Investigation on shrimp feeding of Coilia nasus during its anadromous migration along the Yangtze River
Estuarine tapertail anchovy(Coilia nasus) is a valuable commercial fish in the Yangtze River.It is traditionally believed that C.nasus do not feed during their anadromous migration,but this issue is not yet fully clarified.As a pilot study on feeding of C.nasus during the whole process of anadromous migration,the present study investigated the nekton (e.g.shrimp,fish) in stomach and intestinal contents of C.nasus fish (confirmed as anadromous individuals by otolith microchemical analysis) previously collected in the Yangtze River's estuary,Chongming Island of Shanghai City,sections in Jiangsu and Anhui Provinces,and connected Lake Poyang in Jiangxi Province.The results showed that C.nasus from the river estuary and sections of Jiangsu and Anhui Provinces did not feed nekton and their stomach fullness indices were at 0 degree.In contrast,all C.nasus from Lake Poyang obviously fed nekton and their stomach fullness indices were at 4 to 5 degree.Furthermore,the nektons in stomach contents of the fish were all freshwater shrimps (Macrobrachium nipponense and Exopalaemon modestus),and one and two prawns were found in the stomach contents of 29% and 71% of the C.nasus fish,respectively.However,no nekton residual could be found in the intestinal contents of C.nasus studied.Since the Lake Poyang has been confirmed as a spawning site of anadromous C.nasus,the present study revealed a possible two-step-feeding process for the fish in the Yangtze River,namely,C.nasus may not feed (at least nekton) when moving upstream through the estuary and migratory channel in the sections of Jiangsu and Anhui Provinces,while they may start to feed shrimps of M.nipponense and E.modestus when entering the spawning site in the Lake Poyang.These phenomena may reflect a survival strategy of Yangtze C.nasus,i.e.,the fish keeps fasting to save energy for its long distance anadromous movement,but starts to feed to supply energy for full gonad maturation and finish breeding.