Variations in fish community structure during winter in the southern Yellow Sea over the period 1985-2002

Changes in the structure and composition of the fish assemblages in the southern Yellow Sea were examined based on data collected from bottom trawl surveys in winter over the period 1985-2002. Species diversity showed a decreasing tendency before 1992 and an increasing trend thereafter for the whole fish assemblage. The significant negative correlation of species diversity with relative biomass of anchovy Engraulis japonicus was observed. When the anchovy was excluded from the catches, the species diversity indices fluctuated but were relatively stable, and there was no consistently decreasing or increasing trend. The changes in species diversity indices were mainly due to the variations in the abundance of dominant species anchovy. Multivariate analysis indicated that annual differences in species composition of the whole fish assemblage in the southern Yellow Sea were mainly influenced by abundance of a few key species. Species that contributed most to the similarity within and dissimilarity between year-groups were identified. The size spectra analysis showed that the slopes tended to decrease and the intercepts increased correspondingly prior to 1992, and a reversed trend thereafter for the whole fish assemblage. The result of ANOVA showed highly significant differences for slopes and intercepts for three discrete year-groups. However, for the fish assemblage with anchovy being excluded from the catches, there was a significantly declining trend for slopes and a significantly increasing tendency for intercepts. Significantly negative correlation of slopes with fishing intensity was observed for the fish assemblage without anchovy. Mean individual weight (MIW) of most species has decreased over time as indicated by G-test despite low correlation coefficients for many of the individual correlations.