based upon these individuals, I was able to identify 1
confirmed and 2 suspected spawning sites (Figure 3.13)
Non-spawning habitat was located in 2 discrete areas: the tidally influenced portion
of the lower river (rkm <40) and slightly below Doctortown (rkm 90-95) (Figure 3.14).
This area was categorized as non-spawning habitat because individuals were present
57
throughout the year and attempts to recover eggs at this location were unsuccessful.
This upstream habitat is much deeper (7.6 m) relative to the surrounding stretches of
river (~3 m depth) and Ponar grabs revealed that the substrate is composed of sand
with large numbers of small bivalves and aquatic insects. Movements greater than 2
rkm were not observed in either summering area while river temperatures exceeded
27 oC.
58
59
49.771 mal
49.781 mal
49.791 mal
49.801 mal
49.811 femal
49.831 femal
49.841 femal
49.871 femal
49.881 mal
49.891 mal
49.901 mal
49.921 mal
Days at Lar
Table 3.5. Radio tagged shortnose sturgeon release dates and number of relocations in
the Altamaha River, Georgia. The * denotes an individual tag that did not change
locations after an initial ~100 km upstream migration after tag implantation. ** indicates
a censored individual after x days at large.
geNo.
RelocationsRadio
Frequency Sex Date Released
e 11 Nov. 2004 15 340
e 22 Jan. 2005 0 unknown **
e 16 Dec. 2004 17 328
e 18 Nov. 2004 25 265
e 22 Jan. 2005 1 125 **
e 10 Dec. 2004 22 352
e 22 Jan. 2005 25 326
e 12 Nov. 2004 29 * 327 **
e 14 Jan. 2005 19 277
e 14 Jan. 2005 16 270
e 17 Dec. 2004 14 364
e 6 Jan. 2005 15 345
Suspected spawning area
rkm 160 -170 I-95
Confirmed spawning site (rkm 215)
rkm 110-120
U.S. 1
Figure 3.13. Confirmed and suspected spawning areas of shortnose sturgeon in the Altamaha River, GA (2005).
60
Figure 3.14. Non-spawning habitat of shortnose sturgeon in the Altamaha River, GA (April-December 2005).
61
3 fish 42 relocations (rkm 95-100)
8 fish
I-95
Head of tide
73 relocationsU.S. 1
CHAPTER 4
DISCUSSION
Abundance Estimates
Since shortnose sturgeon were initially listed as endangered in 1967, most
populations have remained depressed (Moser and Ross 1995); however, the
abundance estimate of 6320 (95% C.I. 4387-9249) obtained in this study is nearly ten
times larger than the previous estimate of 650 reported by Kynard (1997—from Rogers
and Weber, unpublished data). My results show that the Altamaha River population of
shortnose sturgeon is probably the fourth largest in the United States, and the largest
south of Chesapeake Bay (Squiers et al. 1982, Hastings et al. 1987, Bain et al. 1995,
NMFS 1998). Although continued study is needed to determine if the population is
remaining stable in the face of ongoing environmental challenges within the Altamaha
watershed, the results of this study suggest that the Altamaha River supports one of the
largest known populations of shortnose sturgeon. Furthermore, small populations in
adjacent rivers may be sustained by immigrating Altamaha individuals in lieu of poor
reproduction by resident individuals (Wirgin et al. 2005). Northern rivers with large
populations (e.g., the Hudson and Delaware rivers) may also supplement adjacent
smaller populations through immigrants and transients (Welsh 2002); however, the
actual genetic contribution to these smaller populations is uncertain. Wirgin et al.
(2005) reported that haplotypes of shortnose sturgeon from the Altamaha and
62
Ogeechee river populations were not significantly different, which suggests at least
some gene flow between populations in these rivers. Further study is necessary prior to
assessing the actual Altamaha River component both in the Ogeechee and other
adjacent rivers.
POPAN-5 is capable of estimating abundance, survival and capture probability and
was chosen because the field data best fit the program. The primary limitation of
POPAN is that because the numbers of individuals not captured are also estimated, the
parameter N includes unknown covariates. Therefore, covariates were not considered
when calculating the population estimate in this study. The best-fitting model in this
study assumed temporal variation in survival and recapture probabilities and constant
immigration. Although these assumptions may have been met, low recapture rates had
a considerable affect on the model and caused the model fit to decrease somewhat.
