长江口桁拖网最小网目尺寸的选择性研究

Selectivity of Minimum Mesh Size of Beam Trawl in Yangzte Estuary

为完善东海区桁拖网最小网目尺寸标准,2017年6月和9月分别于长江口(30°50′~31°24′N,122°12′~122°26′E)近海使用主机功率88.2 kw和总吨位30 t的渔船进行套网法25 mm和35 mm网囊网目的渔获选择性试验,供渔政主管部门制定网目尺寸标准参考。本研究利用Logistic模型分别拟合虾蟹类优势种安氏白虾(Palaemon annandalei)、三疣梭子蟹(Portunus trituberculatus)和葛氏长臂虾(Palaemon gravieri)的选择性曲线,利用极大似然法估算模型参数。结果表明,桁拖网共捕获渔获物47种,包括鱼类23种,虾类13种,蟹类6种,其他类5种;虾蟹类优势种为安氏白虾、三疣梭子蟹和葛氏长臂虾,体长范围分别为17~45 mm、13~130 mm和18~52 mm。在25 mm标准网目尺寸下,安氏白虾、三疣梭子蟹和葛氏长臂虾的选择率为50%所对应的体长或甲宽(L_(50))分别为33.784 mm、30.850 mm和32.930 mm,选择系数(SR)分别为10.100 mm、7.978 mm和9.094 mm。各优势种的平均逃逸率均低于50%,三疣梭子蟹的逃逸率显著低于其他优势种(P<0.05)。研究表明,当网囊网目尺寸由25 mm增大至35 mm时,能够有效释放本研究海域的虾蟹类,建议适当增大现行网目尺寸标准。

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 from17 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(L_(50))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.