Abstract:In this study, 10 typical types of Scomberomorus niphonius gillnets used in the Bohai Sea and the Yellow Sea area were identified and investigated. Based on the analysis of the main parameters of the 10 net types mentioned above, we selected nets with 4 different mesh sizes (110 mm, 115 mm, 120 mm, and 125 mm) to perform a comparative experiment on the fishing boat “Luchangyu-64068” operated in the central and southern Yellow Sea from 10 April to 15 May in 2013. The highest number and weight of fish were caught with net of mesh size 120 mm, and the CPUE was 0.1696 ind/webbing/day and 373.78 g/webbing/day respectively. The fishing efficiency of 120 mm mesh size was also the highest among the 4 studied sizes. Further optimization of main parameters of big-mesh gillnets for S. niphonius were conducted on fishing vessels “Luchangyu-64068” and “Lulaizhouyu-66287” in the central and southern Yellow Sea, offshore of Yantai, Weihai and Laizhou Bay, from 12 April to 27 May in 2015. Using the net of 120 mm mesh size on “Luchangyu-64068”, we caught S. niphonius with the mean fork length of 764.18 mm and mean body weight of 3623.27 g. Among them the longest was 1000 mm; the shortest was 490 mm; the heaviest individual weighed 7500 g, and the lightest was 850 g. The CPUE was 0.1379 ind/webbing/day and 517.65 g/webbing/day. “Lulaizhouyu-66287” employed net of 122 mm mesh size, and the mean fork length and mean body weight of fish caught was 780.47 mm and 4065.17 g respectively. The longest was 910 mm; the shortest was 600 mm; the heaviest individual was 7400 g; the lightest was 2000 g. The CPUE was 0.0778 ind/webbing/day and 283.35 g/webbing/day. Because nets of 120 mm mesh size showed higher fishing efficiency than that of 122 mm, it should be more suitable for catching the spring cohort of S. niphonius. The main parameters of a net of 120 mm mesh size were as follows: 33.00 m×18.54 m in the principal dimension of fishing gear, 500 mesh×175 mesh, 120 mm in mesh size, PA monofilament (Φ=0.48 mm) as the netting material, T-direction hanging ratio 0.55, static buoyancy distribution 267.9 g/m, and buoyancy to sinker-weight ratio 1.768∶1. Due to the replacement of the upper hanging line and float line by decreasing the mesh size of cork line selvedge strip, less net material was need to maintain the major structure and performance of the net. The proper mesh size of cork line selvedge strip should be equal to mesh size of main netting multiplied by T-direction hanging ratio of main netting.
