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Diamondback Terrapin Research

Bycatch in commercial crab traps is believed by many experts to present a significant and unsustainable source of mortality for diamondback terrapins. The Division of Fish and Wildlife, Endangered and Nongame Species Program (ENSP) and Bureau of Marine Fisheries (BMF) have been working to test and modify excluder devices originally developed by Dr. Roger Wood of the Wetlands Institute.

In 1997, ENSP and the Bureau of Marine Fisheries completed testing of two types of terrapin excluder devices modified from Dr. Roger Wood's original design. The results of this study showed that the 5 x 15 cm (2" x 6") rectangular device did not significantly affect the number or size of crabs captured when compared to traditional (no excluder) crab pots. A 5 x 15 diamond-shaped device, however, did appear to reduce the number of larger crabs. The results of this study have been submitted for publication in the North American Journal of Fisheries Management. For a copy of the abstract from a paper click here

Excluders are now required on commercial crab pots set in creeks less than 100' wide or in manmade lagoons. Based on the findings of the ENSP / BMF study and other studies completed in Maryland and Louisiana, the Division of Fish and Wildlife is confident that the excluder devices provide an efficient means to reduce the terrapin bycatch without undue burden on New Jersey's commercial crabbers.

Future work we plan to carry out on diamondback terrapins will look more closely at the their distribution in our coastal estuaries with respect to water depth, distance from shore and association with particular habitats. We also hope to undertake studies to evaluate the cumulative impacts on terrapin populations in New Jersey of road mortality, predation, drowning in crab traps and reduced access to nesting habitat.

Effect of Two Designs of Terrapin Excluder Devices on the Catch of Blue Crabs in Commercial-Style Crab Pots

C. David Jenkins, Jr.
Endangered and Nongame Species Program

Paul G. Scarlett
Bureau of Marine Fisheries

J. Richard Trout, Professor Emeritus
Rutgers University

Abstract. -- We conducted experiments to evaluate the effects on the catch of blue crabs (Callinectes sapidus) of equipping crab pots with excluder devices designed to reduce the bycatch of diamondback terrapins (Malaclemys terrapin). The excluder devices tested were a 5 x 15 cm wire rectangle and a 5 x 15 cm wire diamond. Equal numbers of each of three pot types (rectangular excluder, diamond-shaped excluder, no excluder) were deployed in the waters of Cedar Swamp Creek and the Mullica River -- Great Bay Estuary, New Jersey, during late summer 1997 comprising a total of 934 pot-days. We evaluated the crab catch by examining the average size of crabs, the total number of crabs and the number of crabs in various size classes using repeated measures analysis of variance.

We found no evidence that either excluder affected the average size of crabs captured or the total number of crabs captured. The 5 x 15 cm diamond-shaped excluder reduced the number of crabs > 139 mm captured in the Great Bay trial, but not in the Cedar Swamp Creek trial or when the location data were pooled. We concluded that the 5 x 15 cm rectangular excluder represents an effective and economically efficient means of reducing an important source of adult mortality for diamondback terrapins without reduced catches for commercial crabbers. Continued refinement of the design of excluder devices is possible, and greater understanding of the spatial and temporal distribution of terrapins should help direct future modification of regulations requiring excluder devices on commercial crab pots.

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