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Hairtail(Trichiurus japonicas) is one of the most important kinds of commercial fishes in the northern South China Sea(NSCS).Changes to hairtail catches are not only related to fishing pressure but also to changes in the climatic environment.To understand the influence of climatic environmental factors and fishing pressure on the changes of hairtail catches in the NSCS,we have used long-term fisheries statistical data and climate change data to analyze the changes of hairtail catches between 1956 and 2006 in the NSCS.Hairtail catches in the NSCS can be divided into two parts: the trend of catches caused by fishing pressure and the alteration of catches influenced by climate change.Fitting the trend of catches to the Fox model shows significant relationships between fishing effort and hairtail catches in the NSCS(P<0.01),which in turn demonstrates that growing fishing pressure has significantly affected the changes of hairtail catches in the NSCS.Before 1986,the growth of fishing effort was slow in the NSCS,and correspondingly,the growth rates of hairtail catches increased by a small margin.However,after 1986,with the rapid growth of fishing efforts and improved techniques for fishing,hairtail catches in the NSCS grew continuously and rapidly.After removing the effect of fishing pressure on the changing trends,we found that the alteration of catches had a significant positive partial correlation with precipitation in the southern China,as well as sea surface temperature,summer wind speed and winter wind speed in the NSCS(P<0.04).Alteration of catches also had a significant negative partial correlation with the index of tropical cyclone influences(P<0.03).This correlation showed that the alteration of hairtail catches was also affected by climatic and environmental factors.Rainfall runoff inputs a vast amount of nutrients into the estuary and adjacent waters.However,the monsoon-driven confluence and mixing between coastal and surface waters,as well as vertical mixing between the upper and lower waters,diffuses the distribution of nutrients in the NSCS,aides the efficient use of nutrients,and is conducive to the growth of phytoplankton in the sea.In addition,changes in ocean temperatures directly or indirectly had an effect on the various life stages of marine fish,particularly on the metabolism of fish and the rate of the reproductive cycle.Finally,the tropical cyclone changed the living environment of the hairtail,which would in turn affect its concentration,feeding and migration.Hairtail catches in the NSCS can be highly significantly fitted to fishing efforts and climatic variables(R2=0.958,P<0.01).That is to say,changes of hairtail catches in the NSCS can be confidently attributed to a monotonic growth in fishing pressure and changes of the climatic environment.If the fishing pressure stays unchanged,the future climate changes and human activities that cause the increase of nutrients in the seas are likely to result in an increase in hairtail catches in the NSCS.Moreover,the increased frequency in extreme weather events that is predicted in the future may also cause larger fluctuations of hairtail catches.

The influence of climatic environmental factors and fishing pressure on changes of hairtail catches in the northern South China Sea