Project Title: Trophic Ecology of Kemp's Ridley Turtles in the Ten Thousand Islands Archipelago
Project Manager: Dr. Jeff Schmid
Organization: Conservancy of Southwest Florida (Non-Profit Organization)
Grant Amount: $32,795.00
Completion Date: 2016-06-13
Summary: This research will investigate the trophic status and foraging ecology of Kemp’s ridley turtles inhabiting the Ten Thousand Islands archipelago. Earlier studies found that turtles in this region preyed on tunicates which have not been reported in other diet analyses. Recent stable isotope analyses of Kemp’s ridleys, their crab prey, and components of their foraging habitat characterized the food web in the Charlotte Harbor estuary. Similar studies are needed to elucidate ecological aspects of Kemp’s ridleys in the Ten Thousand Islands that have not been previously investigated and allow for a comparison of the trophic ecology of turtles inhabiting different estuarine systems. Kemp’s ridley turtles will be collected from an area of aggregation via strike netting and will be transported to a shoreside facility for fecal sample collection. Skin and plasma samples will be collected and analyzed for stable isotope composition. Potential prey and habitat components in the study area will also be collected for stable isotope analysis. The objectives of the study are to determine the stable carbon, nitrogen, and sulfur isotopic signatures of skin and plasma samples from Kemp’s ridleys, to further document the food items consumed by turtles in this foraging area, to compare the isotopic composition of turtles with those of food items and habitat components, and to compare the resulting food web in the Ten Thousand Islands with that of the Charlotte Harbor estuary.
Results: In contrast to the diet studies of Witzell and Schmid (2005), Kemp’s ridley turtles in the Ten Thousand Islands have shifted their affinity for the solitary tunicate Molgula occidentalis to that of sponges and a colonial tunicate. To our knowledge, there are no published accounts of sponge in the diet of Kemp’s ridley turtles. The sponge taxa consumed in the present study were tentatively identified as Halichondria sp. and Geodia sp. pending expert identification of voucher specimens. Sponges are the primary prey for hawksbill turtles (Eretmochelys imbricata; Meylan, 1984) and have been identified as a prey item for green (Chelonia mydas; Bjorndal, 1990 and references therein) and, to a lesser extent, loggerhead turtles (Caretta caretta; Dodd, 1988). Geodia sp. was one of the most common sponges in the stomach contents of hawksbills from the Caribbean basin and one of the turtles had consumed sponge from the Family Halichondriidae identified as Hymeniacidon sp. (Meylan, 1984). Among others, the halichondrid sponges Hymeniacidon rugosus and H. sinapium were identified in fecal samples from green turtles in the Gulf of California (Seminoff et al., 2002). Analyses of the stable isotope data presented herein are preliminary and sample collections are continuing in the Ten Thousand Islands study area. More data are needed to elucidate seasonal variability in the isotopic signatures of Kemp’s ridleys, their prey, and the components of their foraging habitat. Future applications of these data will include stable isotope mixing models to calculate estimates of prey contribution to the Kemp’s ridley isotopic signatures and compare these estimates to that observed in diet studies. The preliminary trophic model for this estuarine complex was consistent with the results of the corresponding turtle diet studies. Kemp’s ridley and decapod crustaceans occupied a similar trophic level which indicates crabs were not major food items for turtles in this region. Filter-feeding invertebrates (sponges and tunicates) were the primary prey for Kemp’s ridleys and occurred at a lower trophic level. The 2-3% enrichment of N from prey to predator was in general agreement with the commonly assumed increase of 3.4% per trophic level. Mangroves occupied the lowest trophic level, as expected for a primary producer, and their isotopic signature may be indicative of their leaf litter as the source of nutrients in this estuarine system. However, estuarine food webs are complex and often have multiple inputs of organic matter that were not sampled in the current study (e.g., terrestrial plants, marsh plants, and phytoplankton; Michner and Kaufman, 2007).