Diamondback Terrapin By-Catch Reduction Strategies for Commercial and Recreational Blue Crab Fisheries
The diamondback terrapin (Malaclemys 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. This study looks for ways to preserve terrapin populations while minimally affecting the blue crab fishery through examination of the blue crab fishery and diamondback terrapin spatial habitat overlap.
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:
- Require use of bycatch reduction devices (BRDs) on commercial & recreational crab pots,
- Avoid particular habitats (e.g. small tidal creeks) or establish fishing exclusion zones,
- 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, and
- Promote proper use of gear (e.g. retrieving pots regularly to minimize terrapin mortality).
The Location of Important Coastal Habitats for Terrapins
- Diamondback terrapins live in mesohaline to polyhaline tidal wetlands that provide adequate forage and protection from potential predators. These marshes typically are dominated by Spartina grasses.
- Terrapins consume snails, crabs, bivalves, barnacles, and other invertebrate species they encounter in marshes, in tidal creeks dissecting marshes, and in adjacent subtidal areas that could be sandy bottom, mudflat, or vegetated by seagrass. In addition, terrapins overwinter buried in shallow subtidal mudflats or tidal creeks. Both intertidal marshes and adjacent subtidal areas are used by terrapins throughout their lives.
- As a third habitat requirement, mature female terrapins emerge from the water to nest, and prefer sandy substrates above the level of the highest tides. In the absence of open, sandy beaches, however, terrapins will nest in poorer substrates. Nesting success is mostly dependent on accessibility, the quality of substrate, and the density of nest predators, some of which are subsidized by anthropogenic activity such as residential developed land use.
- 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.
- We defined suitable terrapin habitat on the basis of the connectivity of tidal wetlands, nesting beaches, non-forested riparian lands, shallow water depth & the presence of seagrasses in adjacent subtidal waters.
- In our study area, 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, typically seagrass
Vulnerability to Commercial and Recreational Crabbing: the scope of the derelict crab pot issue
- Terrapin population declines, reduced growth, & changes in sex ratios have been directly attributed to bycatch mortality in commercial crab pots e.g., Dorcas et al. 2007; Wolak et al. 2010.
- 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.
- The amount of derelict pots generally corresponded to the number of active pots in a given area and represents an inherent mortality risk as derelict pots can continue to capture and kill terrapin.
The Total Area of Coastal Shallow Water Habitat: where commercial/recreational crabbing and potential terrapin habitat overlap
- Approximately 15% of the study area was considered to be potential resource conflict areas for terrapin and crabbing.
- Of the suitable terrapin habitat (70km²), 21% (15 km²) was considered vulnerable to crab fishing pressures (10% highly and 11% moderately vulnerable).
Candidate Zones for the Application of Blue Crab Fishery Management: actions to reduce terrapin bycatch
- Three regions in the pilot survey area were considered areas of special concern due to high crab fishing pressure within essential terrapin habitat: Lower York River (Perrin Creek, Cuba Island, Guinea Marshes), Severn River, and Gwynn Island. Further, terrapin occupancy was verified during field surveys.
Potential Relevance - Impacts
- This study sought to balance the conservation of diamondback terrapins while minimizing any economic impact to the blue crab fishery. Delineation of specific resource conflict areas (i.e. terrapin habitat and blue crab fishery) can guide the targeting of discrete fishing pressure relief areas. Fishing pressure relief can be in the form of required use of bycatch reduction devices (BRDs) on recreational and/or commercial pots and fishing exclusion zones. Additional management measures may include public education programs to promote the voluntary use of BRDs, avoidance of particular habitats, proper use of gear (e.g. retrieving pots regularly to minimize terrapin mortality), as well as communicating to recreational boaters the ramifications of severing buoy lines of active crab pots.
- Isdell, R.E., R.M. Chambers, D.M. Bilkovic, M. Leu. 2015. Effects of terrestrial-aquatic connectivity on an estuarine turtle. Diversity and Distributions 21(6):643-653.
- Bilkovic, D.M., Havens, K.J., Stanhope, D.M. and Angstadt, K.T. (2012), Use of Fully Biodegradable Panels to Reduce Derelict Pot Threats to Marine Fauna. Conservation Biology, 26: 957–966. doi:10.1111/j.1523-1739.2012.01939.x (link)
- Morris, A.S., S.M. Wilson, E.F. Dever, and R.M. Chambers. 2011. A test of by-catch reduction devices on commercial crab pots in a tidal marsh creek in Virginia. Estuaries and Coasts: 34:386-390.
- Wolak, M.E., V.A. Ruzicka, G.W. Gilchrist, D.M. Nally, and R.M. Chambers. 2010. Selection imposed by crab fishery shifts turtle body size. Conservation Biology 24:1268-1277.
- Rook, M.A., R.N. Lipcius, B.M. Bronner, and R.M. Chambers. 2010. Conservation of diamondback terrapin and catch of blue crab with a bycatch reduction device. Marine Ecology Progress Series: 409:171-179.
- Havens K.J., D.M. Bilkovic, D. Stanhope, K. Angstadt. 2009. Location, location, location: the importance of cull ring placement in blue crab traps. Transactions of the American Fisheries Society. 138:720–724.
- K. J. Havens, D.M. Bilkovic, D. Stanhope, K. Angstadt, and C. Hershner. 2008. The effects of derelict blue crab traps on marine organisms in the Lower York River, VA. North American Journal of Fisheries Management 28(4):1194-1200.
- Bilkovic, D.M. and M. Roggero. 2008. Effects of coastal development on nearshore estuarine nekton communities. Marine Ecology Progress Series 358:27-39.
- Bilkovic, D.M., M. Roggero, C.H. Hershner, K.H. Havens. 2006. Influence of land use on macrobenthic communities in nearshore estuarine habitats. Estuaries & Coasts 29(6B):1185-1195.
- Bilkovic, D.M., C.H. Hershner, M.R. Berman, K.J. Havens and D.M. Stanhope. 2005. Evaluating Estuarine Indicators of Ecosystem Health in the Nearshore of Chesapeake Bay. In S.A. Bortone (ed.). Estuarine Indicators. CRC Press. Boca Raton, Florida.