Selectivity of Minimum Mesh Size of Beam Trawl in Yangzte Estuary
In order to improve the minimum mesh size standard and provide theoretical basis with the fishery administration department,selectivity tests of 25 mm and 35 mm mesh sizes were carried out in the offshore of Yangtze River Estuary(30° 50'～31° 24'N,122° 12'～122° 26'E)by fishing vessels with a main engine power of 88.2 kW and a gross tonnage of 30 t in June and September of 2017.Logistic selection model was used to fit the selection curves of Palaemon annandalei,swimming crab Portunus trituberculatus and Palaemon gravieri,and maximum likelihood method was used to estimate the model parameters.The results showed that 47 species were totally caught,including 23 species of fish,13 species of shrimps,6 species of crabs and 5 species of other species.Dominant species of shrimp and crab were P.annandalei with body length ranging from 17 to 45 mm,swimming crab with body length ranging from 13 to 130 mm and P.gravieri with body length ranging from 18 to 52 mm.When the standard mesh size of codend was 25 mm,50％retention length(L50)was 33.784 mm in P.annandalei,30.850 mm in swimming crab and 32.930 mm in P.gravieri,and the selectivity ranges(SR)were 10.100 mm,7.978 mm and 9.094 mm,respectively.The scape rate of dominant species of shrimp and crab was all found to be less than 50％,with significantly lower escape rate in swimming crab than that in other dominant species(P＜0.05).The finding indicates that a large number of shrimps and crabs were released when the mesh size was increased to 35 mm,so the minimum mesh size standard of beam trawl in East China Sea needs to expand.
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