Selectivity of crab pots with escape vents and the optimal vent height for reducing the catch of sublegal swimming crab in the East China Sea pot fishery

Abstract

Catching and subsequent discarding of sublegal-sized swimming crab Portunus trituberculatus in the pot fishery may result in increased unaccounted fishing mortality and contribute to overexploitation of the crab stocks in the East China Sea. Developing technical measures and implementing related regulations to improve the size selection of pots to reduce the catch of sublegal crab has become urgent. In this study, we evaluated the size selectivity of pots with rectangular escape vents of different opening heights (25 mm, 28 mm, 30 mm and 33 mm) through comparative fishing experiments and by employing the SELECT model. The results show that the installation of escape vents significantly reduced the catch per pot (in numbers) of total and sublegal swimming crabs, and significantly increased the proportion of legal-sized crabs for most vented pots except the 25 mm vent height. A significant positive linear relationship was found between escape vent height and the 50% retention carapace width, selection range and fishing power. Increasing the height of the escape vent had substantial effect on the exploitation pattern for swimming crab and the pot with a 28 mm height escape vent seemed to be the optimal design, releasing 67.4% of sublegal-sized crab and maintaining 94.8% of legal-sized crabs. Results showing that a pot with an escape vent that significantly reduces sublegal crab catch, while maintaining catch efficiency for legal-sized crabs, is positive for the fishery and supports the implementation of escape vents in the East China crab pot fishery.