Investigating Loggerhead Sea Turtles and Where They Go When Not Nesting in Florida

By Simon Ceriani

Sea turtles, a long-living and highly migratory species, are a conservation concern that is primarily studied on nesting beaches, where they are easily accessible. However, only a small fraction of a female’s life is spent near a nesting beach. Reproductively active females undertake breeding migrations every 1 to 5+ years. These breeding migrations are often from distant foraging areas to their natal nesting beach, where they typically lay several clutches in a nesting season. Protecting sea turtle nesting habitat and monitoring nest counts is essential, but an understanding of where females are when they are not nesting (the majority of their time) is equally vital to ensure their protection.

The last decade has seen a growing interest in using stable isotopes, a type of intrinsic marker, as a tool to study migratory connectivity and identify foraging area locations. Work, combining satellite telemetry and stable isotope analysis, showed that post-nesting, satellite-tracked females migrating to geographically separate foraging areas can be identified by differences in their isotopic values. Thus, researchers have increasingly focused on sea turtle nesting aggregations by sampling nesting females and their nest and have used stable isotope analysis to infer foraging areas of larger numbers of untracked females.

Florida hosts ~90% of all the loggerhead nesting activity in the Southeast USA, yet few research groups encounter nesting females at night. In contrast, thousands of nests are marked to assess hatchling production through an extensive program coordinated by the FWC’s Fish and Wildlife Research Institute (FWRI). In 2013, we began a statewide project aiming to identify loggerhead foraging hotspots and determine inter-annual contribution of foraging areas to the Florida nesting aggregation. The project relies on the support and help of over a dozen Marine Turtle Permit Holders across Florida that collect non-viable eggs from a subsample of loggerhead nests marked as a part of FWRI’s hatchling Productivity Assessment Program. Sampling occurs over the entire course of the loggerhead nesting season and reflects nesting temporal distribution.

A few thousands non-viable loggerhead eggs are then transferred to the Sea Turtle Migration Research lab at FWRI Headquarters in St. Petersburg where interns and volunteers help prepare the eggs for analysis. Stable isotope values of unhatched eggs are then used to infer foraging areas used by females during the non-reproductive season.

This study provides a non-invasive and non-destructive method of sampling a relatively large percentage of the loggerhead population nesting in Florida and represents the most comprehensive geographic assessment of foraging areas to date. Using unhatched eggs to assign females to foraging grounds provides an opportunity to:

  1. Sample at a much larger scale, fostering collaborations among research groups and stakeholders
  2. Obtain information that is more representative at the population level
  3. Begin understanding the relative importance of foraging areas and how each foraging ground contribution changes among years.

Conservation funds are limited and there is a need to prioritize where funds should be spent to maximize conservation outcomes. Understanding relative importance of foraging areas will allow us to make more informed management decisions by focusing mitigation and by-catch reduction measures to areas that are loggerhead hotspots.

The first two years of this work were conducted when the PI was at the University of Central Florida and were funded by the Florida Sea Turtle Grants Program. The Sea Turtle Grants Program is funded from proceeds from the sale of the Florida Sea Turtle License Plate.  Since 2015, the scope and scale of the project augmented thanks to the support of a National Oceanic and Atmospheric Administration (NOAA) – National Marine Fisheries Service (NMFS) – ESA Section 6 grant.