Diversity of fish species and its spatio-temporal variations in Lake Dongting, a large Yangtze-connected lake

To evaluate the fish diversity of Yangtze River-connected lakes, a monthly survey offish assemblage was carried out in the Lake Dongting in the middle basin of the Yangtze River in March-December, 2004 and May, 2005. Three sampling areas along a transect from the Yangtze River mainstream to the lake center, i.e. from lake mouth (Chenglinji) to East Lake Dongting (Yueyang) and South Lake Dongting (Yuanjiang) were selected. Totally 69 fish species belonging to 6 orders, 14 families and 44 genera were recorded. Cyprinids were the richest, accounting for 59.4% of total fish species. Species numbers and diversity indices showed that fish diversity of the lake was high, with great spatial and temporal variations. Generally, higher diversities occurred in spring and summer than in autumn and winter, and in South Lake Dongting than in the other areas. With regard to species numbers, the smaller the distance between sampling area and Yangtze River mainstream was, the earlier the diversity peak was reached. Dominant species were determined as species with the value of dominance over 5000. There were 17 dominant species in the lake among which 80.0% were limnicolous species. With regard to ecological groups, limnicolous species dominated the lake, accounting for 74.0% of the total species, while river-lake migratory species was the least, being 13.0% of the total. In comparison with that of 1970s, fish diversity of Lake Dongting had been dramatically decreased, especially for migratory species, and it was mainly due to the loss of habitats, decreased fry resource and over-fishing. Analysis also showed that fish diversities in Yangtze River-connected lakes were much higher than those of disconnected lakes, indicating that river-lake isolation results in species decreasing. Thus, increasing river-lake connectivities is important to the conservation of fish diversity in the Yangtze River floodplain.