The Schnabel models used in this study assumed a closed population during each
sampling period. This assumption was probably not met given the movements of radio
tagged fish I observed. Furthermore, the estimates from the Schnabel model varied
widely — 46% lower to 30% higher than those generated by the POPAN model, which
suggests that management agencies should not use the simpler Schnabel model for
monitoring future population trends.
Population Age Structure
Age and Growth
In recent years, the marginal pectoral fin ray has become the preferred structure for
estimating the age of sturgeon because of the apparent non-lethality of this technique
63
and because cross sections from it are easier to interpret than those from other calcified
structures (e.g., otoliths or scutes) (Currier 1951, Pycha 1956, Roussow 1957, Dadswell
1979, Brennan and Cailliet 1989, Dovel et al. 1989, Keenlyne and Jenkins 1993, Rien
and Beamesderfer 1994, Morrow et al. 1998). In sturgeons, these rays contain
alternating opaque and translucent bands formed by the differences in summer and
winter growth rates (Currier 1951, Roussow 1957, Dadswell 1979, Nakamoto et al.
1995, LeBreton and Beamish 2000, Paragamian and Beamesderfer 2003). The dark
bands are created by rapid deposition of connective tissue during the summer
(Nakamoto et al. 1995) whereas the translucent bands are formed during the period of
metabolic inactivity that typically occurs during the winter when growth is slow, and
mineralization of the ray is relatively poor (Chilton and Beamish 1982, Nakamoto et al.
1995). Within a typical sturgeon fin ray, the growth increments appear as consistently
spaced rings formed throughout the juvenile phase. After maturity, however, growth
slows causing the growth annuli to become more closely spaced (Dadswell 1979,
Dadswell et al. 1984). This pattern of progressive “tightening” of the annuli in adult
sturgeon often makes interpretation of growth increments more difficult in older
specimens (Dadswell 1979, Dovel et al. 1989, Nakamoto et al. 1995, Fleming et al.
2003).
Age determination of individuals in this study proved difficult, and independent
observers regularly reported different initial age estimates from the same fin ray cross-
section. The age estimates were relatively precise and were comparable to other long-
lived species (Table 4.1). Coefficient of variance is an unbiased estimator of precision
64
Table 4.1. Coefficient of variance (CV) of age estimates for between-observer precision
for reported long-lived species.
Species CV Maximum observed age
White sturgeon A. transmontanus 1 7.8 104
Atlantic sturgeon A. oxyrinchus 2 4.8 42
Shortnose sturgeon A. brevirostrum 3 4.6 14
Monkfish Lophius vomerinus 4 6.3 11
Pacific ocean perch Sebastes alutus 5 4.9 78
Pacific hake Merluccius asper 5 3.2 16
Thorny skate Amblyraja radiata 6 2.8 16
Northern pike Esox luscius 7 1.2 11
Source: (1) Rien and Beamesderfer (1994); (2) Stevenson and Secor (1999); (3) this
study; (4) Maartens et al. (1999); (5) Kimura and Lyons (1991); (6) Sulikowski et al.
(2005); (7) Laine et al. (1991)
65
and allows comparison among species (Rien and Beamesderfer 1994). Among
reported values for 8 other species, 4 had higher values for CV than my study.
Difficulty in estimating age based on fin rays is not uncommon (Kohlhurst et al. 1980,
Rien and Beamesderfer 1994, Nakamoto et al. 1995, Morrow et al. 1998, Fleming et al.
