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) loggerheads 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, longline fishing 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” 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 approximately 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 29°25′04″N, long 85°05′44″W, 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 lherminieri), royal terns (Thalasseus maximus, Fig. 1C), and sandwich terns (Thalasseus sandvicensis, Fig. 1D). As we approached 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 a turtle 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 congregating 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.

View Figure

• Download as Powerpoint
• Download Figure

View Figure

• Download as Powerpoint
• Download Figure

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 dead when 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 upon as 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 loggerheads 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 (Hippocampus 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 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 loggerhead diet samples.

It remains unclear how commonly prey-fish school around loggerheads, but we point to additional observations 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 conclusions 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.