Summary It is hypothesized that fish larvae undertake vertical movements in estuarine waters. To test this hypothesis, a sampling period was selected during times when many spring‐spawning fish reach maximum abundance in coastal waters so that spring and summer variations in larval fish assemblages could be determined in the Yangtze River estuary. Six oceanographic surveys were conducted across the salinity gradient of an inshore (freshwater) and offshore area (brackish intersection) during spring (May) and summer (August) between 2010 and 2012. The fish larval community was dominated by species of Engraulidae, Gobiidae, Champsodontidae and Mugilidae. The pre‐flexion and flexion larval stages of euryhaline marine species, which are dependent on estuaries as nursery areas, were common. The brackish and marine larval assemblage was the most abundant with taxa such as Coilia mystus and Engraulis japonicus accounting for more than 57.3% of the total catch. Spatial differences in the taxonomic composition of larval fish assemblages were evident between the inshore and offshore areas. Additionally, the Yangtze River runoff regulatory functions as affected by the Three Gorges Reservoir operational mode (hydrological alternating operations) showed weak influences on fish and habitat environments. Low salinities from high freshwater inflow limited bay anchovy production in the inshore area.

Summary This study aimed to test the hypothesis that biotic influences play a role in determining estuarine fish assemblages. Thus the mechanisms that regulate the observed spatial niche segregation between fish species with morphological and ecological similarities within estuaries were investigated. Fishes were sampled seasonally at 30 stations along an entire salinity gradient of the Yangtze River estuary from spring 2010 to winter 2011, using bottom trawling (10 mm mesh size in the cod end). A total of 62 species belonging to 28 families were collected. Marine migrants (21) and estuarine species (19) dominated the assemblage and accounted for the highest abundances (marine migrants = 42.5%, estuarine species = 38.3%), with the marine migrants having the highest biomass (57.2%). Canonical correspondence analyses indicated that chlorophyll a, salinity, temperature, and geographic distance were the four main variables influencing the occurrence of fishes within the system. A stable fish assemblage in the upper zone during the wet season (spring and summer) was more obvious than in the dry season. The Pianka index showed a clear spatial segregation in four pairs of tonguesoles species (Cygonobius), and in most pairs of gobiids (Gobiidae), and a high degree of overlapping (>0.60) in only four pairs of gobiid species during the wet season. However, according to the results of the null model the observed segregating or pattern sharing was not caused by interspecific competition.

To investigate the hygienic quality of the lagoon seawater in Shanghai Jinshan City Beach (30°42 22 -30°43 06 N 121°20 40 -121°21 24 E) of North Hangzhou Bay, total coliform and fecal coliform was analyzed monthly from August 2006 to July 2007. The results showed that the densities of total coliform was no more than 1400MPN/L,the concentration is higher in July, August and September, the highest concentration of total coliform reached 1400MPN/L in September, after that, it began to decline. Monthly average fecal coliform was no more than 940MPN/L all the year round, likewise, it was relatively higher in July, August and September, but lower in winter and spring, with a lowest concentration of 20MPN/L,the highest concentration was 940MPN/L in September. From the regression analysis, it showed that coliform was positively correlated with fecal coli-group, their correlation coefficient was 0.7934. In addition, some environmental parameters including physical and chemical properties of seawater quality were also investigated. The environmental parameters were not positively correlated with both total coliform and fecal coliform. According to the Sea Water Quality Standard (GB3097-1997), the quality of seawater in Shanghai Jinshan City Beach was classified as the Second Grade and might be suitable for opening beach recreation park including swimming and other water direct contact activities.