"On the Shell of the Turtle: Identifying Dietary Patterns of the Hawksbill Sea Turtle Using Stable Isotope Analysis of Keratin Tissue.”
The Jumby Bay Hawksbill Project (JBHP) has been monitoring a population of nesting hawksbill sea turtles on Long Island, Antigua since 1987. From 1987-2015, the population exhibited growth (Kendall et al. 2019). However, in recent years, annual nesting numbers have markedly declined with 2018 exhibiting a 15-year record low. As such, there is an immediate need to assess causes of this decline, such as degradation of foraging habitat. Hawksbills forage in coral reef ecosystems and are considered keystone species because their decline has deleterious effects on reef ecosystems. Therefore, understanding the hawksbill’s role in their environment is critical for conservation of their species and coral reef ecosystems as a whole.
Studying the foraging ecology of a long-lived marine species requires innovative techniques. Stable isotope analysis of inert and metabolically active tissues has been used across taxa to better understand the diets of organisms. In particular, δ13C and δ15N values have been widely used to predict location of foraging and trophic level of the marine consumers. Keratin from sea turtle carapaces can provide insights into foraging strategies (Reich et al. 2007). In some turtle species, the carapace can contain a keratin record spanning up to 12 year (Vander Zanden et al. 2010). While isotope applications with sea turtles have recently proliferated, isotope studies of hawksbills are less common, and time-series analysis of Caribbean hawksbill keratin isotope composition is mainly absent from the literature. The archive of trophic history stored in carapace keratin could provide a powerful method for evaluating resource use by hawksbills in reef ecosystems.
When assessing stable isotope values, a trophic enrichment factor (TEF), or the difference between the isotopic value of a predator and its prey, is often necessary to identify the absolute trophic level of a species. Adult hawksbill sea turtles primarily consume sea sponges (Leon and Bjorndal 2002). After four years of successfully tracking hawksbill movement patterns to foraging grounds, the JBHP has identified an area off the western coast of Antigua that hosts at least three hawksbills that nest on Long Island. Sampling potential prey sources from an identified foraging ground would allow for determination of a preliminary trophic enrichment factor for these hawksbills. While analysis of hawksbill stable isotopes without a TEF allows for determination of relative isotopic compositions, inclusion of the TEF will allow for a determination of trophic level of individual turtles within their ecosystem. This TEF has not been determined for the hawksbill sea turtle, making this assessment novel and necessary for future hawksbill stable isotope analysis. Past studies of loggerhead sea turtles have effectively used TEFs based on prey items to compare isotopic niche of sea turtles of varying age classes (Reich et al. 2008). Understanding this connection between the hawksbill and its environment through diet is a critical component in effective conservation of foraging ground habitat.
While stable isotope values through the keratin growth timeframe can provide useful information, placing individual integrations with actual historical timing allows for a better understanding of hawksbill life history. This is achievable through collection and assessment of hawksbill tissue from an individual with a known diet, and a known change in diet. In human care with a known diet this turtle and assessing the tissue for δ13C and δ15N values, I will be able to characterize the timing of isotopic integration in keratin tissue. While the proposed research only includes one individual, future work should involve sampling from multiple hawksbills with known diets to increase sample size. This can then be used to understand the patterns of the Caribbean hawksbills, and place large isotopic shifts in historical time.
Using the determined TEF and timing of keratin integration, I will be able to draw more complete conclusions regarding the population of hawksbills from Long Island. Currently, n=21 individual hawksbills have been tracked to their foraging grounds. By identifying changes in these turtles’ isotopic record, I will attempt to link environmental changes or perturbations. My hope is that this will allow for identification of disturbance events (i.e. and intense hurricane season) in turtle foraging grounds that may disrupt or change foraging. This will also allow for identification of the most vulnerable habitats and can be used to support increased protection efforts for these marine areas.
Aim 1: I will evaluate keratin from the Jumby Bay hawksbill population for patterns in δ13C and δ15N across carapace growth. This will allow for characterization of foraging patterns in individuals and the population as a whole.
Aim 2: I will evaluate δ13C and δ15N values in sponge tissue from Antiguan foraging grounds identified through satellite tracking of nesting females. This will allow for identification of a trophic enrichment factor between sponge tissue and hawksbills and will strengthen further assessment of untracked hawksbills.
Aim 3: I will evaluate δ13C and δ15N values of a hawksbill in human care with a known diet and historical diet switch. This will identify the time of keratin isotopic integration in an adult hawksbill turtle.
Aim 4: I will use the above trophic enrichment factor and isotopic integration rate to draw conclusions regarding the Long Island population and their coral reef ecosystems.
I have collected carapace keratin plugs from nesting hawksbills in 2017-2019, and I will continue sampling in 2020. New tissue will be brought to the Chesapeake Biological Lab under CITES permit #53023C. Tissue samples are carefully collected only during oviposition, and the resulting carapace holes are filled in with a nontoxic quickset epoxy to prevent contamination and ensure proper healing. The hawksbill in human care will be similarly sampled by zoo staff and the samples will be sent to the Chesapeake Biological Laboratory for stable isotope analysis. I will sequentially sample the keratin plugs along the z-axis at 100-μm intervals, analyzing the tissue for δ13C and δ15N. I will identify patterns in isotope composition within to evaluate foraging strategy over the temporal record of the keratin and identify changes in resource use. Sponge tissue samples were collected in 2019, and will be assessed for δ13C and δ15N. Trophic enrichment factor will be calculated using the formula: (ΔX) = δXhawksbill- δXsponge. In addition, I will evaluate foraging strategy of individuals as a predictor of overall reproductive success. Lastly, I will attempt to identify links between coral reef foraging habitat and historical disturbance and shifts in individual trophic patterns.
The first outcome of this research will be a complete isotopic record of keratin tissue for a subset of the Jumby Bay population and the individual in human care. I will determine the rate of keratin isotope integration from the individual in the zoo, and a trophic enrichment factor using the sponge tissue values. Given the model proposed by Vander Zanden et al. (2010), I hypothesize that individuals will display one of three diet patterns in their isotopic record: a generalist, a specialist with no change over time, or a specialist with a change in δ13C and δ15N at a specific point in time I predict that turtles displaying specialist trends will have a higher reproductive output. If shifts in resource use are present in the keratin record, I predict that timed environmental scale phenomena can be identified as driving forces of these shifts.