Alexandra Fireman
"On the Shell of the Turtle: Identifying Dietary Patterns of the Hawksbill Sea Turtle Using Stable Isotope Analysis of Keratin Tissue.”
Project Update:
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1. What were major goals of the project and what specific results were achieved?
The initial goals of my project were:
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 was successful in completing three out of the four goals. I evaluated keratin from 50 individual hawksbill sea turtles for δ13C and δ15N (Aim 1), evaluated sponge tissue samples for δ13C and δ15N from 26 individuals (Aim 2), and used these values to calculate a rough trophic enrichment factor estimate (Aim 4). Due to Covid-19, I was unable to collect a sample to complete Aim 3. However, there are still plans to complete this work in the future.
2. What were the greatest successes and challenges of the project? How were the challenges overcome and if not, why?
The greatest success of this project was creating a dataset of stable isotope values from Caribbean hawksbill keratin. To my knowledge, this is the first project that has used Caribbean hawksbill scute tissue to evaluate long term trends in foraging ecology for the species. The results of this work are published in my Master's thesis, and are currently in preparation for publication in a peer-reveiwed journal. The main conclusions were that hawksbills may be using different resources at different life stages, as evidenced by a narrowing of niche width as turtles age. This conclusion, along with evidence that hawksbills may be foraging for prey items at a lower trophic level in younger life stages, provides strong evidence for inclusion of primary producers in hawksbill diet. The calculation of trophic enrichment factor using three known-origin hawksbills and sponges was within the expected range, and provides a foundation upon which to expand this work.
Unfortunately, Covid-19 provided an unexpected challenge to this research. Due to staff reductions at zoos and aquariums and travel restrictions, I was unable to sample a hawksbill in human care to identify isotopic integration rate in hawksbills. While this goal was not achieved, I believe I've contributed significant work to the field of sea turtle dietary ecology. We are still planning on sampling the hawksbill in human care once pandemic restrictions have been lifted.
3. Who was the audience served (e.g. members, volunteers, children, adults, etc.)?
In the short term, this research is most beneficial for the scientific community and conservation efforts as a whole. This research will be published and shared widely so as to inform future studies of hawksbill ecology and diet. While Covid-19 prevented most educational and outreach programs from continuing last year, when restrictions are lifted, I plan to continue incorporating this research into an established outreach plan. This includes school and camp talks in Antigua, presentations at local government and scientific workshops and meetings.
4. Describe any differences between the project’s anticipated and actual results.
Initially, I expected that hawksbill keratin records would display one of three patterns (as proposed by Vander Zanden et al. 2010), a specialist, a generalist, or a specialist with a change in isotopic value at a certain point in time. These patterns do in fact broadly apply to the actual results of this study. The most interesting individuals are those who had specific changes in δ15N values from less enriched to more enriched over time. These changes could indicate a shift in resource use from lower trophic positions to higher (such as a shift from primary producers to sponges). While the sample size of this study was small, these initial results provide a strong foundation for continued work in this field.
I predicted that there would be statistically significant relationships between stable isotope patterns and overall fecundity of individual hawksbills. However, there were no strong relationships in this study. This lack of relationship between reproduction and stable isotope variance could indicate that 1) untested variables play a more significant role in predicting reproductive success in hawksbills, 2) our sampling and analytical methods are too coarse to distinguish a relationship, and/or 3) isotopic variance doesn’t properly capture the relationship between environmental resource use and fecundity. Concurrently, relatively low variability in the derived fecundity metrics could also account for weak relationships outside of stable isotope analysis.
5. If you were to do this project again, what would you do differently (if anything)?
This project followed standard procedures for sampling keratin and processing tissue for stable isotope analysis. While I wouldn’t necessarily change anything about the study design, there are many ways that this research can expanded. Inclusion of sponge tissue from multiple foraging sites would be an excellent next step for furthering this work. Similarly, comparing stable isotope patterns in Antiguan hawksbills to marine turtles around the Caribbean would provide a more complete understanding of how the species utilizes resources in the region. I hope that these questions can be answered through future work.
Nesting Hawksbill
Sponge Samples
Hawksbill Keratin
Original Proposal:
Introduction
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.
Methods
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.
Expected outcomes
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.
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