Abstract:

During 2003-2011,total three investigating sections including waters of Nanmeng port to Xinhe port with central position in 121°26′06″E longitude and 31°33′41″N latitude,waters of North channel entrance with central position in 122°00′42″E longitude and 31°21′54″N latitude and waters of northern Jiuduansha with central position in 122°00′42″E longitude and 31°21′54″N latitude were set in southern branch waters of Yangtze River estuary.In these sections,four to six ships were arranged every year to research fishery resource of Coilia mystus in fishing season.Based on the investigations,the studies were carried out for biological characteristics,fishing features and annual yield of Coilia mystus,and comparative study with the similar research during 1997-2002 was also performed.The results showed that absolute fecundity in fishing seasons from 2003 to 2011 ranged from 3 404 to 26 850 individuals,and the average amount was 11 554 individuals; the relative fecundity ranged from 347 ind./g to 1 582 ind./g, and the average amount came to 783 ind./g;the diameter of mature eggs fluctuated between 053 mm and 106 mm,with the average value reaching 078 mm.In aforesaid period,total length of sampling groups ranged from 139 mm to 155 mm,and the mean data was 146 mm;standard length ranged from 123 mm to 137 mm,and the mean value was 129 mm;body weight ranged from 120 g to 158 g,and the mean data was 136 g;the coefficient of fullness ranged from 0387 to 0466,and the mean data was 0442.From 2003 to 2011,full length of 3691% random collecting samples were less than 140 mm,and 5709% individuals ranged from 140 to 180 mm.In the same period,body weight of 3957% samples were less than 12 g,while 4534% individuals were 12-20 g.At the beginning of the fishing season,water temperature usually fluctuated from 17℃ to 20℃.The peak fishing season period mainly occurred in later May until the middle of June,and the catch proportion in May of total catch in fishing seasons fluctuated from 30.59% to 83.76%, with the average number reaching 6117%. Fishing ships ranged from 63 to 278 with the average of 141 were used during the investigation and the catch per unit effort ranged from 3679 kg to 5 0235 kg with the average of 2 4412 kg.Total catch in Yangtze River estuary ranged from 232 t to 1 2569 t.The result showed that the downward trend of biological characteristics and annual yield could be easily found after impoundment of the Three Gorges Dam.Compared with the historical data,it could be easily concluded that the Coilia mystus fishery resource is coming to be exhausted,and its fishing value is coming to be lost.Under the unfavorable background,only if the subjective factors with negative influence were improved as soon as possible,resource of Coilia mystus could gain adequate space and time.In the end,protecting proposals were also given,such as making policy of fishing forbidden and establishing protection zones