The status, size, and density of Malaclemys terrapin (Diamondback Terrapin) populations along the Atlantic coast have been reported by most states as unknown or declining. Robust demographic or population data are lacking, with even less information available on their spatial ecology. Spatial capture–recapture (SCR) methods explicitly incorporate spatial processes, providing a formal link between encounter data and space use. Despite the widespread adoption of SCR across ecological disciplines, it has yet to be applied to turtle populations. We present the first application of SCR methods to Diamondback Terrapins by analyzing data collected from two known activity areas in the tidal marsh systems of Wellfleet Bay, Massachusetts. We found that Terrapin detection was positively associated with survey effort at both sites. Detection was also influenced by day of season, tide cycle, the time of tide, survey time relative to the tide, cloud cover, and windspeed. Density and space use differed markedly between the two sites: the estimated density in The Run was 9 individuals/ha with a space use parameter of 309 m, compared to 59 individuals/ha and a space use parameter of 107 m in The Cove. Sex structure was female-biased, with a sex ratio of 0.34 and 0.18 males in The Run and The Cove, respectively. We demonstrate the utility in using SCR methods in turtles, specifically Diamondback Terrapins, to produce comparable estimates of detection and population size and density, while simultaneously providing inference on differential space-use and detection resulting from variation in both behavior and sampling conditions.

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