Studies On Ichthyoplankton Community Structures In The Yangtze Estuary And Its Adjacent Waters

The structures of ichthyoplankton community in the Yangtze Estuary, based on seven surveys about ichthyoplankton and environmental factors, were studied during 1999-2007, species compostion, abundance variety, community diversity and spatio-temporal patterns, The structure variations between before and after Three Gorges Project (TGP) impoundment were discussed. The study results will provide scientific evidence about the effect of Three Gorges Project on the ecological environment and fisheries recruitment resource in the Yangtze Estuary and its adjacent waters. The results are as follows:During May, 1999 and 2001 before Three Gorges Project (TGP) impoundment in the Yangtze Estuary and adjacent sea (30°45′-32°00′N, 122°20′-123°20′E), the Spring ichthyoplankton assemblage was studied. A total of 11 540 ichthyoplankton individuals were taxonomically identified, belonging to 11 orders, 18 families and 32 species, of which Coilia mystus, Engraulis japonicus, Chaeturichthys hexanema, Allanetta bleekeri and Trachidermis fasciatus were dominant species. The ichthyoplankton communities can be classified into three assemblages: (1) estuarine assemblage that dominated by Coilia mystus; (2) coastal assemblage dominated by Allanetta bleekeri and Trachidermis fasciatus; and (3) shelf assemblage featured by Engraulis japonicus and Chaeturichthys hexanema. Salinity, depth, dissolved oxygen, and total suspended particulate matter were major factors influencing the ichthyoplankton assemblage.During May , 2004 and 2007 after Three Gorges Project (TGP) impoundment, salinity, pH and transparency increased, but temperature, chemical oxygen demand (COD), nutrient decreased, the suspended particulate matter (SPM) in riverine water and estuarine mouth became diminished. There were significant differences in temperature, total nitrogen, dissolved oxygen, pH and Chlorophyll a between 2004 and 2007. The distributed patterns of environmental factors in the investigation area were still similar to the environmental situation before the TGP impoundment, but the quantitative differences were lodged in some environmental factors: dissolved oxygen, COD, and SPM were lower, and salinity, total nitrogen, total phosphorus and pH were higher. A total of 1070 ichthyoplankton individuals were taxonomically identified, belonging to 10 orders, 15 families and 25 species, of which Allanetta bleekeri, Coilia mystus, Trachidermis fasciatus, and Engraulis japonicus were dominant species, contributing 94.57% of the total abundance. The ichthyoplankton abundance of 2004 and 2007 rapidly decreased, with 10% of total abundance before TGP impoundment. Salinity and SPM content were major factors influencing the ichthyoplankton assemblage. Then, the ichthyoplankton were classified into three assemblages: estuarine, coastal and shelf assemblage, same as before TGP impoundment. But the distribution areas of the assemblages are shifted: the space of coastal assemblage was reduced and the distribution area of shelf assemblage extended to the estuarine mouth. In all environmental variables, salinity and SPM content were important influence factors, driving the ichthyoplankton assemblage variance between before and after TGP impoundment.Spatial and temporal variations of ichthyoplankton community structures before and after impoundment of TGP in the Yangtze Estuary and adjacent sea were analyzed during Spring of 1999,2001,2004 and 2007. 39 species, consisting of freshwater, brackish, coastal and marine forms, were collected in trawl nets. The dominant species comprised Engraulis japonicus, Coilia mystus, Allanetta bleekeri, Trachidermis fasciatus and Chaeturichthys hexanema. The dominancy of Engraulis japonicus and Coilia mystus reduced in 2004. There was significant dissimilarity within four year's ichthyoplankton community structures, and the dissimilarity in 2004 was higher, comparied with 1999 and 2001. Similarity of percentage analysis showed that Engraulis japonicus and Coilia mystus represented the highest contributions in the community structure variations. Moreover, the similiarity index of ichthyoplankton was highest between 2004 and 2007 with the value 71.17%; Ichthyoplankton community diversities of before and after impoundment of TGP was significantly different in the Yangtze Estuary, and the diversity of after impoundment evidently decreased. A series of environmental disturbance, which happened in the Yangtze Estuary and adjacent water during 1999-2007, directly or indirectly aroused the variation of ichthyoplankton community structures.The composition and diversity of ichthyoplankton species in the Yangtze estuary and adjacent waters were analyzed based on four seasonal surveys during 2007. A total of 6,463 individuals, representing 12 orders, 28 families and 45 species, were collected. Engraulidae was the most abundant family comprising 76.5% of all catch. The ichthyoplankton community was composed of four ecological guilds: freshwater (2 species), brackish (14 species), coastal (10 species) and marine (16 species), of which marine species was the most abundant (74.6% of total abundance). Engraulis japonicus, captured in every season, contributed most to total ichthyoplanton abundance. Species composition varied between seasons; in Spring, Allanetta bleekeri was the dominant species, while in Summer E. japonicus dominated, Autumn dominant species were E. japonicus and Hemisalanx prognathus. These dominant species contributed more than 89% of total catch in different seasons. Species number, abundance and diversity peaked in Summer, were lower in Spring and Autumn, and were least in Winter. These seasonal variations resulted from both migration associated with fish spawning and the environmental influence. A persistent spatial structuring of the estuarine community in three area: estuarine, coast and shelf , while changes in distribution area of each group were related to changes in the input of the freshwater input in different seasonal periods. Salinity was the main factor associated with spatial differences distribution, and temperature has the strongest influence on seasonal structure of ichthyoplankton community. Both spawing migration and feeding migration appear as important factors affecting the temporal structure of the estuarine community.