The Study Of The Minimum Cod-end Mesh Size Of Pair Trawl In The Yellow Sea

With the rapid development of anchovy fishery since 1990 s,the fishery of pair trawl based on the midwater trawl has become one of the important fisheries in the Yellow Sea.Because of larger trawl gears and smaller mesh size of cod-end gradually,the destruction of fishery resources and ecological environment is becoming increasingly serious.The increasing proportion of juvenile fish and smaller average body length of main economic species have aroused widespread concern in society and attracted great attention of the relevant departments.For the protection and rational utilization of fishery resources,the government has strengthened the management of marine fishing gear.The Ministry of Agriculture issued "Announcement of the Ministry of Agriculture on the minimum mesh size system of marine fishing accepted fishing gear and transitional fishing gear ",and then pair trawl was included in the marine fishing transitional gear and the minimum mesh size of cod-end was tentatively scheduled for 54 mm.The research of trawl was mainly concentrated in the bottom trawl gears,but less study on the pair trawl.There are not enough data to support and set minimum mesh size of cod-end of pair trawl in the Yellow Sea at present,so it is necessary to carry out the research on the mesh size of pair trawl in the Yellow Sea to provide a reference for determining the ownership after the transitional period and the minimum mesh size of cod-end.In order to support the implementation and management of Natural Marine Fishing Gear Catalog and have a research of cod-end mesh size of pair trawl with double wings and single cod-end,selectivity experiments with five different cod-end meshes of 40 mm,54 mm,60 mm,70 mm and 90 mm were carried out by using cover-net method in Haizhou Bay in the southern Yellow Sea in October 2015,September 2016 and August 2017.Because of error of the production in the net gear factory,the corresponding measured values of five different cod-end meshes are 30 mm,47 mm,48 mm,61 mm and 81 mm respectively which are used for calculation.The study analysed the composition of total catch and the distribution of body length of economic species,determined the dominant species of catches by IRI,and conducted the research on selectivity of mesh size.And the study analysed the escape rate of main catches,the release effect of juvenile fish and yield under different mesh size to provide a reference on the minimum mesh size of cod-end from the respect of ecological effectiveness or economic effectiveness.Finally a balance of ecological effectiveness and economic effectiveness was made with the mathematical modeling method.The results are as follows: 1.The dominant species of catches in different months is different.There are six dominant species: Scomberomorus niphonius,Stromateoides argenteus,Trichiurus lepturus,Pseudosciaena polyactis,Thrissa kammalensis and Engraulis japonicus in August.There are five dominant species: S.niphonius,S.argenteus,T.lepturus,E.japonicus and Decapterus maruadsi in September,and P.polyactis is important species.Then E.japonicusis the dominant species,S.niphonius,S.argenteus,P.polyactis,T.kammalensis and D.maruadsi are important species,and T.lepturus is common species.2.Body length of main economic species is smaller and smaller,and particularly the proportion of juvenile fish of the resources of T.lepturus and P.polyactis is still high(up to 75%~99%and92%~97% respectively).But the proportion of juvenile fish of main economic species decreases as the time goes on.3.It is estimated that the corresponding mesh sizes of cod-end allowed for main commercial fishes are 118.5 mm for S.argenteus,92.1 mm for T.lepturus,and 60.4 mm for P.polyactis.According to the results of other experimental studies in recent years,it is suggested to take the average value of the results of previous years to manage,which is more representative and more practicable.Therefore the cod-end mesh size is 111 mm to capture the large fish species,such as S.argenteus,S.niphonius;And the cod-end mesh size is 67 mm to capture the small and medium fish species,such as T.lepturus and P.polyactis.4.With the increase of cod-end mesh size,the escape rate of fish increases gradually and the release effect of juvenile fish grows significantly because the escape of fish species mainly depends on the escape of juvenile fish.Considering the ecological effectiveness,when the mesh size reaches 81 mm,the adult fish of S.argenteus and S.niphoniusare still not escaped,while the escape rates of juvenile fish of S.niphonius,S.argenteus,T.lepturus and P.polyactis are 20%,49%,82% and 94% respectively.And lower age of the resources of T.lepturus and P.polyactis is serious,so the protection for these two resources should be strengthened.5.Considering the economic effectiveness,the catch of anchovy is relatively few,the quality of catch is high and the income is high in August.When the mesh size is not less than 61 mm,the escape income in the cover-net will make up the cost of the fishing boat.In September,the catch of anchovy is also few and the income is high.By the actual trawl,the escape income of in the cover-net is relatively low and the highest percent of escape income is 5.2%.In October,the catch of anchovy is higher and the income is lower.Under every mesh size,the escape income in the cover-net can make up the cost of the fishing boat to a certain extent,and the escape income is not less than 11.8%.6.Besides,there is an effective solution to balance of ecological effectiveness and economic effectiveness by the mathematical model.When the mesh size is less than 56 mm,the release effect is remarkable.When more than 56 mm,the loss of economic effectiveness is significant.The following management suggestions are given: 1.The mesh size of 111 mm and 67 mm should be a scientific reference on the minimum cod-end mesh size of pair trawl to capture S.argenteus,S.niphoniusor other large fish species and T.lepturus,P.polyactis or other small and medium fish species respectively by the study on the selectivity of mesh size to protect the fishery resources in the Yellow Sea.2.Then closed time should be extended appropriately to reduce the pressure on the growth and reproduction of fish.3.Based on the ecological protection under the present situation of fishery resources in the Yellow Sea,the mesh size of cod-end needs to be enlarged to more than 81 mm.4.In the view of economic effectiveness,48 mm should be a reference of the mesh size in non-anchovy flood period;while anchovy trawlers should hold the chartered fishing permit issued by the local government department in the anchovy flood period.5.On the comprehensive analysis of ecological effectiveness and economic effectiveness,the mesh size of 56 mm should be a scientific reference to formulate the standard of the minimum cod-end mesh size of pair trawl in the Yellow Sea.