a loggerhead sea turtle (caretta caretta) preying on fish within a mixed species feeding aggregation...
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A Loggerhead Sea Turtle (Caretta caretta) Preying on Fish Within a Mixed-Species
Feeding Aggregation
Author(s): Shigetomo Hirama and Blair Witherington
Source: Chelonian Conservation and Biology, 11(2):261-265. 2012.
Published By: Chelonian Research Foundation
DOI: http://dx.doi.org/10.2744/CCB-0918a.1
URL: http://www.bioone.org/doi/full/10.2744/CCB-0918a.1
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8/12/2019 A Loggerhead Sea Turtle (Caretta Caretta) Preying on Fish Within a Mixed Species Feeding Aggregation 2012 Chel…
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Chelonian Conservation and Biology, 2012, 11(2): 261–265g 2012 Chelonian Research Foundation
A Loggerhead Sea Turtle (Caretta caretta)
Preying on Fish Within a Mixed-Species
Feeding Aggregation
SHIGETOMO HIRAMA1
AND BLAIR WITHERINGTON1
1Florida Fish and Wildlife Conservation Commission, 9700 South
A1A, Melbourne Beach, Florida 32951 USA [[email protected];
ABSTRACT. – Loggerhead sea turtles (Caretta caretta)
feed predominantly on marine crustaceans, mollusks,
and other hard-shelled macroinvertebrates. In this
note, we report on a subadult loggerhead observed to
be feeding on live fish swarming around the turtle in a
prey-fish ball. The observation suggests that logger-
heads occasionally prey upon certain pelagic fish
species other than those encountered as carrion or
fishing bait, and may explain accounts of turtles
impaled by billfish (Istiophoridae) rostra.
Loggerhead sea turtles (Caretta caretta) inhabit
tropical, subtropical, and temperate waters worldwide.
As they become large immature and adult turtles,
loggerheads occupy predominantly benthic habitats in
neritic waters (Bolten 2003). In Florida (USA) logger-
heads forage within coastal inlets (Butler et al. 1987),
lagoons (Ehrhart et al. 2007), bays (Lutcavage and
Musick 1985; Schroeder and Foley 1995), and on
deepwater (.
10-m–deep) offshore reefs (Girard et al.2009) in the Gulf of Mexico and Atlantic Ocean. Due to
difficulties associated with offshore research, loggerhead
foraging in habitats distant from land is not well studied
relative to foraging in coastal habitats. When loggerhead
diet has been described in the literature, the information
has come principally from analyses of gut contents in
shore-stranded turtles.
Gut contents of stranded loggerheads have included
predominantly marine crustaceans, mollusks, and other
hard-shelled macroinvertebrates (Lutcavage and Musick
1985; Dodd 1988; Ruckdeschel and Shoop 1988; Burke
and Standora 1993; Plotkin et al. 1993; Lazar et al. 2010).
In addition to these naturally occurring prey items, longlinefishing baits (Revelles et al. 2007) and discarded fish
(White 2004) have also been recorded from loggerhead gut
contents.
Given their apparent preference for slow-moving
prey and their limited capacity for rapid movement (S.
Hirama and B. Witherington, pers. obs.), loggerheads
would not be expected to eat live, fast-moving pelagic
fishes. However, many pelagic fishes form compact, ball-
shaped schools that may change their vulnerability to
predation (Pitcher and Parrish 1993). A prey-fish ‘‘ball’’
NOTES AND FIELD REPORTS 261
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typically contains small fish close to the water’s surface
(Marchal and Lebourges 1996). The schooling of these
prey fish is thought to confuse predators and increase prey
survivorship (Partridge 1982; Milinski 1984; Magurran and
Pitcher 1987). In the present article, we describe a
loggerhead sea turtle that was a focal point
of a prey-fish ball, allowing the turtle to feed on the
constituent fish.Observation Note. — On 30 July 2009, at approxi-
mately 1900 hrs, we were returning to port after
conducting offshore vessel transects for oceanic juvenile
sea turtles in the northern Gulf of Mexico. Approximately
26 km from shore, lat 29u259040N, long 85u059440W,
in water 18 m deep, we observed a large immature
loggerhead (approximately 70-cm straight carapace
length) at the surface (Fig. 1). Above the turtle, seabirds
in a mixed-species flock were diving and feeding on fish.
