Variation in fish community structure in Beibu Gulf before and after La Niña event

La Niña events will affect the life history and diversity of fish, leading to the succession of fish community structure in different sea areas. To understand the potential impact of La Niña events on the fish community structure in the Beibu Gulf, based on the bottom trawl survey data of fishery resources in July 2006 and July 2008, we analyzed the variation in the fish community structure in the Beibu Gulf before and after La Niña event. The results show that the dominant species were Acropoma japonicum and Trachurus japonicus in summer of 2006, and T. japonicus, Decapterus maruadsi and A. japonicum in summer of 2008. T. japonicus and A. japonicum were the common dominant species in the two years. The diversity index (H') and evenness index (E₁ and E₂) did not differ significantly between years, but the richness index (D') differed significantly. After the La Niña event, the density of warm-water fish resources, such as A. japonicum, decreased, while that of warm-water fish resources, such as T. japonicus and D. maruadsi, increased. Besides, T. japonicus and D. maruadsi moved towards the coast of the Beibu Gulf. The diversity index of the Beibu Gulf coastal sites (C1–C22) was significantly different before and after the La Niña event (P<0.05). The results of cluster analysis and non-metric multidimensional scale ranking show that the fish community spatial distribution was more stable, and the main difference was in the western coast of Hainan Island. The La Niña event resulted in a decrease in water temperature and an increase in primary productivity in the Beibu Gulf, causing an increase of the density in small pelagic fish stocks such as T. japonicus and D. maruadsi. In conclusion, when the fishing pressure is relatively stable, the significant variation in the fish community structure in the Beibu Gulf is closely related to the La Niña event.