Structural and quality changes in the spawning stock of the Chinese sturgeon Acipenser sinensis below the Gezhouba Dam (Yangtze River)

The Gezhouba Dam, the first dam on the Yangtze River, blocks the migration route of the Chinese sturgeon and other migratory fish species since 1981. In order to understand its ecological effects on the Chinese sturgeon population and to evaluate the effectiveness of the current protective measures for this fish, annual temporal spawning stocks were captured during the spawning season from 1982 to 2003., It is concluded that the structure and quality of the spawning stocks have changed. The average length and weight of females during the period 1999 and 2003 was higher ( by 8.3% and 42.4%, respectively) when compared to the period between 1986 and 1990; likewise the average age had increased by 13.8% in 1997-2000 compared to the period 1981-1982. For the males, the average changes for length and body weight were similar ( 9.2% and 38.1%, respectively), while average age of the arriving spawning migrants had increased by 36.8%. The sex ratio ( F: M) had also changed from 1.13 +/- 0.32( mean +/- SD) in 1982-1994 to 3.48 +/- 1.27 in the years 1995 to 2003. The male gametes quality ( time of active vigorous and quick movements of spermatozoa) had declined by about 396% during the period 2001 to2003 compared to the year 1983 while up to 1995-1996 this decline was about 93% only. The observed overall trend is considered to represent an extremely dangerous situation with a smaller number of very old females and a high number of males lacking well-functioning gametes. This observation points to the fact that the natural population of Chinese sturgeon in the Yangtze River is not only are declining but also aging with little recruitment of younger spawners, indicating that the current protective measures had no obvious positive results yet on the stock structure.