2003). Several authors have questioned the validity of estimating the age of sturgeons
based on the first pectoral fin ray, citing either low accuracy or an underestimation of
age based on fish of known age (Rien and Beamesderfer 1994, Paragamian and
Beamesderfer 2003, Hurley et al. 2004, Whiteman et al. 2004). Numerous studies of
North American sturgeons; however, have used this method; Beamish and McFarlane
(1983); Rossiter et al. (1995) for lake sturgeon A. fulvescens, Brennan and Caillet
(1991), Rien and Beamesderfer (1994), and Paragamian and Beamesderfer (2003) for
white sturgeon A. transmontanus, Stevenson and Secor (1999) for Atlantic sturgeon A.
oxyrinchus, and Hurley et al. (2004) for pallid sturgeon Scaphirhynchus albus. For
shortnose sturgeon, some studies using stocked fish or mark-recapture experiments
have attempted to validate the method (Fleming et al. 2003, Paragamian and
Beamesderfer 2003). Age validation was not possible in this study because known age
fish were not available. Despite these difficulties, age determinations from the marginal
fin ray method provide the best non-lethal means available to obtain information on
growth, recruitment, and mortality of shortnose sturgeon.
The current age structure of the shortnose sturgeon population of the Altamaha
River suggests that individuals from southern populations may not be as long-lived as
their northern counterparts. Fleming et al. (2003) reached a similar conclusion
regarding shortnose sturgeon on the Ogeechee River, where he estimated maximum
66
age to be less than 16 years. Shortnose sturgeon from southern populations, also
appear to grow much faster than in northern rivers (Dadswell et al. 1984), as evident
from both this study and that of Fleming et al. (2003). Dadswell et al. (1984) suggests
that faster growth of shortnose sturgeon in southern rivers may be offset by smaller
maximum size; however, age and growth data from both the Altamaha and Ogeechee
rivers seem to disprove this theory. Individuals larger than 1000 mm TL were routinely
captured in this study, with the largest measuring 1166 mm TL.
The current size structure of the Altamaha shortnose sturgeon population may be
interpreted in several different ways. First, the relatively high abundance of juveniles
suggests that the population is recovering. If this is the case, then the number of adults
captured should increase and the numbers of juveniles gradually decrease with time as
the population approaches equilibrium. On the other hand, the data could suggest that
mortality between the juvenile and adult stages is unusually high, possibly a result of
incidental mortality associated with the commercial shad fishery and the simultaneous
spawning migration of shortnose sturgeon. If this is the case, then the large 2001
cohort would become indistinguishable as they were harvested over the proceeding
years. Another possible interpretation is that adults are less constrained to the tidally
influenced portion of the river than are juveniles and therefore are underrepresented in
my catch data. Capture data from 2003-2005 indicates that at most, 40% of the
resident population are adults; a potential increase of 30% from results reported by
Flournoy et al. (1992).
The catch curves used to derive the mortality estimates of shortnose sturgeon in this
study were based on several assumptions: 1) mortality was constant across the
67
sampling periods, 2) recruitment was constant across the sampling periods, 3) that
mortality was equal among ages, and 4) that all age/size classes were adequately
represented within the sample. The age-frequency histogram indicates that the latter
two assumptions may not have been met. A potential source of bias in the catch curve
data was the low sample size of fish older than age-10. A larger sample size of adults
might have resulted in a lower mortality estimate. By representing older cohorts, a more
representative distribution of adult size classes relative to age could have elevated the
right side of the catch curve, thereby decreasing the angle of the regression line and
subsequent mortality estimate. While I can not discount the possibility that adult
shortnose sturgeon were inhabiting areas where I did not sample, the lack of samples of
older fish was likely a result of a lack of those individuals within the population rather
than a result of sampling bias. Other large fish captured during the course of this study,
as well as reported preferred habitat in other studies on southern rivers (Flournoy et al.
1992, Weber 1996, Collins et al. 2001) is further evidence that adults are not as
abundant in the Altamaha river population. In 2005, an unusually large 2001 cohort
began to affect the mortality rate estimates by artificially inflating the portion of the curve
represented by age-4 fish. The violation of the assumption of constant recruitment may
have resulted in a higher mortality estimate in 2005. When age-4 fish were excluded
from the catch curve analysis the mortality rate slightly decreased from 0.384 to 0.344.
