Preliminary study on community structure of mesopelagic species in the central South China Sea during autumn and winter

Based on the data collected by midwater trawl survey in the central South China Sea in autumn and winter of 2014, the community structures including composition, dominant species, and biodiversity index of mesopelagic species were analyzed, and 241 mesopelagic fish species were recorded during the survey, including 196 fish species, 21 cephalopod species and 24 crustacean species. The number of mesopelagic species in winter was higher than that in autumn; the catch per unit effort ( CPUE) in autumn, by contrast, was slightly higher than that in winter. According to the index of relative importance (IRI), 25 dominant populations (IRI100) in autumn were confirmed, including five dominant species which are Chauliodus sloani, Ceratoscopelus warmingii, Diaphus brachycephalus, Brama japonica, Diplophos taenia (IRI500); and there are three dominant species ( IRI500) among 26 dominant population (IRI100) in winter, including Oplphorus typus, C.sloani, Histioteuthis pacifica. There is no significant difference in IRI between autumn and winter. The Margalef index, Pielou index and Shannon-Weiner index calculated fromindividual number of mesopelagic species in winter were higher than those in autumn, but the Simpson index in winter was lower than that in autumn. The Margalef index and Shannon-Weiner index calculated by biomass in winter were higher than that in autumn, but the Pielou index and Simpson index was lower than that in autumn. Based on abundance-biomass comparison (ABC) curve analysis, the status of mesopelagic species community in the central South China Sea was evaluated. The result indicates that mesopelagic species community in autumn has been seriously disturbed and that in winter has been medium disturbed.