Abstract: Deep longline surveys were conducted in the South China Sea from June 2010 to February 2013. The composition, quantity and seasonal variation of fish species were reported using hooking rate and catch per unit effort as index. A total of 33 species were identified, of which six species including Thunnus obesus, Lampris guttatus, Alepisaurus ferox, Scoliodon Palasorrah, Taractichthys steindachneri, and Xiphias gladius were the most abundant ones taking 75.56% of the total weight, and of which five species such as Alepisaurus ferox, Gempylus serpens, Taractichthys steindachneri, Dasyatis atratus, and Thunnus obesus were the most abundant ones taking 82.59% of the total number. The species compositions of the catches did not change with season, but more Thunnus alalunga, Xiphias gladius and Lepidocybium flavobrunneum were caught in winter. The average hooking rate was higher in central western Nansha waters than in other sea areas in all four seasons. The average CPUE was higher in central western Nansha waters in spring and summer, in western Xisha waters in fall and in southeastern waters off Hainan Island in winter. Fish species could be classified into shallow-water group and deep-water group according to catching depth, and 87.33% of the catches came from 80 to 280 m water depths. There are two reasons for low hooking rate with comparison to previous surveys and productions. One is that survey area coverage was not large enough to make us find better fishing ground; the other is that we could not adjust fishing gear and method effectively without knowledge of fish behavior in deep South China Sea . We suggest paying more attention to fishing ground and fishing technology in future surveys.

Key words: tuna, longline, species composition, quantitative distribution, South China Sea