The Variation Of The Community Structure Of Nekton In The Northern South China Sea

The South China Sea is one of the marginal seas in China, which lies to the western Pacific with tropical and subtropical monsoon climates. It has a long coastline and contains lots of islands. The nekton community is highly diversity and most of the cluture have the almost are early in sexual maturity, long egg-laying season ,strong reproductive capacity, fast population complementary speed, short lifecycle and quickly replenish rate. The research area of this paper is shallow than 200m along the continental shelf of northern South China Sea, which not contained the Beibu gulf.With the boats and stress enhancing, the food chain of nekton and the environment of nekton have been influenced even destroyed. Overfishing has caused the apparent decline of the fishery resource. The community structure has been greatly changed and the catch rate of the high nutrition level species decline sharply. We studied the datas of 1964-1965, 1997-1999 and 2006-2007 about the nekton which contains fish, cephalopods and crustaceans in order to find the mechanism of the influence. The results are as following:1.Compared with the datas of 1964-1965, the composition of fishes of the northern South China Sea is changing. In the interannual composition of fish species, the 1997-1999 and the 2006-2007 show that there were 283 and 268 species in common ; in the subrogation, the rates are63.99% and 63.24 ;in the similarity, the species similarity are 36.01%and 36.76%. Compared with the data of 1997~1999 ,the species of the same with 2006-2007 in four seasons are 148,155,170,107; the species similarity are 31.97%,31.83 %,35.79 %,30.57%;the species of layered water which contain five levels( shallow than 30m,30-60m,60-90m,90-120m and 120-200m )is also rejiggered,the subrogation of the two years is from 61.57 % to 71.19% and the similarity is from 28.81 % to 38.43%.2. There are several factors can reflect on the changes of the number of animals swimming, such as the climate, the fishing pressure and so on. There are palpable differences of catching rate during different years. The average catch rate of the whole area of 1997~ 1999 is only 28Kg/h, to 2006 ~2007, catch rates have rebounded to 81kg /h . Catch rates did not change apparently during the season, but there are large fluctuations between years. Changes in catch rates have greater volatility in the region from 1964~ 1965 to 1997 ~ 1999, catch rates have occurred at each depth interval of the northern South China Sea. From 1997 to 1999 to 2006 to 2007, nearly 10 years, after the fishing moratorium and other fishery resources protection measures, the catch rates in all regions has been significant recovery. In the bottom trawl catches, mainly by the fish, cephalopods and crustaceans composed of three groups. The seasonal weight composition of the three groups has certain laws. Fish catch rates were the lowest in the fall, cephalopods, crustaceans, catch rates were highest in summer and autumn. Cephalopods were the highest share of weight in summer, the percentage of crustaceans cases were highest in autumn. Different times from the northern South China Sea survey, three groups have a more significant percentage change. The composition of cephalopods, crustaceans significant increase in the percentage share of catches. Because the life cycle of cephalopods is relatively short and the population could replenish quickly, after declining in the high-quality fish, cephalopods, the replacement of a variety of fisheries resources.3. The data of 2006~2007 shows a better catching rate because the midsummer moratorium from the 1999 in the South China Sea. Compared with 1997~1999 and 2006~2007. But the quality is not obviously raised. South China Sea district begins the fishing moratorium system since 1999, data from 2006 to 2007 shows the more obvious effects moratorium system. However, the quality of resource has not improved catches, except the region that 30 meters shallow not worse than the catch from 1997 to 1999, other water depth area of the fish catch was even worse than the level of 1997 ~1999. Species richness of nekton community from 2006 to 2007, in water depth shallow than 60m higher than the 1997 to 1999, were lower than in the 60m to deep levels from 1997 to 1999. But the diversity index of fish community was higher than those of fish from 1997 to 1999 and from 1964 to 1965. It can show that the northern South China Sea continental shelf s, community structure changes taking place during the nearly 40years.4. The results also showed that there are certain effects by the depth of living under the stress of catching. Trichiurus haumela,Parargyrops edita,Taius tumifrons Upeneus bensas,Nemipterus virgatus are find moving towards more deep; The water depth distribution of Loligo duvaucelii Orbigny,Saurida tumbil became shallow. The water depth distribution of Priacanthus tayenus,Psenopsis anomala are relatively stable..This paper studies the community structure, the species and the diversity of nektons in the northern continental shelf of South China Sea. And this will provide a clear understanding about the fishery resources of the change during the past 40 years. We hope that it can be a theoretical basis for the rational using and protecting.