Diving behavior and energetic strategy of leatherback sea turtles during internesting intervals on St. Croix, US Virgin Islands

Female leatherback sea turtles (Dermochelys coriacea; hereafter referred to as leatherbacks) nesting on St. Croix in the U.S. Virgin Islands in the spring lay 5-6 clutches, with an ~10-day internesting interval at sea between clutches. Although leatherbacks feed on gelatinous zooplankton during the winter, foraging and prey preference during the internesting interval has remained speculative. The goal of this study was to use miniature, animal -borne video and data recorders and satellite telemetry to monitor the diving behavior and movements of leatherbacks during the internesting interval on St. Croix. Our hypothesis was that foraging is rare and opportunistic during the internesting interval. Our results show that leatherbacks make two types of dives: Shallow Transit (ST) and Deep Transit (DT). ST dives were short (1.14 min), shallow (3.0 m). and associated with slow (0.6 m sec(-1)), subsurface swimming to reduce drag with no periods of gliding descent. DT dives were longer (13.42 min), deeper (84.0 m), and covered a 10-fold greater distance (446 m) than ST dives but at the same slow speed. The deepest and longest DT dive was 531 m in depth and 28.9 min in duration, but only 4% of DT dives were deeper than 200 m. Leatherbacks spent 94% of the time at sea making ST and DT dives with short resting periods at the surface. Six species of gelatinous prey were identified from recorded video, but feeding was rare and opportunistic indicating fasting during the nesting season. The estimated energy expended while at sea for five internesting trips over six weeks was equivalent to the catabolism of 28.2 kg of fat. An additional 12 kg of fat was needed for egg production, resulting in a total energy requirement equivalent to 40.2 kg of fat (~12% of body mass). Our results indicate that the purpose of the internesting interval is not foraging and that leatherbacks are capital breeders, which fast during the nesting season after acquiring energy reserves (i.e., fat) during the non-breeding season at higher latitudes where foraging effort may be more efficient.