Assessment of diamondback terrapin presence and abundance in Bogue Sound, NC
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Abstract
The diamondback terrapin (Malaclemys terrapin) inhabits salt marshes, coves, and tidal creeks along the Eastern and Gulf coasts of the United States from Cape Cod, Massachusetts in the north to Corpus Christi, Texas in the south (Hart and Lee, 2006; Ernst and Lovich, 2009). Terrapins forage primarily on small species of crustaceans and mollusks in shallow, flooded marshes at high tide (Tucker et al., 1995) and retreat to deeper waters or bury in the mud of the intertidal zone during low tide (Spivey, 1998; Harden et al., 2007; Southwood et al. 2009). Long-term terrapin studies at various locales within their range provide evidence of population declines, primarily due to factors such as habitat loss, road and boat mortality, nest predation, and mortality due to fisheries interactions (Siegel and Gibbons, 1995; Gibbons et al., 2001). Diamondback terrapins are currently listed as a “Species of Concern” in North Carolina, but a recent report by the Wildlife Resource Commission’s Scientific Council on Amphibians and Reptiles recommends re-classifying this species as “Threatened” (Dorcas et al., 2011). Additional monitoring is necessary in order to establish the terrapin’s conservation status and identify the primary threats facing this species in coastal North Carolina. Of particular concern is the potential for incidental capture and mortality of terrapins in the economically-important nearshore blue crab fishery. Terrapins that are captured in crab pots cannot reach the surface to breath and will drown within 45-300 minutes, depending on environmental conditions at the time of capture (Crowder et al., 2000). Male and immature female terrapins are more vulnerable to capture in crab pots, as they fit through trap openings more easily than the larger adult female terrapins (Roosenburg et al., 1997; Gibbons et al., 2001). Selective removal of young terrapins of both sexes and adult males may result in shifts in population demography that threaten the long-term viability of the population (Dorcas et al., 2007). Bycatch mortality is perceived to be a significant contributor to terrapin population decline (Roosenburg et al., 1997; Wood, 1997; Dorcas et al., 2007; Hart and Crowder, 2011), and this topic has captured the attention of both conservation groups and management agencies. The North Carolina Blue Crab (Callinectes sapidus) Fishery Management Plan (Blue Crab FMP, NC Division of Marine Fisheries, currently under revision) states that research to determine the distribution of terrapins and to assess the impacts of bycatch reduction devices (BRDs) on terrapin and crab catches is essential in order to devise proper regulations and management strategies to prevent interactions between terrapins and crab pots (draft Blue Crab FMP Amendment 2, Section 11.12). Regulations being considered by the Advisory Committee to the Blue Crab FMP include requiring the use of BRDs on all crab pots during the height of the terrapin active season (March through June) in specified areas (Stephen Taylor, personal communication), but a greater knowledge of terrapin distribution is required before spatially-based recommendations can be enacted. We propose to assess diamondback terrapin distribution and abundance at various locations in the Bogue Sound region historically utilized by terrapins. We will use modified crab pots at sites with relatively high terrapin abundance to investigate the impacts of two configurations of BRD on terrapin and crab catch rates.
