Virginia Institute of Marine Science
Diamondback terrapin (Malaclemys terrapin) is considered a keystone species for its influence on community structure of tidal marshes. Terrapins exhibit strong habitat and nest site fidelity, and have relatively small home ranges (< 2 km), so that sub-populations tend to be spatially discrete. Terrapins rely on open water, wetlands, and adjacent uplands at various stages of their life-cycle, so the quality and connectivity of these habitat patches is critical to population persistence. Terrapin is listed in Virginia as a species of "Very High Conservation Need" based on threats due to nest predation and drowning of adults in crab pots. Terrapin population declines, reduced growth, and changes in sex ratios have been directly attributed to bycatch mortality in commercial crab pots. Our overall project goal was to characterize essential terrapin habitats toward development of bycatch reduction strategies for managing commercial and recreational blue crab fisheries. In a pilot study area surrounding the mouth of the York River, Virginia, our approach was to 1) geospatially define suitable terrapin habitat based on natural features, 2) integrate spatial datasets to develop a "Vulnerability Index" of terrapin habitats and define potential resource conflict areas where crab pots correspond to essential terrapin habitat, and 3) conduct terrapin and crab pot counts in habitats with varying suitability to test predictions. Suitable terrapin habitat (full connectivity among habitat metrics) accounted for over 50% of all terrapin observations, and another 45% of observations occurred in areas where only one habitat metric was absent. In 96% of these cases, the absent metric was SAV presence. In contrast, full habitat connectivity was determined for only 5% of areas where terrapins were absent. Within the pilot study area during a two year retrieval program, 2872 derelict pots were removed. Of these, 22% were within shallow waters (≤ 2 m) where terrapins typically reside. Of the suitable terrapin habitat (70km2 ), 21% (15 km2 ) was considered vulnerable to crabbing pressures (10% highly and 11% moderately vulnerable). Approximately 15% of the study area was considered to be potential resource conflict areas for terrapin and crabbing. Candidate zones for the targeted application of blue crab fishery management actions to reduce terrapin bycatch include the Severn River, Perrin River, Guinea Marshes, and south of Gwynn Island. The integration of spatial information on terrapin habitat and crabbing pressure in a single framework will allow managers to identify areas where terrapins are most likely to encounter threats and target conservation efforts in those areas. In resource conflict areas, there are several management options that can be used in combination 1) Require use of bycatch reduction devices (BRDs) on commercial & recreational crab pots 2) Avoid particular habitats (e.g. small tidal creeks) or establish fishing exclusion zones 3) Educate – design public education programs to • promote the voluntary use of BRDs, and • communicate to recreational boaters the ramifications of severing buoy lines of active crab pots 4) Promote proper use of gear (e.g. retrieving pots regularly to minimize terrapin mortality). With further refinement to improve the predictability of terrapin occupancy, the terrapin habitat vulnerability model is transferable to all coastal areas where diamondback terrapins occur and where blue crabs are commercially and recreationally fished—from southern New England to Texas.
Diamondback Terrapin, Keystone Species, Bycatch, Virginia, Fisheries
Final Report to Virginia Sea Grant, National Oceanic and Atmospheric Administration (NOAA) Agency Award Number: NA10OAR4170085
Bilkovic, D. M., Chambers, R., Leu, M., Havens, K. J., & Russell, T. (2012) Diamondback Terrapin Bycatch Reduction Strategies for Commercial and Recreational Blue Crab Fisheries. Virginia Institute of Marine Science, College of William and Mary. https://scholarworks.wm.edu/reports/1270