Evolution of fish community structure, cause analysis and implications for <i>ex-situ</i> conservation of Yangtze finless porpoise in Tian-e-Zhou Oxbow of Yangtze River

Abstract

长江天鹅洲故道是长江江豚首个迁地保护区，其中充足而适口的饵料鱼类资源是长江江豚种群健康发展的保障。为此，根据2017年10月-2021年4月对长江天鹅洲故道进行的7次鱼类调查数据，结合禁捕前的历史数据，对其鱼类群落结构演变进行了分析，并针对长江江豚饵料鱼的供应状况对天鹅洲故道内鱼类资源管理提出了建议。调查期间，共采集到鱼类57种，其中，蒙古鲌（Culter mongolicus）、鲢（Hypophthalmichthys molitrix）和鳙（Aristichthys nobilis）等大中型鱼类生物量明显增加，而䱗（Hemiculter leucisculus）和似鳊（Pseudobrama simoni）等小型鱼类生物量呈现下降趋势。鱼类群落优势种逐渐由太湖新银鱼（Neosalanx taihuensis）、短颌鲚（Coilia brachygnathus）和䱗等小型鱼类转变为鳙、蒙古鲌和拟尖头鲌（Culter oxycephaloides）等大中型鱼类，尤其是新近出现的拟尖头鲌种群发展迅速。同时，短颌鲚和䱗等小型鱼类的体重在减小，而鲢、鳙等大中型鱼类规格在持续提高。长江江豚以小型鱼类和大中型鱼类的幼鱼为食，其资源下降将威胁长江江豚种群的维持和发展。为保障天鹅洲故道内长江江豚的食物供应，建议适当地捕捞鲢、鳙、蒙古鲌和拟尖头鲌，减缓鲢、鳙对小型鱼类的食物竞争压力，以及降低蒙古鲌和拟尖头鲌对小型鱼类的捕食压力；同时丰水期加强故道与长江的连通，引入更多长江干流繁殖的小型鱼类资源；并通过建设人工鱼巢以改善故道内䱗等产粘性卵小型鱼类的繁殖条件，增加小型鱼类资源量。;Tian-e-Zhou Oxbow is the first habitats for the ex-situ reserve established for the Yangtze finless porpoise. The rich and abundant forage fish resources are the guarantee for the healthy development of the Yangtze finless porpoise population. Therefore, the change of fish community structure was analyzed with combined of seven fish surveys data in Tian-e-Zhou Oxbow of Yangtze River from October 2017 to April 2021 and historical data before the fishing ban. In view of forage fish of Yangtze finless porpoise, some suggestions were provided for the scientific management of fish resources in Tian-e-Zhou Oxbow. During the investigation, 57 fish species were collected. The biomass of large and medium-sized fish such as Culter mongolicus, silver carp and bighead carp increased significantly while the biomass of small-sized fish such as Hemiculter leucisculus and Pseudobrama Simoni showed a declining trend. The dominant fish species gradually changed from small-sized fish such as Neosalanx taihuensis, Coilia brachygnathus and Hemiculter leucisculus to large and medium-sized fish such as bighead carp, Culter mongolicus and Culter oxycephaloides, in particular, the newly emerged Culter oxycephaloides population developed rapidly. Meanwhile, the body-weight of small-sized fish such as Coilia brachygnathus and Hemiculter leucisculus was decreasing, but the body-weight of large and medium-sized fish was increasing. Because the Yangtze finless porpoise feeds on the small-sized fishes and juveniles of large and medium-sized fishes, the decline of their food resources would threaten the maintenance and growth of the Yangtze finless porpoise population. In order to ensure food supply for the Yangtze finless porpoises in Tian-e-Zhou Oxbow, it suggested that rational capture of silver carp, bighead carp, Culter mongolicus and Culter oxycephaloides should be taken into consideration in order to reduce the food competition pressure of silver carp and bighead carp and the predation pressure of Culter mongolicus and Culter oxycephaloides on small-sized fishes. Besides, another measures to increase the small-sized fish resources like strengthening the connectivity of the oxbow with Yangtze River in the wet season to introduce more small-sized fish resources from Yangtze River and laying artificial fish nests in the oxbow to improve the breeding conditions for the small-sized fish species with adhesive eggs such as sharply should be carried out.