Study On Hatching Date And Growth Of Juvenile Grass Carp From The Middle Reaches Of Yangtze River Using Otolith Daily Increment Technology

Grass carp (Ctenopharyngodon idellus) is one of the most famous cultivated species in China. It was predicted that the Three Gorges Project would affect to a great extent on the spawning grounds of the four major domestic carps, including grass carp, due to the hydrological changes downstream the dam after the completion of the reservoir. In June 2003, the Three Gorges reservoir began to store at 135 meter, and after November 2003, the reservoir ran at 139 meter. Based on the observation of microstructure and daily increment on otolith of juvenile grass carp sampled from Xintankou section and Dongting Lake in 2003 and 2004, the hatching date, the spawning date, and the growth were studied with the purpose of discussing whether the spawning date was affected by the incipient storage of the Three Gorges reservoir. The main results were as follows: 1 The primordium, growth core and daily increment on the lapillus of grass carp could be observed clearly under microscope. There were low optical density areas and optically dense areas alternately on the otoliths. Secondary growth core could be found in a few otoliths indicating dissociative primordium might exist in otolith sac. First feeding check was easily distinguished on most of the otoliths. 2 Short radius and long radius of lapillus was linearly correlated. The relationship between long radius of lapillus and body length was linear, whereas exponential relationship was found between long radius of lapillus and body weight of juveniles. 3 The growth model of body length and weight fitted into both linear and exponential functions, but the latter was better. The index in exponential model was approximately 3 indicating the growth of juvenile grass carp was equally between body weight and body length. The juveniles from Dongting Lake grew faster than those from Xintankou section of the Yangtze River. 4 In 2004, the date of the first spawning seemed to occur on May 12, and the last date appeared to be July 9. The earliest spawning date of the grass carp in 2004 came a little later than those in recent years before the water storage of the Three Gorges reservoir, however, compared with the record of 1960s, it obviously was delayed. 5 The date that water rising in 2004, which could suffice the requirement of fish reproduction, was not distinctly delayed comparing with those in years before water storage, expressing that the changes of water level was not the main reason causing the postponement of spawning. Hence, the water temperature might be a limit factor. Further studies were need to ascertain whether the change on water temperature was led by the incipient storage of the Three Gorges reservoir, or it was only ascribed to natural variety. The main conclusion of the present paper were: 1) Juvenile grass carp in Dongting Lake grew faster than those in the main section of the Yangtze River. 2)After the incipient storage of the Three Gorges Reservior in 2003, grass carp still spawned in the middle section of the Yangtze River,but the date of spawning was delayed to some extent.