Given that shortnose sturgeon are long-lived and that the population has been
protected for over 30 years, I believe that adults should have been found in higher
numbers. Telemetry data indicated that although adults do make long spawning runs
into the upper reaches of the river where I did not sample, they did not spend much time
68
there outside of the spawning season, presumably because they prefer the tidally
influenced habitats of the lower river. Similar behavior for shortnose sturgeon also has
been documented in other studies in southern rivers (Collins and Smith 1993, Rogers
and Weber 1994, Moser and Ross 1995, Weber et al. 1998). Therefore, I believe my
data are reasonably representative of the current age-structure and that a particularly
strong year class may be the reason for the relatively high abundance of juveniles
observed in this study. A total annual mortality rate of about 30% is the highest ever
reported for any shortnose sturgeon population. In contrast, Secor and Woodland
(2005) reported a mortality rate of 22% for the Hudson River population.
Gear Selectivity
Several methods are used currently to sample sturgeon populations. Passive
entanglement gears often are the most effective, but they tend to be size selective
depending on the mesh sizes used and how they are deployed (Hamley 1975). This
bias is minimized most often by fishing “gangs” of different mesh sizes or by using
experimental nets that have been constructed using several mesh sizes (Hamley, 1975,
Kieffer and Kynard 1993, Foster and Clugston 1997, Haxton 2002). Direct selectivity
estimates can be derived through mark-recapture studies if the size distribution of the
population is known (Hamley 1975). In this study, however, I employed an indirect
method for estimating size selectivity because my recapture rate was low (9.6%) and
because the size distribution of the population was unknown. The indirect method that I
used required 2 assumptions. The first is that the degree of size selectivity depends on
the geometry of a fish relative to a specific mesh size as described by Baranov (1948).
69
Hence, the width of any given selectivity curve (i.e., the size range of fish captured by
any single mesh size) is proportional to the mesh size being fished (Hamley 1975). The
second assumption of the indirect method is that each mesh size used is equally
exposed to all fishes (Hovgård 1996). Because I used a variety of mesh sizes in
standardized nets that were deployed with equal effort in each period, this assumption
was met (Hovgård and Lassen 2000).
Because the population size-structure was unknown I standardized each selectivity
curve to 1 (Hamley 1975). However, this distorted the selectivity curve for each net,
especially for the 10.2 cm gill net. A total of 52 individuals were captured in this net,
and the length frequency histogram had 3 peaks occurring at 200 mm intervals, with
each peak comprised of only 5 fish. The effective range of fish sizes captured by the
10.2 cm gill net may be overstated by the curve. Because of the small number of
captures in this mesh size, the modes at 375 mm and again at 850 mm more likely
exaggerates the effective size range of fish captured by this mesh size. Given these
problems I used the combined catch of trammel nets and experimental gill nets in my
analysis of the population size-structure.
In comparing size distributions shortnose sturgeon captured in each gear type, I
found that trammel nets and experimental gill nets caught a slightly wider size
distribution of fish than did the 10.2 cm and 15.2 cm gill nets when fished in tandem.
The implications of this analysis are that single-mesh gill nets can be used to efficiently
sample specific size classes of sturgeon (e.g., juvenile cohorts), whereas experimental
gill nets and trammel nets provide a much more representative sample of the
population. Although this comparison showed that each gear may impart some size
70
selective bias to my catch data, the effect was probably not biologically significant.
Furthermore, the results suggest that using both trammel nets and experimental gill nets
may provide the best representation of the true population size.
Analysis of CPUE data from each gear type showed that trammel nets are more
efficient than gill nets. In 2005, I attempted to compensate for the disparity in the size
distribution between my trammel and single-mesh gill nets by fishing experimental gill
nets constructed from 3 different mesh sizes, but twice the number of these nets were
required to catch the same number of shortnose sturgeon as were caught in a single
trammel net. Furthermore, the smallest mesh in the experimental gill nets increased
bycatch to levels similar to those of trammel nets. This finding is noteworthy because a
primary argument against the use of trammel nets in fisheries sampling is that gill nets
are thought to produce less bycatch. Because site specific variables such as current
velocity and bottom structure may affect sampling efficiency of both gill and trammel
nets, sturgeon researchers should evaluate both types of gear in specific study areas
before implementing a long term sampling program.
