DIVERSITY OF FISH COMMUNITY STRUCTURE IN THE SPRING-TIME OFFSHORE WATERS AT THE YANGTZE RIVER ESTUARY

Based on the bottom trawl survey data in May, 2007 and May and June, 2008 in the Changjiang River estuary offshore waters, the species composition, variation of biomass, diversity of community structure and size spectra of dominant species were analyzed. There were 187 species of fishes, which were mainly of warm temperate and warm water species, and to a less extent offshore species and coastal species. Trichiurus haumela was the absolutely dominant species in the three surveys, and its biomass was beyond 50% of the total catch. The other dominant species, excluding Pseudosciaena polyactis, Psenopsis anomala and Pampus argenteus, were small-size noncommercial species. In addition, cold temperate species Champsodon capensis became a dominant species in the Changjiang River estuary offshore waters. Higher diversity of fish community structure mainly distributed around the 50-m isobath in May 2007, and between the 50-m and 100-m isobath in May and June 2008. The higher diversity position in the Changjiang River Estuary in May 2008 moved northeastwards relative to the position in May 2007. The size spectra of fish community were mainly from 3 to 33cm. In particular, the size spectra of dominant species, such as Trichiurus haumela, Pseudosciaena polyactis and Trachurus japonicus, were narrower; and their populations were characterized by small size individuals, especially in June, 2008, indicating that the recruitment population dominated the biomass of fishery resource in the Changjiang River estuary offshore waters. The succession and heterogeneity of distribution in fish community were the results of natural selection under the control of outside disturbance, and were also a strategy that were helpful in ensuring fish survival, reproduction and maintaining the equilibrium of the ecosystem.