Diamondback terrapins (Malaclemys terrapin) are long-lived turtles that exist as continuously distributed geographic populations along North America's Atlantic and Gulf coasts. Residing in salt marshes, mangroves, and tidal tributaries, the terrapin is the only North American turtle that lives exclusively in brackish water. One of the top predators of benthic macrofauna in the estuarine food chain, terrapins may play an important ecological role, and may thus be particularly suitable for monitoring as an indicator species. Additionally, the terrapin is a species of conservation concern. The historical harvest resulted in population crashes that, coupled with disappearing coastal wetlands, have greatly reduced the numbers of terrapins across their range.

Because the vital rates and population structure for terrapins are poorly understood, we initiated an in-depth mark-recapture study within the Big Sable Creek system of Everglades National Park, southwest Florida. Short-term project goals are to characterize habitat use and movement patterns of individuals, and to estimate the size and geographic extent of the population. Long-term project goals are to compare the habitat use, demographic features, and genetic profiles of selected Atlantic and Gulf coast populations of the species.

To date, we have conducted three weeklong sampling trips to the Big Sable Creek system to capture, mark, and recapture terrapins. Captures of terrapins have been concentrated in the upper reaches of creeks in the system. On each sampling trip, we surveyed the named creeks and their navigable branches systematically for terrapins at AM and PM low tides. We used dip nets to capture turtles, with new moon tides providing the best conditions for capture success.

Over the course of the first year of our mark-recapture study, we marked the first 50 terrapins in November 2001, an additional 96 individuals in June / July 2002, and 64 new turtles in December 2002. Thus far we have recorded 210 unique individuals of which 104 are females and 106 are males, for a population sex ratio of 1:1.

Recapture locations have been clustered around capture sites, suggesting that terrapins display extreme site fidelity, even across seasons--recapture locations are often only meters away from original capture sites. Our current recapture rate is 32.4 percent. The summary recapture statistics together with the number of marked animals per sampling trip (Table 1) allow us to make initial estimates of population size.

We used the Schnabel population estimator because it is appropriate for closed population studies with multiple mark and recapture periods. We assumed that every individual in the population has the same capture probability for a given sampling occasion, but that the capture probabilities can vary among sampling periods. We plan to update the population estimate after each of our next two sampling periods, scheduled for May and November 2003.

The Big Cape Sable terrapin population presently consists primarily of adult animals. Analysis of the age composition data revealed that 80 percent of females and 92 percent of males captured to date are in the adult life stage. We have recorded no females less than 5 years of age and no males less than 4 years of age, suggesting we have not encountered any juveniles during our surveys. Despite considerable and repeated efforts to sample as far up each creek as possible (to the limit of canoe penetration), we have not yet found young animals in the population.

We will continue to mark and recapture terrapins in the Big Sable Creek system throughout 2003. Additionally, we will study the movement of individual females by radiotracking 10 tagged individuals. We also hope to use satellite telemetry to better understand the movement of nesting females once they leave the Big Sable Creek system to find dry upland habitat suitable for egg-laying. Future analysis of microsatellites obtained from genetic samples will help to further define population structure and gene flow among individuals. These data will help to define the extent of a mangrove terrapin population and will delineate an ecologically and genetically relevant management unit for terrapin conservation.

Contact: Hart, Kristen, U.S. Geological Survey, Center for Coastal and Watershed Studies, 600 Fourth Street South, St. Petersburg, FL and Duke University, Nicholas School of the Environment Marine Laboratory, 135 Duke Marine Lab Road, Beaufort, NC 28516, Phone: 252-504-7571, Fax: 252-504-7648, kristen.hart@duke.edu

(This abstract was taken from the Greater Everglades Ecosystem Restoration (GEER) Open File Report 03-54)

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