The seabird species included black terns (Chlidonias
niger , Fig. 1D), Audubon’s shearwaters (Puffinus lhermi-
nieri), royal terns (Thalasseus maximus, Fig. 1C), and
sandwich terns (Thalasseus sandvicensis, Fig. 1D). As weapproached the feeding site, we observed that the seabirds
were feeding on fish swimming out from water and onto
the loggerhead’s partially exposed carapace (Fig. 1A).
We recorded a series of 114 images with a Canon single
lens reflex camera with a 400-mm focal length, f5.6
telephoto lens. Approximately 20 m from the turtle, we
entered the water to observe the loggerhead from beneath
the surface. Upon reaching the turtle, we recorded 82
images with a Canon Power Shot A710IS camera within
an underwater housing. From our observations and
recorded images, we noted that the school of prey fish
was in a compact spherical ball similar to that described
in the literature (Clua and Grosvalet 2001), except that aturtle was at its center (Fig. 2). We observed 6 species of
fish near the loggerhead. The prey fish aggregation
consisted of 3 species, round scad ( Decapterus punctatus;
Carangidae) and sardines ( Harengula jaguana and
Sardinella aurita; Clupeidae). We observed that predator
fish, little tunnies (Euthynnus alletteratus), were congre-
gating and striking from below in order to feed on the
prey fish at the perimeter of the ball. We also observed
five sharksuckers (Echeneis sp.) circling and occasionally
attaching to the turtle, and lesser amberjacks (Seriola
fasciata) also maneuvering tightly around the turtle. The
loggerhead repeatedly made lateral head movements with
its mouth held open. On at least 3 occasions, the turtle
closed its mouth on a fish that had swum between its jaws,
allowing the turtle to crush the fish and swallow it. These
consumed fish could not be specifically identified, but
were one of the 3 prey species, Decapterus punctatus,
Harengula jaguana, and Sardinella aurita.
Discussion. — Schools of prey fish decrease their
surface-area-to-volume ratio by forming a ball swarm,
and further limit their exposure to fish predators by
swarming near the water’s surface. However, this surface
concentration made the prey fish we observed susceptible
to predation by seabirds. Seabirds commonly associate
with dolphins and tunas, and in oceanic waters these birds
may rely on prey fish available at the surface because of
these subsurface predators (Ashmole and Ashmole 1967;
Au and Pitman 1986). We hypothesize that the prey-fish
school we observed received additional antipredator
advantages in being sheltered by the loggerhead. The
presence of the turtle likely prevented the approach of predators from the side of the school occupied by the
turtle, and when at the center of the ball, the loggerhead
likely prevented predators from striking through the
school. In our observed case, although the prey fish
may have benefitted from their proximity to the turtle, a
trade-off was predation by the turtle.
Direct observations of loggerheads feeding on food
items in the wild are rare. Although there are some first-
hand records of interactions between loggerheads and
their prey (e.g., Sauls and Thompson 1994), most
information on loggerhead foraging comes from ex post
facto examination of ingested items. One might infer that
most fish found in loggerhead digestive tracts were deadwhen eaten, given that most fish can easily escape the
sluggish movements limiting a loggerhead’s predatory
capacity (Shoop and Ruckdeschel 1982; Lutcavage and
Musick 1985). For example, Plotkin et al. (1993) inferred
that fish parts found in the digestive tract of stranded
loggerheads were from fish eaten as carrion because the
guts also contained large numbers of the scavenging
gastropod Nassarius acutus. Seney and Musick (2007)
found Atlantic menhaden ( Brevoortia tyrannus), Atlantic
croaker ( Micropogonias undulatus), striped bass ( Morone
saxatilis), bluefish (Pomatomus saltatrix), and seatrout
(Cynoscion sp.) in digestive tracts of loggerheads. These
authors reasoned that most of these fish had been fed uponas they were either alive or freshly dead, having been
restrained or killed by commercial fishing nets. A low
frequency of scavenging mud snails in the gut contents
supported this observation. In contrast, Revelles et al.
