The community structure and species diversity of fishes in the Jiulong Estuary were evaluated based on two trawling surveys conducted in May and November, 2009. Totally 35 fish species were identified, belonging to two classes, 12 orders and 22 families. According to their habitats, three species are pelagic fish, seven near-demersal fish and 25 demersal; and 10 species are estuarine and 25 marine. Based on their temperature adaptions, 22 species are warm water fish, and the rest 13 are temperate warm water fish. According to their feeding habits, seven species are lower carnivorous fish and 28 middle carnivorous fish. Arius sinensis, Coilia mystus and Odontamblyopus rubicundus were dominant species in the samples of May, and A. sinensis, C. mystus and Johnius belengerii were dominant species in the samples of November. The weight densities were 471.53 and 143.97kg·km-2, and the individual densities were 40811 and 3804ind-km~2 in May and November, respectively. The Shannon-Wiener index, Pielou index, Margalef index and Trophic index of the fishes in the Jiulong Estuary were 1.20, 0.39, 3.11 and 3.55, respectively. Compare to the survey data conducted in the same periods in 1985, the fish community structure and species diversity changed largely, and the trophic index deceased significantly in the Jiulong Estuary. Overfishing, environmental pollution and habitat loss are suggested to be the main factors contributing to the decline of fish diversity in the Jiulong Estuary.%2009年5月和11月对九龙江口进行了两个航次的拖网渔业资源调查,探讨了该河口鱼类群落结构和物种多样性特征.结果表明,两次拖网调查共鉴定鱼类35种,隶属于2纲12目22科.其中中上层鱼类3种,近底层鱼类7种,底层鱼类25种；暖水种22种,暖温种13种；河口性鱼类10种,海洋性鱼类25种；低级肉食性鱼类7种,中级肉食性鱼类28种.中华海鲇Arius sinensis、凤鲚Coilia mystus和红狼牙鰕虎鱼Odontamblyopus rubicundus 是5月的优势种,中华海鲇Arius sinensis、凤鲚Coilia mystus和叫姑鱼Johnius belengerii是11月的优势种.5月和11月鱼类重量密度分别是471.53kg·km-2和143.97kg·km-2,5月和11月尾数密度分别为40811ind·km-2和3804ind·km-2.九龙江口鱼类多样性指数为1.20,均匀度指数为0.39,丰富度指数为3.11,营养级指数为3.55.与1985年同期的调查资料相比,九龙江口鱼类群落结构和物种多样性发生了很大的变化,平均营养级明显降低.九龙江口鱼类多样性下降的主要原因可能是过度捕捞、环境污染和栖息地丧失等.
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