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Marine Turtle Newsletter 55:2-5, © 2001

Post-Hatchling Loggerhead Turtles Eat Insects in Sargassum Community

James I. Richardson¹and Phil McGillivary^2
1Institute of Ecology, University of Georgia, Athens, Georgia 30602 USA;
²NOAA, NOS Ocean Applications Group, Monterey, California 93942-5005 USA

On 22 August 1981 at 1900 hrs, the R/V Cape Florida stopped to make a hydrographic station at the west wall of the Gulf Stream (29°55'N, 30°00'W), 93 km east of St. Augustine, Florida. Water depth was 100 m. A convergence along the front, extending over the horizon in both directions, had concentrated rafts of Sargasssum weed into patches 5-50 m² in size. More than half of the patches contained one to three or more post-hatchling stage loggerhead sea turtles (Caretta caretta). As the vessel drifted through about 30 such patches, we observed at least 20 small loggerheads. The turtles were resting quietly on the floating Sargassum with their heads near the surface of the water. When approached by the vessel, they dived beneath the Sargassum and swam vigorously. Because we were able to examine only a small area (<5000 m² of the frontal convergence), we cannot speculate on the extent of this population of young turtles. However, along the Gulf Stream front between Florida and the Carolinas there is the equivalent of several km² of this Sargassum habitat.

Two dead turtles (45 and 47 mm carapace length) were collected floating in the Sargassum. Both had bites around the neck and eyes, probably from the orange file fish (Aluterus schopfi), but there was no obvious cause of death. Their guts contained four categories of material (Table 1). Macroalgae and marine invertebrates associated with the Gulf Stream and the Sargassum rafts accounted for roughly half of the items. A third major category consisted of terrestrial insects and the fourth of indigestible items, including string fibers, a feather, and tar balls. The predominate size of ingested food items (1-5 mm) indicates a preference and lower limit for prey size selection by neonate loggerheads. A yolk about 10 mm diameter was present in both animals.

There has been speculation about what food is available to post-hatchling turtles in the open ocean (Carr 1967; Frick 1976) and the ability of the turtle to reach food after it leaves its natal beach (Kraemer and Bennett 1981). Both turtles reported here contained a substantial reserve of yolk while actively feeding in the open ocean. The presence of insects in the gut suggests a source of food not previously considered. Insects are regularly collected in the air at heights up to 5,000 m, and insects like those ingested by the turtles have been caught in flight far out to sea, carried there passively by moving air masses such as high pressure systems (Johnson 1969).

Sargassum rafts, while providing camouflage and refuge, contain potential food in greater abundance than does the surrounding water. The affinity of post-hatchling turtles for convergence lines, both here and elsewhere (Carr and Meylan 1980; Carr 1987), may indicate a requirement by these animals to capitalize on food sources in addition to what is produced by the Sargassum community (Carr 1986). Aerial insect plankton, settling on the ocean's surface, would accumulate in convergencies and supplement the food in the Sargassum. Furthermore, the planktonic community beneath the Sargassum in the region of the Gulf Stream front is substantially more productive than that of either the core of the Gulf Stream or the water of the outer continental shelf. This higher planktonic productivity results from intrusions of nitrate-rich North Atlantic Central water from the west wall of the Gulf Stream up to within 50 m or less of the surface along the edge of the continental shelf (Atkinson 1977; Yoder et al. 1981). August is usually a period of prevailing southerly winds which enhance the upward and westward movement of the nutrient-rich water from the west wall of the Gulf Stream by creating an Ekman effect. August is also a period of more consistently enhanced planktonic productivity on the outer continental shelf of northern Florida (Atkinson et al. 1978). These conditions increase the amount and diversity of food available to turtles floating in the Sargassum.

Table 1. Identifiable items removed from the gut of two post-hatchling loggerhead sea turtles collected 93 km east of St. Augustine, Florida. Particle sizes were measured to the nearest 0.5 mm. S = found in stomach; I = found in intestine; P = "present". Size refers to particle size and/or body length of prey; number in parentheses indicates sample.