Based on the catch data of set net in Yangtze estuary during 1999-2007, the paper analyzed the catch composition and diversity. 1 060 488 fish individuals were collected, representing 32 species which belonged to 28 genera, 22 families and 11 orders. The most of fish species attributed to Sciaenidae. The density index CPUE decreased year by year.Coilia mystus, Collichthys lucidus, Harpodon nehereus were dominant species. The size of fish become smaller, younger, and the community structure tended to be simple according to investigate results. The marine fishes were the dominated species among the catch, and the number of migratory fishes were scare. Average characters of diversity showed that Margalef's index was 1.283 6, Shannon's index was 1.053 2, Pielou's index was 0.409 1, Simpson's index was 0.471 6. Margalef's index, Shannon's index, Pielou's index were declining, but Simpson's index was increased during the past nine years. The paper also discussed the factors which caused the decline of the fish species diversity.%根据1999～2007年长江河口区定置张网的渔获物组成,分析了渔获种类组成及其多样性.共收集鱼类样品1 060 488尾,隶属于11目,22科,28属,共32种,其中以石首鱼科鱼类最多.资源密度指数CPUE呈现年下降趋势.相对多度表明,优势种出现年际变化,凤鲚(Coilia mystus)、棘头梅童鱼(Collichthys lucidus)、龙头鱼(Harpodon nehereus)为优势种.调查结果显示,河口鱼类群落趋向个体小型化、低龄化、简单化发展.鱼类区系组成以海洋种类占优势,而洄游种类数量逐年降低.多样性指标特征值平均为:丰富度指数1.283 6,香浓-维纳多样性指数为1.053 2,均匀度指数为0.409 1,优势度指数为0.471 6.其中丰富度指数、香浓-维纳多样性指数、均匀度指数均逐年下降,优势度指数则有逐年增加趋势.同时,也探讨了引起长江口鱼类群落物种多样性降低的因素.
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