Stable isotope analysis suggests large numbers of loggerheads nesting in Florida remain in U.S. coastal waters

For a creature that spends nearly all its life in the sea, studying sea turtles proves a fickle prospect for their terrestrial researchers. Sea turtles spend 99 percent of their time in the ocean, but biologists focus 99 percent of their efforts studying sea turtles on the nesting beach where they are more easily accessible; in fact, the best data available on sea turtles are nest counts. In terms of conservation, managers cannot protect or research sea turtles if they do not know where they are when at sea. Florida accounts for approximately 87 percent of the Northwest Atlantic loggerhead turtle (Caretta caretta) nesting population and the majority of nesting is concentrated on FL central east and southeast coast. Before this study, only few females had been satellite tracked from east FL nesting beaches and followed to their foraging areas (non-nesting areas). This project aims to identify where a representative sample of the loggerhead turtle population nesting in FL is when they are not on the nesting beach.

The study has three main objectives: one, to identify geographic foraging area hot spots of the overall loggerhead aggregation nesting in Florida and by management unit; two, examine annual variation in the relative contribution of each foraging hotspot to the overall aggregation nesting in Florida and to each management unit; and three, evaluate whether female foraging area location relates to reproductive output (i.e., clutch size, hatching and emergence success and breeding frequency) and nesting phenology (i.e., time of arrival to the nesting beach).

Simona Ceriani, Ph.D., led the study published in Scientific Reports, in conjunction with three UCF Department of Biology researchers: John Weishampel, Llewellyn Ehrhart and Kate Mansfield. Michael Wunder of the University of Colorado, a specialist in the analysis of stable isotopes, was also a key member of the team.

UCF Marine Turtle Research Group 2016 summer intern, Victoria Tackett, separating loggerhead nest content in different categories and collecting whole unhatched egg for the project at the Archie Carr National Wildlife Refuge, Florida. Photo credit: Katrina Phillips.

Sea turtles spend almost all their lives in the ocean, and therefore researchers only have easy access to them on the nesting beach. Stable isotope analysis provides a cost-effective way to see where a large number of females have been in the water. For example, if many loggerheads spend their time in the summer off the Delmarva Peninsula near scallop beds, researchers can compare their foraging habitat with fishery data (where and when the fishery operates) and develop management plans such as seasonal closure, no fishing zones, or modify fishing gears in order to reduce fishery-turtle interactions. Another example is offshore drilling exploration; knowing where many turtles are can help managers make informed decisions and decide which offshore drilling sites will have the least impact on sea turtles.

How does stable isotope analysis work? Isotopes are everywhere in the environment, including in the air, food and water that we consume. As we eat food, drink water and breathe the air of a particular geographic location, the isotopes of that specific location imprint a chemical signature on us. This is as true for sea turtles as it is for humans. The specific isotopes that accumulate in loggerheads give the researchers a unique isotopic “fingerprint” of where geographically the loggerheads have spent the most time, feeding and swimming in specific areas.

Over a nine-year period, researchers examined the unique isotopic “fingerprint” of more than 700 individual loggerhead turtles that nested at the Archie Carr National Wildlife Refuge south of Melbourne (east central Florida). Coupled with satellite tracking on a subset of turtles, the team mapped these signatures across the western Atlantic from the waters off Nova Scotia (Canada) to the Yucatán Peninsula (Mexico). The 258-acre Archie Carr NWR is one of the most significant areas for loggerhead nesting in the world, and the most significant area for green turtle nesting in North America. The study showed that most Archie Carr NWR females do not leave the coastal waters that fall under the United States jurisdiction. “Since most loggerheads nests on Florida’s beaches and remain in the United States what we do on our beaches and waters can have a really positive or a detrimental impact. We are the steward for this species,” Ceriani said.

One goal of this research project is to demonstrate the viability of low-cost indirect tracking methods, specifically, stable isotope analysis, for conservation efforts aimed at loggerhead turtles as well as other sea turtles. As resources are limited, there is a need to prioritize where funds should be spent to maximize conservation benefits from management activities. The use of stable isotope analysis to infer female foraging areas (i.e., where females are coming from and what proportion of females come from each area), combined with the increasing evidence suggesting female fidelity to foraging areas throughout adult life, provides new opportunities to identify foraging hot spots, examine hot spot dynamics and assess potential threats occurring at foraging areas.

Ecological Associates Inc. senior project manager, Niki Desjardin and biologist, Ryan Dulfer, conducting a loggerhead nest excavation and collecting whole unhatched egg for the project in Hutchinson Island, Florida. Photo credit: Ecological Associates Inc.

Stable isotope analysis costs only $10 per turtle, as opposed to $5000 required for individual GPS satellite tracking. Both the more expensive satellite tracking, and much more affordable stable isotope analysis were used in conjunction in this study. Stable isotope analysis was used for a broader portrait of where the loggerheads were spending their non-breeding time while at sea, while satellite tracking was used on certain individual turtles for more precise data on locations and to validate the isotopic method. Stable isotope analysis, while very effective, isn’t as precise as pinpoint GPS tracking. A stable isotope study may only be able to tell the researchers that a loggerhead was, for example, off the coast of South Carolina, but not the exact GPS coordinates that the latter tracking method provides. This dual tracking method is important because if the researchers only used GPS tracking for, say, 10 loggerheads, then that data might not portray an accurate portrait of the whole species, since the sample size is so small. If you asked 10 tourists in Florida where they are from, and eight of them said New York, does this mean 80% of Florida tourists are from New York? Certainly not. Thus, the need for this dual-pronged approach, to obtain a better representation of the whereabouts of the true loggerhead (and Florida tourist) population.

To discover where loggerheads are during the non-nesting season, Ceriani used several commonly collected tissues (blood, skin, fresh eggs and unhatched eggs) collected simultaneously from nesting females and developed a common currency for stable isotope analysis studies conducted on the nesting beach. All these tissues were found to be equivalent and can be used to identify the feeding area used by loggerheads. One of the most important results from this study is that researchers can use non-viable, unhatched eggs for stable isotope analysis. Collecting unhatched eggs allows researchers to conduct this important research project without sacrificing viable eggs or disturbing the nesting female.

This project is long-term, and Ceriani plans to continue this project for many more years. The team has a cost-effective method to monitor foraging areas directly from the nesting beach and it is something that needs to be done long-term. The next steps are to refine the geographic assignment models so that we have higher spatial resolution, conduct periodic telemetry studies to evaluate temporal isotopic consistency, assess the stability of isotopic baseline, and incorporate additional markers to refine spatial resolution.