Habitat and Seasonal Movements
Habitat Use in Summer
During summer months when water temperatures exceeded 27 oC, shortnose
sturgeon were not captured below the fresh-saltwater interface. All captures during this
time occurred in areas deeper than the surrounding river stretches, with a maximum
depth of 12.8 m. Similar habitats were used by shortnose sturgeon during the summer
on the Savannah River (Collins et al. 2001), Ogeechee River (Weber 1996) and in a
71
previous studies on the Altamaha River (Flournoy et al. 1992). In the latter study, the
authors postulated that deepwater habitats may contain coolwater springs that provide
important thermal refugia for shortnose sturgeon during the summer months. In this
study, however, we could detect no differences in bottom temperatures in the area
between rkm 22-23. Hence, I believe that the summer aggregation of shortnose
sturgeon in this area likely resulted from the combined effects of increasing temperature
and the progressive intrusion of the salt wedge that essentially “drove” shortnose
upstream to freshwater habitats. In fact, I observed the upstream movement of several
tagged fish into this area coincident with the progressive intrusion of the salt wedge.
Hence, I believe that summer aggregations of shortnose sturgeon at rkm 22-23 occur
simply because this area contains the first deepwater habitat above the salt wedge that
forms during summer, especially during periods of low discharge. Once the fish arrived
at this site, most remained there until water temperatures began to decline in late
summer or fall.
During the summer of 2005, water temperatures were 1-2 oC degrees cooler, and
discharge was much higher than in summer of 2004. Increased rainfall during the
summer months largely diminished the upstream intrusion of salt water in that year.
Not surprisingly, shortnose sturgeon were captured throughout the summer as far
downstream as rkm 15.4 and capture rates at rkm 22-23 were much lower than in
summer of 2004. Movements of radio-transmittered fish also showed that shortnose
moved freely throughout the lower portion of the river during summer 2005.
72
Habitat use in Fall, Winter, and Spring
Adult shortnose sturgeon actively moved throughout the river and estuary during the
cool months of the year. Areas of summer aggregations were abandoned by early fall
and individuals became scattered throughout the estuary. Captures of shortnose
sturgeon occurred over a variety of substrates (e.g., mud and sand) and at salinities of
0.0 - 26.1‰. Catches of more than 5 individuals were rarely obtained from any single
site, except in late January when 11 adults were captured at rkm 7-8. Subsequent
laparoscopic examination of these individuals revealed the presence of both gravid
females and ripe males. This aggregation of pre-spawning adults in the lower river
during late winter suggests that shortnose sturgeon may gather in specific estuarine
staging areas just prior to making their upstream spawning migrations. Although this
behavior has been documented previously for shortnose sturgeon on the Hudson River
(Bain et al. 2000), it has not been observed in a southern population and it has never
been found to occur in an estuarine environment. The importance of this behavior is
unknown; however, it may help to synchronize spawning migrations and the
reproductive cycles of males and females.
Spawning Habitat
Temperature appears to be a key environmental factor governing the timing of
spawning (Dadswell et al. 1984). Numerous studies from throughout the range of
shortnose sturgeon have shown that spawning occurs at temperatures between 9o and
12 oC (Meehan 1910, Heidt and Gilbert, 1978, Dadswell 1979, Taubert 1980a, Buckley
and Kynard 1981). This study documents the first confirmed spawn by shortnose
73
sturgeon in the Altamaha River. While water temperatures in the Altamaha at this time
were consistent with preferred spawning temperatures of shortnose sturgeon reported
in other studies, spawning substrates in the Altamaha were different than those
previously described. Substrates of both confirmed and suspected spawning sites of
shortnose sturgeon in this study typically consisted of course sand. Although Rogers
and Weber (1994) suggested that shortnose may spawn at nearby sites containing
large rock and gravel substrates, these areas were not found in this study. Although I
sampled substrates in several sites in the headwater reaches of the river, it is possible
that shortnose may spawn at several specific sites in this area of the river. Further
studies are needed to identify specific spawning habitats of shortnose sturgeon in this
area of the Altamaha System, including the lower reaches of both the Ocmulgee and
Oconee Rivers.
