The shrimp beam trawl fishery in the northern South China Sea is characterized by high by-catch and poor selectivity problems.Among by-catch species,banded scad(Caranx(Atule)kalla)is economically the most important on...The shrimp beam trawl fishery in the northern South China Sea is characterized by high by-catch and poor selectivity problems.Among by-catch species,banded scad(Caranx(Atule)kalla)is economically the most important one.The aim of this study is to improve the selective properties of codends for banded scad.Size selectivity of 2 traditional diamond mesh codends,with mesh size 25 and 30 mm(defined as D25 and D30,respectively),and 4 new combined square and diamond mesh codends,with25 mm square-mesh and 25 mm diamond-mesh(S25+D25),30 mm square-mesh and 25 mm diamondmesh(S30+D25),35 mm square-mesh and 25 mm diamond-mesh(S35+D25),and 35 mm square-mesh and18 mm diamond-mesh(S35+D18),were tested for banded scad in shrimp beam trawl fishery of the South China Sea.A total of 54 valid hauls were finished using the covered codend method,and 5750 banded scad were caught.Selective parameters were obtained using the logistic equation with the maximum likelihood method,by incorporating the between-haul variation.The results show that the present minimum mesh size for shrimp beam trawl in the South China Sea,25-mm diamond mesh size,is insufficient to release immature banded scad.The S35+D25 codend is proved to be the most eff ective codend to release immature banded scad,and its 50%retention length(L _(50))is larger than the minimum landing size(MLS=63.5 mm)of banded scad.However,the 95%confidence intervals of L _(50) overlap among the three combined mesh codends,the S25+D25,S30+D25 and S35+D25 codend.For a sustainable exploitation of banded scad,larger mesh sizes are suggested to be investigated for the combined mesh codends.展开更多
The size selectivity and usability of three codends were quantified and compared for the first time in the inshore Northern shrimp(Pandalus borealis)trawl fishery of Iceland using the covered codend method:a conventio...The size selectivity and usability of three codends were quantified and compared for the first time in the inshore Northern shrimp(Pandalus borealis)trawl fishery of Iceland using the covered codend method:a conventional diamond-mesh codend(T0),conventional square-mesh codend(T45),and a 90◦turned mesh codend(T90)constructed of four panels and with shortened lastridge ropes.Fishers,wanting to increase the average-individual size of captured shrimp,had requested the T90 codend to be compared with conventional codends for consideration in the fishery.Results showed that,on average,the T45 and T90 codends had better size selectivity than the T0 codend in terms of releasing individuals smaller than 13 mm carapace length(Minimum References Size;MRS).The T90 codend retained significantly less Northern shrimps between 9 and 19 mm than the T0 codend and between 15 and 19 mm than the T45 codend.No significant difference of size selectivity between T45 and T0 codends was observed.All three codends presented high retention ratios of Northern shrimps above MRS(>63%)for the population encountered.However,the T0 codend was not effective at sorting out small Northern shrimps;at least 86%of Northern shrimps smaller than 13 mm were retained in the T0 codend if encountered.Catches from T45 and T90 codends had a lower proportion of shrimp below MRS.Since discarding of undersized Northern shrimps is prohibited in Iceland and fishers wanted to catch on average larger shrimp,using the novel T90 codend would enable fishers to use their quotas more efficiently.展开更多
基金Supported by the Special Fund for Agro-scientific Research in the Public Interest(No.201203018)
文摘The shrimp beam trawl fishery in the northern South China Sea is characterized by high by-catch and poor selectivity problems.Among by-catch species,banded scad(Caranx(Atule)kalla)is economically the most important one.The aim of this study is to improve the selective properties of codends for banded scad.Size selectivity of 2 traditional diamond mesh codends,with mesh size 25 and 30 mm(defined as D25 and D30,respectively),and 4 new combined square and diamond mesh codends,with25 mm square-mesh and 25 mm diamond-mesh(S25+D25),30 mm square-mesh and 25 mm diamondmesh(S30+D25),35 mm square-mesh and 25 mm diamond-mesh(S35+D25),and 35 mm square-mesh and18 mm diamond-mesh(S35+D18),were tested for banded scad in shrimp beam trawl fishery of the South China Sea.A total of 54 valid hauls were finished using the covered codend method,and 5750 banded scad were caught.Selective parameters were obtained using the logistic equation with the maximum likelihood method,by incorporating the between-haul variation.The results show that the present minimum mesh size for shrimp beam trawl in the South China Sea,25-mm diamond mesh size,is insufficient to release immature banded scad.The S35+D25 codend is proved to be the most eff ective codend to release immature banded scad,and its 50%retention length(L _(50))is larger than the minimum landing size(MLS=63.5 mm)of banded scad.However,the 95%confidence intervals of L _(50) overlap among the three combined mesh codends,the S25+D25,S30+D25 and S35+D25 codend.For a sustainable exploitation of banded scad,larger mesh sizes are suggested to be investigated for the combined mesh codends.
文摘The size selectivity and usability of three codends were quantified and compared for the first time in the inshore Northern shrimp(Pandalus borealis)trawl fishery of Iceland using the covered codend method:a conventional diamond-mesh codend(T0),conventional square-mesh codend(T45),and a 90◦turned mesh codend(T90)constructed of four panels and with shortened lastridge ropes.Fishers,wanting to increase the average-individual size of captured shrimp,had requested the T90 codend to be compared with conventional codends for consideration in the fishery.Results showed that,on average,the T45 and T90 codends had better size selectivity than the T0 codend in terms of releasing individuals smaller than 13 mm carapace length(Minimum References Size;MRS).The T90 codend retained significantly less Northern shrimps between 9 and 19 mm than the T0 codend and between 15 and 19 mm than the T45 codend.No significant difference of size selectivity between T45 and T0 codends was observed.All three codends presented high retention ratios of Northern shrimps above MRS(>63%)for the population encountered.However,the T0 codend was not effective at sorting out small Northern shrimps;at least 86%of Northern shrimps smaller than 13 mm were retained in the T0 codend if encountered.Catches from T45 and T90 codends had a lower proportion of shrimp below MRS.Since discarding of undersized Northern shrimps is prohibited in Iceland and fishers wanted to catch on average larger shrimp,using the novel T90 codend would enable fishers to use their quotas more efficiently.
