Fish diversity in southwestern seas of Nansha Islands and the mouth of Beibu Bay

Based on fixed-point survey data in southwestern seas of Nansha Islands and the mouth of Beibu Bay in the spring and autumn of 2012 and 2013, we calculated the composition of fish species, index of relative importance (IRI) of each species, Margalef richness index (D), Shannon-Wiener diversity index (H'), Pielou evenness index (J'), replacement index and migration index of the fish community.Results showed that a total of 504 species were identified involving 2 classes, 31 orders, 129 families, and 294 genera in the surveyed areas.There were 301 fish species in the mouth of Beibu Bay and 357 species in southwestern seas of Nansha Islands.The number of dominant species in autumn was higher than that in spring, and the number in the southwestern seas of Nansha Islands (SS) was higher than that of the mouth of Beibu Bay (SN), with an obvious seasonal fluctuation.All diversity indexes during the spring survey were higher than those of the autumn survey because of recruitment in spring and the movement of fish species to the deep sea in autumn.However, all diversity indexes of SS were higher than that of SN due to the larger effects of sea temperature and currents in the southwestern seas of Nansha Islands.Results of the replacement index and migration index showed that the fish community structure in autumn was less stable than that in spring and the community structure in the both seasons deviated from equilibrium because of fish migration and immigration and emigration of fishes with different thermophily.The integrated results showed that the species diversity and stability of fish community structure of SS were greater than those of SN.The fishery allowable catch shouldbe controlled in a reasonable range and the fishery resources of the central and southern South China Sea should be exploited reasonably under the national supported policies.The stability of fisheries in the South China Sea is based on the high species diversity, therefore, it is also important to protect the diversity of species.