In northern rivers, spawning migrations appear to begin in late fall with individuals
overwintering just downstream of the spawning grounds (Dadswell 1979, Buckley
1982). In hypothesizing that this two-step migration pattern may also be used by
shortnose sturgeon in the Altamaha, Rogers and Weber (1994) suggested that
spawning adults may require additional time to accumulate the energy reserves needed
to make long upstream migrations. In this study, I did find congregations of pre-
spawning adults in the lower estuary during early winter, but their upstream migrations
were rapid and continuous once they began. Although two-step spawning migrations
can not be ruled out for Altamaha River shortnose sturgeon, this pattern was not
observed in this study or in previous studies of shortnose sturgeon in other southern
rivers (Hall et al. 1991, Collins and Smith 1993, Weber 1996).
74
Management Implications
This study has documented the largest known shortnose sturgeon population in the
southern United States. Based on the criteria specified by the Shortnose Sturgeon
Recovery Team (NMFS 1998), the Altamaha River population may be a possible
candidate for future delisting from the Federal Endangered Species List. Under the
terms of the Endangered Species Act, delisting of an endangered species (or a
population of that species) can be considered only when sufficient evidence exists that
the population is large enough to prevent extinction and that the loss of genetic diversity
is unlikely (NMFS 1998). Before any such action is considered, however, several
additional years of monitoring should be completed to ensure that the Altamaha
population maintains or increases its current abundance and complex age structure.
Although the results of this study have provided important new information about the
adult population of shortnose sturgeon in the Altamaha system, additional studies are
still needed to better understand recruitment mechanisms and habitat requirements of
juveniles. Furthermore, the minimum population size required to protect genetic
diversity has not been determined for any shortnose sturgeon population. Given that
southern populations exhibit a shorter life expectancy and are subject to greater
environmental stochasticity than their northern counterparts, delisting of any southern
population is probably premature at this time.
In Georgia, the commercial fishery for American shad operates on several coastal
rivers, including the Altamaha, from 1 January until the end of March. Because this
fishery coincides with annual spawning migrations of shortnose sturgeon, the incidental
75
take of spawning shortnose sturgeon could be substantial, especially where fishermen
employ bottom set gill nets. Previous studies by Collins et al. (1996) showed that
mortality of shortnose sturgeon captured in set gill nets was 16%; however, similar
studies in the Altamaha River have not been attempted
Illegal harvest of shortnose sturgeon is also a potential threat to the shortnose
sturgeon on the Altamaha River. Kynard (1997) reported that illegal harvest of
shortnose sturgeon in southern rivers was widespread and that 80% of radio-tagged fish
in the Cooper River (S.C.) were taken by poachers. Given that shortnose sturgeon
pass through many remote reaches of the Altamaha River during their annual spawning
migrations, additional studies are needed to quantify illegal harvest and to assess this
potential threat.
Summary
Data collected in this study shows that the Altamaha River population of shortnose
sturgeon is substantially larger than previously thought; however, the estimated
mortality rate raises concerns that undocumented sources of mortality may be limiting
further population growth. Growth rates of individual fish are higher than those reported
for northern populations but maximum lifespan is substantially shorter, although this
could be a result of anthropogenic factors. Unlike shortnose sturgeon in northern rivers,
adults exhibit one-step spawning migrations in the Altamaha from early January through
March. Spawning migrations, both upstream and downstream, are rapid and extensive
with most fish moving at least 200 rkm in each direction. After spawning, most adults
return to the brackish, or tidally influenced freshwater of the lower Altamaha. As
76
summer approaches and temperatures rise above 27 oC, the fish seek deep freshwater
habitats just upstream of the salt-freshwater interface. During periods of high water
temperature and low discharge, activity is reduced and the fish form large aggregations
in these deepwater areas
Based on the information obtained in this study, the shortnose sturgeon population of
the Altamaha River appears to be in the midst of a robust recovery. Although the
annual mortality rate of the population was surprisingly high, ~30%, the population
appears to be increasing as indicated by the relatively high abundance of juveniles
present in the catch data from both year of this study. Nonetheless, additional studies
are needed to identify and quantify potential threats from both legal and illegal fishing,
and to better understand factors that affect the abundance and survival of juvenile
shortnose sturgeon in this system.
77
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