The effect of escape gaps on trap selectivity in the United Kingdom crab (Cancer pagurus L.) and lobster (Homarus gammarus (L.)) fisheries

• Published in: ICES Journal of Marine Science
• Main Author: Brown, C. G.
• Format: Text
• Language: English
• Published: Oxford University Press 1982
• Subjects: Articles; Homarus gammarus
• Online Access: http://icesjms.oxfordjournals.org/cgi/content/short/40/2/127; https://doi.org/10.1093/icesjms/40.2.127

The current United Kingdom national minimum size is 115 mm carapace width for crabs, and 80 mm carapace length2 for lobsters. Large numbers of undersized crabs and lobsters, which have to be discarded, are caught in traps. The use of escape gaps would reduce the quantity taken and therefore reduce landing of and damage to undersized shellfish. The object of this series of experiments was to find the size and shape of escape gap which would retain legal-sized crabs and lobsters but allow undersized ones to escape. For crabs, observations show that escape-gap selection depends upon body length and depth rather than carapace width; for lobsters, selection depends upon body width and depth rather than carapace length. Laboratory experiments were conducted to decide the best size and shape of gaps and the most suitable were tested in field trials under normal fishing conditions. For crabs the rectangular gap sizes tested were 38 × 74 mm, 38 × 115 mm, and 42 × 74 mm: for lobsters the gap sizes tested were 42 × 100 mm and 42 × 74 mm. All escape gaps resulted in significant decreases in numbers of undersized crabs and lobsters taken. The most successful crab escape gap measured 38 × 74 mm. This reduced the undersized catch to 34 % and increased the commercial-sized one to 125 %. The use of two such gaps per trap further increased its efficiency. The most successful lobster escape gap measured 42 × 100 mm. This reduced the undersized catch to nil and increased the commercial one to 350 %. For a mixed crab and lobster fishery the most suitable practical gap was that of 42 × 74 mm.