Species composition and quantitative distribution of crabs in the northern continental shelf of South China Sea

The species composition and distribution status of crabs were evaluated based on the data of four seasonal fishery resources surveys in the northern continental shelf of South China Sea from 2006 to 2007.The results showed that there were 51 species belonging to 10 families and 24 genera in the area,most of which were tropical warm-water species belonging to South China-Southeast Asia subregion of Indian-western Pacific fauna.Depending on their adaptability,all crab species were categorized into three ecological groups: tropical euryhaline group,eurythermic euryhaline group,and tropical halophile group,with predominant tropical euryhaline species including Portunus argentatus,P.sanguinolentus,P.tweediei,Charybdis feriatus,Charybdis hongkongensis,Calappa philargius,and Charybdis miles,C.truncata,by Pinkas.The catching rate of the predominant species(2.99 kg/h)was around 81.69% in total catching rate of crab species(3.66 kg/h).The economic crab species Portunus sanguinolentus,P.trituberculatus,P.haanii,P.pelagicus,Charybdis feriatus,C.miles,and C.japonica belonging to Portunidae(2.37 kg/h) had the catching rate of around 64.75% of total catch rate(3.66 kg/h).The crabs were stocked at a density of 97.47 kg/km2,including the economic crabs at stocking density of 63.20 kg/km2.The maximal distribution area of the crabs were found in the transect D(125.60 kg/km2) and the minimal in the transect C(70.93 kg/km2).The maximal biomass of the crabs were observed in autumn(181.60 kg/km2) and the minimal in winter(50.13 kg/km2).The most aggregation of crabs species were mainly distributed in depths of 10-60 m with the peak(222.13 kg/km2) in depth of near 10 m,negative relation with the water depths.