Variations in spawning stock structure of Acipenser sinensis within 24 years since damming of Gezhouba Dam

The Chinese sturgeon, Acipenser sinensis, is an anadromous fish. The migration pattern is only given for the population entering the Yangtze River. The spawning migration route was blocked by construction of the Gezhouba Dam in 1981 and the spawning ground, which has been found since then, is now located in the reach about 7?km below the dam. The variations in the spawning stock structure in the reach about 30?km below the dam from 1981 to 2004 during the spawning season were investigated. The sex ratio (F∶M) gradually decreased from 1.10∶1 during 1981-1983 to 0.63∶1 during 1987-1989, and then increased to 5.86∶1 during 2003-2004. The average body length of the females increased from 263.1?cm during 1990-1991 to 276.7?cm during 1999-2001, increasing by 5.2%; the average body weight increased from 202.4?kg during 1990-1992 to 237.4?kg during 2003-2004, increasing by 14.5%; the average age fluctuating between 21.1 and 22.7 years during 1990-2001 was older than that during 1981-1989, which was fluctuating between 19.0 and 20.7 years. The changes were greater in males than those in females. The average body length and body weight of the males decreased from 205.1?cm and 89.8?kg during 1981-1983 to 197.5?cm and 72.7?kg during 1987-1989, decreasing by 3.7% and 19.0%, respectively, and then increased to 229.4?cm and 120.1?kg during 2003-2004,increasing by 16.2% and 65.2%. The average age of males decreased from 15.4 years during 1981-1983 to 13.3 years during 1987-1989,decreasing by 13.6%, and then increased to 17.6 years during 1996-1998, increasing by 32.3%. In the individuals collected and age-determined during 1993-2004, there were 28 samples were born during 1981-1989. The results showed that the sturgeon did not lose their homing instinct as the migratory route was shorted off 622?km to 1?166?km since the damming in 1981. This verified that the new spawning ground below Gezhouba Dam was efficient since there was no restocking into the river in some years during 1980s.