(2007) were unable to explain the origins of clupeid fish,
which were the highest-frequency item in digestive tracts
from 19 dead loggerheads in the western Mediterranean.
Tomas et al. (2001) found that teleost fishes were the
principal constituent within gut contents of 54 logger-
heads captured by trawls in the western Mediterranean.
The ingested fish included 13 species, including two
clupeids (Sardina pilchardusa and Alosa alosa). The
authors concluded that the clupeids had been discarded
bycatch, but that the short-snouted seahorse ( Hippocam-
pus hippocampus), a slower-moving benthic fish also
found in some of the turtle stomachs, may have been
consumed alive (Tomas et al. 2001).
One question about loggerhead feeding behavior
concerns the extent to which they are opportunistic
feeders or are selective of targeted food items (Dodd
1988). Although we do not present a sample sufficiently
large to answer this question, we do offer an observation
of opportunistic feeding on live prey fish in which a
262 CHELONIAN CONSERVATION AND BIOLOGY, Volume 11, Number 2 – 2012
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Figure 1. A surfacing subadult Caretta caretta, with Decapterus punctatus, Harengula jaguana, Sardinella aurita, and Seriola fasciata swimming close to the turtle’s carapace (A, B), and Thalasseus maximus (C), Thalasseus sandvicensis, and Chlidonias niger (D) flying over prey fish.
Figure 2. Underwater images of a subadult Caretta caretta with Decapterus punctatus, Harengula jaguana, Sardinella aurita,Echeneis sp., and Euthynnus alletteratus.
NOTES AND FIELD REPORTS 263
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loggerhead exhibited predatory behavior that would not
seem to be effective in feeding on its more commonly
encountered benthic prey species. In the loggerhead we
observed, success in capturing prey fish seemed to be
linked to the swarming tactic used by the prey fish, a
tactic presumed to reduce predation by larger fish. We
propose prey-ball feeding as an explanation for the
occurrence of tightly schooling pelagic fish in loggerheaddiet samples.
It remains unclear how commonly prey-fish school
around loggerheads, but we point to additional observa-
tions that could be explained by these events, such as
interactions between billfishes (Istiophoridae) and sea
turtles. Frazier et al. (1994) describe four confirmed
cases of turtles (loggerhead; olive ridley, Lepidochelys
olivacea; leatherback, Dermochelys coriacea; green
turtle, Chelonia mydas) being impaled by billfish rostra.
In their interpretation, the authors concluded that the
interactions may have occurred incidentally due to prey
fish (the billfishes’ target species) congregating around
the turtles.We suggest two areas of caution in drawing conclu-
sions from fish found in the gut contents of loggerhead sea
turtles. One is that certain pelagic fish species may
occasionally be preyed upon while alive and are not
always encountered as carrion or fishing bait. A second
caution is that researchers are probably correct in
identifying other fishes found in loggerhead diet samples
to be the result of net-catch, discarded bycatch, or fishing
bait, especially if there is additional supporting evidence.
ACKNOWLEDGMENTS
The observation was made during work funded by
grant NA09NMF4720040 from the NOAA, National
Marine Fisheries Service (NMFS), and by Florida’s
Marine Resources Conservation Trust Fund. Work was
carried out under a NMFS Permit for Scientific Purposes
under the Endangered Species Act (permit 14726). The
authors are grateful to D. Adams and E. Matheson for their
comments and assistance with fish identification. We
thank J. Boyett, B. Crowder, A. Foley, R. Hardy, A.
Meylan, and T. O’Meara for providing comments on the
manuscript.
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Received : 26 October 2011
Revised and Accepted : 29 December 2011
Handling Editor : Jeffrey A. Seminoff
NOTES AND FIELD REPORTS 265