Individual study: Impact of a bycatch reduction device on diamondback terrapin Malaclemys terrapin and blue crab Callinectes sapidus capture in crab pots in Patuxent River, Maryland, USA
Roosenburg W.M. & Green J.P. (2000) Impact of a bycatch reduction device on diamondback terrapin and blue crab capture in crab pots. Ecological Applications, 10, 882-889
In this study, the efectiveness of a bycatch reduction device (BRD), developed by Roger Wood at Stockton State College, that reduces the capture of turtles in commercial crab pots was tested. The two main objectives were: to determine the dimensions of a BRD that would be most effective at preventing diamondblack terrapin Malaclemys terrapin from entering crab pots; and to evaluate the impact of the BRD on the number and size of crabs caught.
Study site: The study was undertaken in 1996-1997 on the Patuxent River, a tributary of Chesapeake Bay.
Crab pots: Standard crab pots (60 x 60 x 60 cm) with and without 11-gauge galvanized wire BRDs were used in low terrapin density areas (open river, 1-4 m deep). Tall crab pots (60 x 60 x 120 cm) with and without 11-gauge galvanized wire BRDs were used in high terrapin density areas (shallow creeks, <1 m deep). Three different BRDs were used: 4 x 10, 4.5 x 12 and 5 x 10. Pots with no BRD attached were used as controls. A total of 28 pots were trialed in 1996, and 30 in 1997. The number and size of the crab pots and BRDs used are shown in Table 1 (attached). Crab pots were used for a total of 50 d in 1996 and 42 d in 1997, but the number of days each pot type was used varied.
Crab & terrapin measurements: Caught crabs were categorised into standard commercial grades and were measured for length, height and width, and sexed. Size (weight, straight-line plastron and carapace length), sex and age data from captured terrapins was also recorded.
The results indicate that the 4.5 x 12 cm bycatch reduction device represents the most feasible solution for preventing terrapins and other bycatch from entering crab pots, whilst be acceptable to crab fishers. The 4.5 x 12 cm BRD (19 terrapins captured) was considerably more effective at reducing the number of terrapins (reduced terrapin bycatch by 82%) caught than the 5 x 10 cm BRD (56 captured, reduced terrapin bycatch by 47%). Neither the 4.5 x 12 cm or 5 x 10 cm BRDs reduced the number or size of crabs caught.
Whilst the 4 x 10 BRD effectly prevented all incidental terrapin captures (0 captures) it reduced the crab catch considerably in both standard and tall pots, excluding the largest size class of crabs. This BRD would therefore be unacceptable to crab fishers.
The smaller, fixed opening created by a BRD may infact increase effectiveness by keeping crabs trapped inside the pot. Additionally, the wire funnel entrance into crab pots is flexible, such that crabs may be able to move the wire and escape, but installation of a BRD rigidly fixes the aperture of the funnel. Furthermore, the BRD excludes other bycatch, such as conchs and spider crabs, which have been shown to reduce blue crab catch.
Conclusions: Attachment of the 4.5 x 12 cm BRD on crab pots both for commercial and recreational crab fishing is recommended as a low-cost effective method to reduce terrapin mortality whilst not impacting on the efficiency of pots at catching blue crabs.
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