Fish Community Structure In The Changjiang Estuary And Adjacent Waters

The characteristics of the fish assemblage and the influences of environmental factors in the Changjiang Estuary and adjacent waters were studied based on fish abundance data sampled from 1998 to 2004. Multivariate methods were adopted, including community ordination such as principal components analysis (PCA), detrended correspondence analysis (DCA) and canonical correspondence analysis (CCA), and two-ways indicator species analysis (TWINSPAN). Biological community structure and environmental factors were analyzed respectively to reveal their spatial distributions, temporal dynamics and seasonal variations. The influence degree of environmental factors on the fish community was especially discussed. The analyses results showed:Environmental factors including water depth, salinity, turbidity, transparency, nutrient and suspending matters, which formed a synthetic spatial gradient from coastal to pelagic area, were the most related to fish abundance. Higher nutrient concentration and primary productivity provided plenty of food to euryhaline and brackish-water fishes, and higher turbidity as well as dissolved oxygen made it good refuge for the fishes. Fishes in pelagic area were richer in species diversity but less abundant. The characteristics of the fish assemblages in southern and northern Changjiang Estuary were distinguished in certain seasons, and the borderline was approximately 31°30′N.Ecological and environmental factors temporally changed from 1998 to 2004, and drove the fish community succession. Before the first impoundment of the Three-Gorges Reservoir in June 2003, environmental factors influencing the fish community included water depth, bottom temperature and bottom dissolved oxygen, while those after the first impoundment included bottom and surface pH, surface temperature and surface dissolved oxygen. The water-retaining in the reservoir reduced fresh water input into the estuary, and caused the increase of salinity. A great amount of sands had sedimentated in the reservoir, and the total suspending matters in the estuary were decreased, which made the illuminative condition for phytoplankton growth improved. In correspondence, fish assemblage characteristics in both May and November had changed significantly, while marine and planktivorous fish became relatively more important in fish assemblages.Environmental factors in winter were characterized by low temperature and high dissolve oxygen concentration, while those in summer were opposite. Surface pH in spring is significantly higher than that in other seasons and temperature lower than that in autumn. Environment conditions in autumn were close to the average value of the each factor. The resultant seasonal succession happened in fish community, which appeared in fluctuation of fish abundance, replacement in species composition and dominant species, as well as periodical rise or fall in the abundance proportions of the functional groups.