休闲渔船拖网标准化改造前后渔获物中游泳动物群落结构分析

为了解休闲渔船体验式拖网标准化改造对乐清湾游泳动物群落结构的影响，分别在2016年网具改造前和2017年网具改造后的春、夏季于乐清湾海域进行游泳动物底拖网调查。结合相对重要性指数、种类更替率、生态多样性指数和多元统计分析等方法，对网具标准化改造前后渔获物中游泳动物的组成进行分析。结果表明：网具标准化改造前后游泳动物群落组成发生了一定的变化，2年春季共有优势种为三疣梭子蟹（Portunus trituberculatus）、小黄鱼（Larimichthys polyactis），夏季共有优势种为三疣梭子蟹、小黄鱼、龙头鱼（Harpodon nehereus）和刀鲚（Coilia nasus）。网具标准化改造后：春季渔获物中游泳动物的平均丰度由5.91×10⁴ ind./km²降低至2.72×10⁴ ind./km²，夏季平均丰度由9.76×10⁴ ind./km²降低到3.45×10⁴ ind./km²；春季平均生物量由310.87 kg/km²提高至356.07 kg/km²，夏季由444.90 kg/km²提高到515.90 kg/km²；渔获物中的小黄鱼、花鲈（Lateolabrax japonicus）和刀鲚等重要经济种的平均体长显著大于改造前，部分站位的三疣梭子蟹的甲宽和小黄鱼的体长显著增大；重要经济种的幼体比例降低，小型饵料生物种类的优势度有所下降。综上所述，休闲渔船体验式拖网标准化改造对重要经济种类的幼体和饵料生物有一定的保护效果，有利于对乐清湾渔业资源的可持续利用和生境保护，但渔获物中幼体数量仍占有较高的比例，说明网具规格需要进一步改进。该结果可为休闲渔船的管理提供参考。

Analysis of the community structure of nekton in the catch before and after the standardized transformation of the recreational fishing boat trawl

Trawling activities of recreational fishing boats are part of recreational fishery activities. Fishing gears such as trawls on recreational fishing boats were required by the government to be standardized prior to summer fishing moratorium of 2017. The composition of catches would vary with the change of fishing gear and mesh size, so the standardized transformation of trawl may have a positive effect on fisheries resources and their habitats. To investigate the effects of recreational fishing boat experiential trawl standardized transformation on nekton community structure, bottom trawl surveys on composition of nekton community were conducted in Yueqing Bay. The surveys were conducted in the spring and summer of 2016 before the transformation and the spring and summer of 2017 after the transformation. The difference in average body length of captured nekton samples was tested by T-test. The composition of nekton and the effectiveness of trawl transformation were analyzed by combining some parameters such as index of relative importance, biodiversity index, and species replacement. The results showed that there was difference in species composition of the nekton via the transformation of trawl, with the evident species replacement. Portunus trituberculatus and Larimichthys polyactis were the mutual dominant species in spring through the reconstruction, and the common species were Exopalaemon carincauda. P. trituberculatus, L. polyactis, Harpodon nehereus and Coilia nasus were the mutual dominant species in the summer through the reconstruction. The number of species increased by 22 and decreased by 18, with species replacement rate of 58% among two seasons of spring. During summer, 25 species increased and 29 species decreased, and the replacement rate was 75%. Both replacement rates exceeded 50%. The abundance of nekton reduced but the biomass improved with the standardized transformation. After the transformation of nets, the average abundance of swimming animals in the spring catch decreased from 5.91×10⁴ ind./km² to 2.72×10⁴ ind./km² and the average abundance in summer from 9.76×10⁴ ind./km² reduced to 3.45×10⁴ ind./km². The average biomass increased from 310.87 kg/km² to 356.07 kg/km² in spring, and from 444.90 kg/km² to 515.90 kg/km² in summer. The average body length of several important economic species was significantly larger than that before the transformation.L. polyactis, Lateolabrax japonicus and C. nasus were greatly larger after the transformation; the body length of L. polyactis and the carapace width of P. trituberculatus increased significantly. In addition, the proportion of nekton was still high after the net transformation. In spite of that, the juvenile proportion of some important economic species fell a bit after thetransformation. For instance, as the mutual species among spring, the juvenile proportion of L. japonicus was 100% and 67% before and after the nettransformation, which decreased by 33%. But the proportions of some dominant species such as P. trituberculatus and L. polyactis were still above 90%. The dominance of prey species declined. For example, the Gobiidae''s index of relative importance decreased markedly. The standardized transformation of experiential trawl nets of recreational fishing boats had some protective effect on juvenile of important economic species. This measure was conducive to the sustainable development of fishery resources and the protection of habitats in Yueqing Bay, which promoted the development of recreational fishery to a certain extent. However, in order to protect fishery resources and habitats, the proportion of juvenile in catches still needs to be reduced.At present, the fishing activities of recreational fishing boats in China are not restricted by the system of summer fishing moratorium. Furthermore, the selectivity for catches of experiential trawls is poor. The negative influence on fishery resources and aquatic ecosystems cannot be ignored.Therefore, it is extremely urgent to formulate more scientific and reasonable management methods of recreational fishing gear. The result provides a theoretical basis for the management of recreational fishing boats.