Sea Turtles in the Dry Tortugas: Tracking Movements of Endangered Species in Florida's Coral-Reef Habitats
Kristen Hart and Keith Ludwig of the USGS Florida Integrated Science Center (FISC) office in St. Petersburg participated in two research cruises in 2008 to study patterns of habitat use by endangered sea turtles in and around the National Park. The cruises were conducted in May and August 2008 on board the merchant vessel (M/V) Fort Jefferson. Hart and Ludwig used the ship's tender (the Livingston, a 14-ft center-console catamaran skiff with a 25-horsepower motor) as a workboat.
Dry Tortugas National Park, part of the Florida coral-reef tract, harbors both soft-bottom (seagrass) and hard-bottom (coral and sponge) habitats that are important for several species of sea turtles. In January 2007, the designation of a new Research Natural Area in the Park set aside 46 mi2 as a no-take preserve, presenting an opportunity to address the efficacy of the Research Natural Area in protecting endangered species and their habitats.
Hart's research effort focuses on quantifying patterns of sea turtle habitat use; her team employs capture-recapture and satellite- and acoustic-tracking techniques to determine the amount of time endangered sea turtles spend in and around the various "no fishing" zones of Dry Tortugas National Park. The team also samples turtles for genetic material and will use molecular genetic methods to reveal connections between Dry Tortugas sea turtles and others sampled previously at various locations throughout south Florida and the Caribbean.
This sea turtle project complements a high-resolution underwater digital-imaging project led by Dave Zawada of the FISC St. Petersburg office. Zawada and Hart will work together to identify areas of Dry Tortugas National Park that are frequently used by tagged sea turtles, and then they will characterize the benthic (sea floor) cover in those areas, using the USGS Along-Track Reef-Imaging System (ATRIS; see related Sound Waves articles, "Sea-Floor Survey Off Key Largo, Florida, Using Along-Track Reef-Imaging System (ATRIS)" and "Along-Track Reef-Imaging System (ATRIS) Used to Survey the Sea Floor in Dry Tortugas National Park, Florida"). Zawada will also lead an effort to characterize bottom cover in areas of the Park currently classified as "unknown," as well as along the boundaries of the Research Natural Area.
During fieldwork conducted in October 2007 (also aboard the M/V Fort Jefferson), Hart and Ludwig logged the locations of sea turtle sightings. Hart used this record to plan the team's in-water capture efforts for 2008, since there were several "hotspots" where sea turtle sightings were concentrated. At present, Hart's record of Dry Tortugas sea turtle sightings consists of 105 locations representing 105 individual turtles observed in Park waters from October 2007 through August 2008.
In May 2008, Hart and Ludwig tagged their first Dry Tortugas sea turtles, three female loggerheads (Caretta caretta) nesting on East Key. Loggerheads are listed as Threatened under the Endangered Species Act (see NOAA Fisheries Office of Protected Resources - Marine Turtles), and Dry Tortugas loggerheads represent a genetically distinct subpopulation. The turtles were outfitted with satellite tags capable of relaying daily latitude and longitude locations to a central computer, as well as acoustic tags that transmit to a stationary array of acoustic listening stations deployed around Dry Tortugas National Park. The satellite tags relay data only when they are out of the water, which occurs when the tagged turtles are at the water surface or on land. In contrast, the acoustic tags relay data only when submerged; their placement on the carapace assures that the tags are submerged when the turtles come to the surface to breathe as well as when they are feeding or resting underwater (where they actually spend most of their time). The acoustic tag's signal is recorded any time a tagged turtle passes within range of an acoustic receiver, of which there are approximately 85 within Dry Tortugas National Park. The battery life of satellite tags should allow them to transmit daily positions of sea turtles for as long as 1 year, whereas the battery configuration of acoustic tags should allow them to transmit considerably longer, for as long as 4 years. Since May, Hart has been satellite-tracking the movements of the three tagged loggerheads within, around, and outside the boundaries of the Park on a daily basis. These three individuals are the first sea turtles from Dry Tortugas National Park ever to be equipped with such tags.
In August 2008, Hart and Ludwig initiated their in-water capture efforts and caught 26 juvenile sea turtles of two different species23 green turtles (Chelonia mydas) and 3 hawksbills (Eretmochelys imbricata)using dip-net and hand-capture methods. In the Florida region, both green turtles and hawksbills are listed as Endangered under the U.S. Endangered Species Act. None of the turtles captured had fibropapillomasfibrous, lobe-shaped tumors that affect other populations of green turtles in coastal areas adjacent to peninsular Florida. With a limited number of tags on hand, Hart chose to outfit all three hawksbills with satellite tags and two with acoustic tags in order to track their movements around the study site. (Juvenile hawksbills have never before been satellite-tracked in U.S. waters.) She also placed an acoustic tag on one of the juvenile green turtles. By tracking the movements of several different species of sea turtles, Hart can determine turtle habitat-use patterns and pinpoint particular resources in Dry Tortugas National Park that may serve to sustain the local sea turtle populations; loggerheads forage on such food items as spiny lobsters and crabs, hawksbills consume sponges, and green turtles graze on seagrasses and marine algae.
Although the ecology and movements of sea turtles using mainland U.S. nesting beaches are relatively well known, little is known about the habitat requirements or movements of juvenile sea turtles of any species in their aquatic environment. Globally, only a handful of studies have ever focused on satellite-tracking of hawksbills, and most of those have targeted nesting hawksbills. Similarly, knowledge of the ecology and movements of adult sea turtles using remote U.S beaches, such as those in the Dry Tortugas, is also limited. Thus, the tracking of loggerheads and hawksbills will provide extremely useful information for Federal recovery plans for these endangered species. In the future, Hart will also track green sea turtles in Dry Tortugas National Park to gain a more thorough understanding of their use of the seagrass resources in and around the Park.
Through this research, the USGS will provide a more comprehensive understanding of endangered sea turtles' use of National Park resources over time. Such information will be instructive in forming management strategies that benefit endangered species and the habitats and resources upon which they rely.
To learn more about the conservation status of sea turtles in the United States, visit the "Marine Turtles" Web site hosted by the National Oceanic and Atmospheric Administration (NOAA) Marine Fisheries Service's Office of Protected Resources. To learn more about Hart's studies of sea turtles, visit URL http://sofia.usgs.gov/people/hart.html.
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