Using sea turtles to find seagrass: Tracking juvenile Chelonia mydas with satellite telemetry in the southwest coastal Everglades, Florida, USA

Kristen M. Hart and Carole C. McIvor¹

¹US Geological Survey, Florida Integrated Science Center, St. Petersburg, FL, USA

Green sea turtles (Chelonia mydas) are a federally endangered species. Threats to their survival include loss of nesting beach habitat, loss of eggs to nest predators, mortality due to by-catch in commercial fishing gear, and disease. Whereas the ecology and movements of adult green turtles are reasonably well understood, relatively little is known about the habitat needs and movements of juveniles and subadults. We recently initiated a tagging study on juvenile and subadult C. mydas in the Big Sable Creek (BSC) mangrove complex of the southwest coastal Everglades National Park (ENP) to determine habitat use in BSC and visitation patterns to ENP. A 5-year sightings record of green turtles in the remote stretches of the study site is comprised of 50+ individual records, and we postulate that these difficult-to-access areas may provide previously unknown foraging and possibly nursery grounds for this endangered species. We are currently establishing activity and habitat-use patterns through the combined use of mark-recapture, satellite tracking, and molecular genetic techniques. Here, we present tracks of a tagged turtle to indicate locations of seagrass and algal cover in the western Everglades, where benthic habitat has not yet been mapped.

Fig. 1. Study site in South Florida, Everglades National Park. [larger image]

Discovering the relationships among sea turtle populations is a global research priority. By determining the connectivity of herbivorous green sea turtles (Chelonia mydas) of ENP, our research will enable practitioners to better understand the range of the populations they manage, recognize distinct populations and identify regional management partners, and further understand green sea turtle population biology and habitat needs. Specifically, we will determine visitation patterns to and residence time in the BSC study site for green turtles outfitted with satellite tags. This information will be of value to ENP managers who currently have no information on green turtle use of Park resources.

Thus far, we have captured 4 individuals (Fig. 2) and recorded observations of green sea turtles ranging from 10 to 60 cm carapace length in the mangrove tidal creeks of the BSC. We have also collected GPS coordinates of sightings for 50+ different green sea turtles (Fig. 3)—many of these locations have been in the headwater regions of the study site, approximately two kilometers from the Gulf coast. In this headwater habitat, there is a surprising amount of submerged algae-covered logs that are remnants of old red mangroves (Rhizophora mangle) (Fig. 2) and clear, salt-water seeps (Fig. 3):

Fig. 2. Photos of the first green turtle captured as part of this study (captured Nov. 15, 2006, 31cm straight carapace length (SCL) and submerged algae covered logs in the headwater regions of one of the creeks in the study site. [click on images above to view larger versions]

Fig. 3. Green sea turtle sightings, captures, and seep locations in the Big Sable Creek complex, Everglades National Park. [larger image]

• Currently, our capture methods include using dip nets, pound nets, and entanglement nets.
• We take standard straight and curved carapace measurements, administer inconel flipper tags and PIT tags, and sample each individual for diet and genetic composition.
• We are deploying Wildlife Computers™ SPOT5 satellite tags on 15 individuals captured in and near the BSC complex. All data derived from satellite tags will be filtered for plausible swim speeds, location quality, and erroneous locations. Kernel density plots will be developed for each turtle after tracking ceases.

• To date, we have outfitted 3 turtles with satellite tags (Table 1).
• Raw data for one juvenile green turtle (Fig. 4) indicates concentrations of satellite-derived locations at the entrance to the BSC complex.
• Because no coverage for seagrass exists for the Western Everglades, we postulate that areas of dense green turtle location points may indicate areas of seagrass and algae beds. As well, we hypothesize that points outside the complex may represent additional seagrass habitats.

Fig. 4. Green sea turtle locations relayed by satellite for “Muffin” between 3/14/07 and 5/3/07 [larger image]

• Relatively short deployments may be indicative of battery failure or tag loss.
• A high proportion of locations are of lower quality location classes (i.e., A, B, Z) with larger error.
• Repeated visits of multiple turtles to specific areas needs to be investigated—we plan to survey turtle location “hotspots” for presence of seagrass and algae.

We thank the U. S. Fish and Wildlife Service, the U. S. Geological Survey, Everglades National Park, the Marathon Sea Turtle Hospital
Eugenia Naro-Maciel, Caroline Good, Betsy Boynton, Gary L. Hill, Noah Silverman, Lisa Eby, Paula Gillikin, Selina Heppell, BJ Reynolds, and Adam Brame. The authors wish to acknowledge use of the Maptool program for analysis and graphics in this paper. Maptool is a product of SEATURTLE.ORG. (Information is available at www.seaturtle.org)