Distribution and community structure of fish in Obitsu-gawa River Estuary of inner Tokyo Bay, central Japan

The distribution and community structure of fish in Obitsu-gawa River Estuary of inner Tokyo Bay, central Japan was studied from May to December 2005 and March to April 2006. A total of 19,006 individuals, represented by 25 species and some unidentified species under family Clupeidae, Cyprinidae, Gobiidae, Hemiramphidae, Mugilidae, Platycephidae, Pleuronectidae and Triglidae were collected. Family Gobiidae had the most number of taxa with 13 genera and 10 species. Greatest fish abundance happened in August and secondarily in April and May. Species richness was evident in the warmer months particularly in May (17 taxa), August (21 taxa), September (15 taxa) and October (17 taxa). Marine teleosts significantly contributed to the species richness and abundance of fish, which corresponded to 52.9% (10,046 individuals) of the total catch while the estuarine fishes were the second most abundant group with 33.5% (6,372 individuals) of the total catch. Species dominance was a coherent feature of this community. The proportional contribution of marine teleosts to the fish community decreased with increase distance upstream while that of estuarine fishes increased with increase distance upstream. The developmental stages of gobies range from larvae to adult but juveniles constitute 77.06% of the total sample. The distribution of developmental stage of estuarine gobies was influenced to a greater extent by variation in monthly water temperature and station or the interaction of both. Adult estuarine gobies had the tendency to aggregate in the middle estuary reflecting their high tolerance to a wide range of water salinity inherent in this station but avoided the lower estuary most likely due to the predominance of high salinity waters.