Size selectivity of combined square and diamond mesh codends of shrimp beam trawl for banded scad Caranx ( Atule ) kalla in the northern South China Sea

Abstract

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, with 25 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 and 18 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 effective codend to release immature banded scad, and its 50% retention length ( L50 ) is larger than the minimum landing size (MLS=63.5 mm) of banded scad. However, the 95% confidence intervals of L50 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.