Investigation into the cause of eruptive death of fish occurring in Xuanwu Lake, Nanjing

南京玄武湖近年来死鱼事件频发,对该湖水生生态系统的长期监测结果表明,湖水富营养化和沉积物污染十分严重;但生态毒理学研究表明,湖水和沉积物等对鱼类尚未见有急性毒性作用.根据玄武湖鱼类死亡症状,从玄武湖鱼体上分离到一种致病力极强的鱼类病原菌──嗜水气单胞菌(Aeromonas hydrophila),研究了该病原菌的致病特性,并系统地分析了水质主要污染指标、水体富营养化、藻类种群动态、沉积物污染及其氧化还原层(Eh,+200mV)等多种因素对玄武湖鱼类暴发性死亡的发生和流行的影响.研究结果表明,近年来玄武湖频频发生的暴发性死鱼多系病原菌感染鱼体引起的鱼类暴发性出血病(鱼类环境病)所致,而该湖水体长期处于严重富营养和有机污染状态以及年久未清的沉积物等是这种鱼类环境病的诱发因素.;Although the result of long-time detail ecological monitoring shows that the pollution of water and sediment is serious, there is no acute toxicity to fish in the aquatic ecosystems of Xuanwu Lake. This was proved by toxicity tests(in situ and in laboratory) and simulation tests (simulated the lake water and sediment). The pathogenic bacteria, Aennnonas hydrophila, were isolated from silver and variegated carp of the lake, and is believed to be the responsible pathogen. From the results of determining germ virulence and observing the symptoms of fish inection it can be concluded that the cause of fish eruptive death is a kind of bacteriosis caused by A. hydrophila. The results of comprehensive analyses have drawn the following conclusion. In the summer when the redox shielding level(Eh, +200mV) is close to the surface of the sediment, the introduction of pathogenic bacteria from the sediment into the water occurs readily. The dominant population, Merismopedia, is not a preference bait for fish. On the contrary, it makes fish struggle along on the verge of starvation during the summer. In addition, terrible pollution of water and sediment may not only destroy the antibody systems of fish but also effect the dynamic of pathogenic bacteria. Therefore, this bacteriosis is an environmental disease.