Environmental Impact On The Fish Assemblage Structure In Adjacent Sea Area Of The Yangtze River Estuary

Fish community characteristics during three periods2004-2007,2009-2010and2011werediscussed according to the data from the surveys conducted in the Yangtze River Estuaryadjacent sea2004to2011. The relationships of fish assemblage structure and environmentalfactors were studied by synecological analysis methods. Variation characteristics of fishassemblage before and after Three Gorges reservoir impoundent were tried to explore, and theresponse mechanisms of fish assemblage assemblages to Yangtze River runoff and sedimentwere analyzed based on comparing with the historical data of1980s and1998-2002.Fish captured in spring of2004and2007were identified to36species25orders9families,and the dominant species were Trichiurus lepturus, Setipinna taty, Engraulis japonicus,Trachurus japonicus, Larimichthys polyactis, Scomber japonicus. The fish community could beclassified into four assemblages:(1) South high-temp assemblage that represented by Scomberjaponicus;(2)South low-temp assemblage dominated by Trichiurus lepturus;(3) Northhigh-temp assemblage featured by Trachurus japonicus and (4) North low-temp assemblage waspredominant by Setipinna taty. The CCA ordination among the four assemblages and theircorrelated environmental variables revealed that water depth, bottom total phosphorus, surfacetotal suspended material (TSM), bottom salinity and surface temperature were major factorsinfluencing the fish assemblages in the area, in which depth, salinity and TSM drove the spatialvarieties of fish assemblage structures, and temperature impacted their temporal varieties.Comparing with1999-2001, fish community had performed significant variations during2004-2007: fish resources declined in fish species amount and biomass, succession happened inthe dominant species; Water depth, salinity and temperature remained the impacts on the fishassemblages, and TSM became important driving factor for their spatial structure in the YangtzeRiver Estuary adjacent sea.All the fish captured in spring of2009and2010，were indentified to42species30orders12families. The dominant species were Larimichthys polyactis, Chelidonichthys kumu, Harpadonnehereus, Setipinna taty and Pampus argenteus. The estuary environmental conditions were influenced by variation of Yangtze River runoff and sediment to sea, leaded to change in fishassemblage structures. In spring of2009, according to environmental condition of salinity inbottom water(TP-b), total phosphorus in surface water(TP-s) and Chlorophyll a(Chl a), the fishassemblage was divided into4groups. The first was coastal waters mainly inhabited by estuaryspecies, with indicator species of Chelidonichthys kumu and Coilia nasus. The second one wasoffshore aggregated by demersal fishes, with indicator species of Larimichthys polyactis. Thethird group was the central area inhabited by pelagic fishes, with indicator species of Minousmonodactylus and Trachurus japonicus, and had the highest primary production. The last wascentral south area inhabited by Engraulis japonicus, with the highest salinity in surface water. Inspring2010，with the increase of Yangtze River runoff and sediment content, the fish assemblagestructure was turned into restricted by sediment contents from limitation by nutrition condition.The fish assemblage was classified into3assemblages:(1) North coastal area, was mainlyinhabited by estuarine species and indicated by species of Setipinna taty and Vespiculatrachinoides, with the lowest salinity and highest sediment content;(2) North offshore area, wasinhabited by pelagic fish and indicated by Pampus argenteu and Lophius litulon, with highersalinity and sediment content in surface water;(3) South area, was inhabited by demersal fishand swimming fish and indicated by Harpadon nehereus and Larimichthys polyactis, with thehighest depth,salinity and primary production.In spring of2011, all the fish captured were identified to36species25orders9families,with dominant species of Coilia mystus and Larimichthys polyactis. The fish assemblage, whichwere mainly impacted by salinity in surface water and temperature in bottom water, wasclassified into3communnities:(1) Castal area was indicated by Coilia mystus, with the lowesttemperature and salinity, higher TSM and nutrients;(2) North offshore area was indicated byVespicula trachinoides, with the environmental character of medium salinity and temperatureand higher pH and COD(chemical oxgen demand);(3) South offshore area was indicated byEngraulis japonicus and Trichiurus lepturus, with higher temperature and salinity. In autumn of2011,43species29orders9families were captured, the dominant species were Trichiuruslepturus, Harpadon nehereus, Larimichthys polyactis, Pampus argenteus and Setipinna taty. Thefish assemblage structure was mainly effected by depth, total phosphorus and temperature in bottom water, and was classified into three communitie. The three communities were:(1) Northcostal area was indicated by Harpadon nehereus, with lowest depth, lower salinity, and highercontent of nutrition and sediment;(2) Centra area was indicated by Trichiurus lepturus, withmedium depth and salinity, higher pH and COD;(3) South area was indicated by Larimichthyspolyactis, with the highest depth and salinity.Compared with the fish assemblage of1980s and2000s, the fish assemblage structure in2011had changed greatly with declined fish species numbers, variation of dominant species anddeclined fish resources, and the fish assemblage in autumn had taken more obvious variation.The environmental condition had changed significantly, which caused the variation of fishassemblage. In spring, the fish assemblage had taken great change between1986,2001and2011,which was caused to some extent by the time-scale variation of TSM-b and dissolved oxygen inbottom water(DO-b) and the space variation of pH in bottom water and temperature in bottomwater. In autumn, temperature, TSM-b and Chl a were the main environmental factors that haddrove the variation between1985,2000and2011.