Use of Dry Tortugas National Park by threatened and endangered marine turtles
Dry Tortugas National Park (DRTO) harbors several key benthic habitats that are important for marine turtles. Threatened loggerhead turtles (Caretta caretta) forage in hard-bottom areas on spiny lobsters and crabs; endangered hawksbill turtles (Eretmochelys imbricata) forage on reefs and consume sponges; and endangered green turtles (Chelonia mydas) graze on seagrasses and marine algae. The sandy beaches of DRTO provide suitable nesting habitat for all three species.
The majority of nesting activity at DRTO consists of nest construction on East and Loggerhead Keys by loggerhead and green turtles. In order to monitor the immigration and emigration of targeted species in the Research Natural Area (RNA), we characterized the populations of the three sea turtle species in DRTO and quantified the proportion of time individuals spent in the RNA as compared to other areas of the park. We examined turtle data with respect to the RNA and DRTO boundaries.
We initiated our on-going turtle tagging and tracking project in 2008. To distribute capture effort within park boundaries, we captured nesting turtles on East and Loggerhead Keys, and captured turtles in the waters near Bush and Garden Keys, Northkey Harbor, and Pulaski Shoal. Our total area patrolled to catch turtles has expanded over time as determined by suitable turtle capture conditions, with more area being incorporated outside the RNA than inside the RNA. In addition to capture efforts, we recorded sightings of turtles as we patrolled the park by boat and marked the locations with a Global Positioning System (GPS) receiver. Sightings could include one or more turtles at a single location.
Turtles were captured by intercepting reproductive females on nesting beaches and catching turtles in the water with rodeo (diving from boat to snorkel-capture turtles), hand-capture, and dip-netting methods. We individually marked each turtle with internal passive integrated transponder (PIT) tags and external flipper tags. We took standard length, width, and mass measurements and obtained blood and tissue samples for genetic, isotopic, and dietary analyses. We attached acoustic and/or satellite-telemetry tags to a subset of turtles.
Seven acoustic receivers were placed by the U.S. Geological Survey (USGS) in the northeastern region of the park; all but one of these were placed outside the RNA. These locations were chosen based on spatial gaps in the network of receivers deployed by the Florida Fish and Wildlife Conservation Commission (FWC) and Mote Marine Laboratory (MML) (see Chapter 4, this volume). We received data from the FWC/MML receiver array, which included more than 80 receivers spread throughout the park in all management areas and outside the park to the southwest.
Using satellite and acoustic telemetry techniques, we determined daily locations and movement patterns for tagged turtles, calculated home ranges and core-use areas, and statistically summarized the extent of overlap of these areas with that of the RNA. We used the kernel density estimation technique (KDE) (Worton 1989; White and Garrott 1990) to determine “hotspots” of turtle activity in the park. KDE is a method used to identify one or more areas of disproportionately heavy use (core-use areas) within a home-range boundary, with appropriate weighting of outlying observations. We also compared the number of turtle-days within and outside the RNA using only days the turtles spent inside the park (DRTO turtle-days). We combined these data sets to determine residence times inside the park and locations of core-use areas for all three species.
It is essential to understand the locations, movements, estimated population size, preferred habitats, and status (for example nesting, foraging, number of juveniles and adults, etc.) of marine turtles within DRTO to effectively manage activities that affect these imperiled species. Our capture and tagging results summarized here can inform decision-makers by providing key information on these population characteristics. Our spatial habitat-use information provides details on locations and areas within and outside the park that turtles select, regardless of capture site.