南海区双船大网目拖网网囊网目对带鱼的选择性

大网目拖网是我国海洋渔业捕捞带鱼的重要渔具。为提高拖网网囊选择性, 本研究在南海区采用套网法对双船大网目拖网 4 组网目尺寸(30、35、40 和 45 mm)的网囊进行实验, 在双重拔靴法的架构下, 拟合选择性参数、比较选择性的差异、估算捕捞方式指标。结果表明, 随着网目尺寸的增大, 网囊对带鱼的 50%选择体长增大, 对带鱼幼鱼个体的选择率显著下降, 带鱼幼鱼的渔获比例也呈下降趋势。结合带鱼最小可捕规格(23.0 cm)管理规定, 结果还表明网目尺寸为 40 mm 的网囊选择性较差, 其对带鱼的抛弃率大于 93%。为了进一步提高带鱼拖网渔业的资源养护水平, 还需要开展更多的选择性实验。

Codend size selectivity of pair trawls with a large mesh-size in the forefront section for largehead hairtail in the South China Sea

Bottom trawls with large mesh sizes in the forefront section are commonly used to target the largehead hairtail (Trichiurus lepturus), which is one of the most ecologically and economically important fish species in marine capture fisheries in China. In this study, the size selectivity of four diamond-mesh codends with mesh sizes of 30, 35, 40, and 45 mm was tested using the covered codend method in pair trawls with large mesh sizes at the forefront section of the South China Sea. The experimental codends were termed D30, D35, D40, and D45 according to their mesh sizes. Catch data were analyzed using the double-bootstrapping technique to account for uncertainties from both within- and between-haul variations, compare the size selectivity of tested codends, and estimate the exploitation pattern indicators for largehead hairtail. The results demonstrated that by increasing the mesh sizes of the codends, the 50% retention length (L50) of the largehead hairtail increased, whereas the capture probability and exploitation pattern indicators for undersized individuals decreased. For instance, the L50 was 12.22 cm for the D30 codend, and increased to 13.24 cm for the D45 codend. The retention proportion of undersized individuals was 57.10% for the D30 codend and decreased to 40.06% for the D45 codend. The differences in the L50 and exploitation pattern indicators between the tested codends were not statistically significant. Differences in the retention probability of undersized individuals in some length ranges between the codends were statistically significant. Considering that the mesh size used in trawling fisheries is usually small, often close to 30 mm, our results demonstrated that increasing the mesh size from 30 to 40 or 45 mm would improve size selectivity for the target species. The L50 values increased, whereas the retention probability of the undersized largehead hairtail decreased. However, the size selectivity of the codend with a mesh size of 40 mm (D40), currently regulated by the government, was poor for the target species under the management regulation scenarios of the minimum landing size (23.0 cm). Consequently, over 93% of the largehead hairtail were undersized and thus discarded. This finding suggests that more selectivity studies are needed to further improve the selective properties and achieve sustainable development of the bottom trawl fishery. Additionally, research on the minimum landing size for largehead hairtails is warranted to supplement the minimum mesh regulations in the South China Sea.