Project Title: Diet characterization of green, loggerhead, and Kempís ridley turtles in Florida
Project Manager: Dr. Erin Seney
Organization: University of Central Florida (Research and Educational Institute)
Grant Amount: $6,291.00
Completion Date: 2018-07-18
Summary: The diet of an organism provides valuable information on its habitat, ecosystem role, and potential for interaction or competition with human activities. While the sea turtle diets are understood generally, specific dietary information is limited to very few foraging areas. Strandings represent a relatively untapped opportunity to quantitatively assess diets of Floridaís three most common sea turtle species, the green turtle (Chelonia mydas), loggerhead (Caretta caretta), and Kempís ridley (Lepidochelys kempii). This project will collect gastrointestinal contents from stranded turtles and use established data collection and analysis techniques to: (1) characterize green turtle, loggerhead, and Kempís ridley diet in Florida; (2) examine spatial, temporal, and size-based trends and interspecific overlap in diet; and (3) as possible, complement the stateís examinations of anthropogenic interactions. Gastrointestinal tract contents will be collected during semi-annual Florida Fish and Wildlife Conservation Commission necropsy sessions and subsequently be sieved, sorted, and identified to the lowest possible taxonomic level. Percent occurrence, dry weight, and number will be calculated for each food type in loggerhead and Kempís ridley samples, whereas percent frequency, weight, and volume will be calculated for green turtles. Percent index of relative importance, a compound diet metric that combines frequency, weight, and count data or frequency and volume data, will be used to examine contributions of different food types to each speciesí diet. Spatial, seasonal, and size-based analyses will provide valuable information for sea turtle management and conservation, including food and habitat requirements for each species. Results will also enable comparisons to live sea turtle diet studies and may be used to complement other ongoing research and examine any interaction or competition with fisheries.
Results: The purchases, travel, and data collection proposed for the project have all been completed. Large equipment (freezer, drying oven), supplies, and analysis software were purchased, and three sampling trips were completed (September 2016, April 2017, and November 2017). There were delays due to the timing of state necropsy sessions vs. the grant, as well as cancellation of one session due to Hurricane Irma. Loggerhead (Caretta caretta) and Kempís ridley (Lepidochelys kempii) samples from all sessions have been sorted, dried, and weighed, and preliminary data have been collected from the green turtle (Chelonia mydas) samples from all trips. Dr. Seney and one volunteer traveled to St. Petersburg, Florida to collect diet samples during the 17-18 September 2016, 29-30 April 2017, and 18-19 November 2017 Florida Fish and Wildlife Conservation Commission (FWC) sea turtle necropsy sessions. Whole gastrointestinal (GI) samples were collected from a total of 148 sea turtles across 3 species: 66 green turtles, 46 loggerheads, and 36 Kempís ridleys. These turtles comprised all Atlantic coast green turtle and statewide loggerhead and Kempís ridley carcasses necropsied that had both (1) a complete GI tract and (2) any non-fluid GI contents. Travel to the third session (November 2017) also facilitated collection of swab samples for a complementary (UCF-funded) genomic characterization of stranded sea turtle microbiome. Results indicated Kempís Ridley had a strong preference for crabs, which were consumed by 45 of the 48 Kempís ridleys (94%). Commonly-observed (>20% occurrence) walking crabs included flame box crab (Calappa flammea), spider crab (Libinia spp.), calico box crab (Hepatus epheliticus), purse crab (Perseophona mediterranea), and stone crab (Menippe spp.). Swimming crabs included blue crab (Callinectes sapidus) and Florida lady crab (Ovalipes floridanus). Some size-based differences were detected, with blue crab disproportionately consumed by individuals <45 cm SCL, and flame box crab, calico box crab, stone crab, and lady crab favored by larger ridleys. Horseshoe crab (Limulus polyphemus), bony fishes, and large (>2 cm) gastropods were consumed by relatively few ridleys (<20%) but were consumed by both size classes. Live bottom species (tunicates and worm tubes) were encountered in very few samples (6%). One commonly-occurring prey type, flame box crab, has not been reported in published studies, but these were limited in geographic and temporal scope. Size-based differences in diet, however, have been observed for west-central Florida ridleys (Servis et al. 2015) and may be related to ontogenetic changes in growth rate and habitat (Schmid and Barichovich 2006). Consumption of fish, even at a relatively low level, is of concern, given its potential connection to anthropogenic sources. This diet characterization provides a foundation for future studies, which will enable better understanding of Kempís ridley ecology, spatiotemporal diet variation, and potential anthropogenic interactions, ultimately informing management decisions. We analyzed digestive tracts from 65 loggerhead sea turtles (Caretta caretta) that stranded along the Atlantic and Gulf coasts of Florida, USA between 2011-2017, with straight carapace lengths (SCL) ranging from 34.0-103.1 cm (avg=77.4 cm, SD=13.6 cm). NS was subjectively scored on a scale of 1 (severe muscle and fat atrophy) to 5 (no atrophy, excessive fat). Archived wet and dry samples, as well as all relevant accompanying data, were provided by the National Marine Fisheries Service (NMFS), and new sample collection was conducted during 2016-2017 in cooperation with the Florida Fish and Wildlife Conservation Commission. Food items were identified to the lowest possible taxonomic level and counts of individual prey items were estimated. Of the 65 turtles sampled, NS of 8 was categorized as poor (NS1), 17 as thin (NS2), 14 as fair (NS3), and 26 as good (NS4). Preliminary results from 49 samples indicate that diet differences between NS 3-4 and NS 1-2 were minimal; additional samples and dry weights will be integrated to further explore this outcome.