从连续到偶发：中华鲟在葛洲坝下发生小规模自然繁殖

中华鲟(Acipenser sinensis Gray)为国家一级重点保护野生动物,典型的溯河洄游产卵鱼类。历史上,中华鲟在长江上游及金沙江下游产卵,由于葛洲坝修建阻隔了其洄游通道,1981年以后在葛洲坝下形成了比较稳定的产卵场,1982―2013年,每年均有自然繁殖发生。由于其栖息生境退化,每年洄游进入长江的中华鲟繁殖亲本逐年减少,2013―2015年连续3年在已知葛洲坝下中华鲟产卵场未监测到中华鲟自然繁殖活动。2016年11―12月的野外监测发现,中华鲟在宜昌葛洲坝下已知产卵场发生了自然繁殖。其中底层网具采集到中华鲟鱼卵(卵膜)67粒、仔鱼22尾;解剖食卵鱼发现,10尾食卵鱼类共摄食中华鲟卵454粒;水下视频观测到5处中华鲟卵黏附底质位点。根据采集到的鱼卵发育期及采集位点推算,产卵时间为2016年11月24日凌晨,产卵场位于葛洲坝大江电厂以下约300 m的江段内,产卵日水温为19.7℃,流量为6610 m3/s,水位为39.7 m。

From continuous to occasional: Small-scale natural reproduction of Chinese sturgeon occured in the Gezhouba spawning ground, Yichang, China

The critically endangered Chinese sturgeon (Acipenser sinensisGray), a large anadromous fish, is strictly protected in Chinese waters, where it is deemed to be a national treasure. Mature individuals enter the Yangtze River estuary from July to August, before entering their spawning ground the following year from Octo-ber to November. Historically, sturgeon spawned in the upper reaches of the Yangtze and lower reaches of the Jinsha Rivers, but construction of Gezhouba Dam (the first dam in the Yangtze mainstream) blocked this migratory route. A relatively stable spawning ground has since developed below this dam. From 1982, the Yangtze River Fisheries Research Institute has conducted annual monitoring of natural breeding of sturgeon within the newly formed spawning ground. Between 1982 and 2013, sturgeon spawned annually, though both fecundity and fre-quency of reproduction declined. Whereas the number of mature fish arriving at the spawning ground annually exceeded 2000 individuals in the early 1980 s (near the closure of Gezhouba Dam), it also dropped to fewer than 100 individuals in recent years. Prior to impoundment of the Three Gorges Reservoir (1984?2003), sturgeon spawned on average 1.8 times per year, though this decreased to 1.1 times during impoundment (2004?2012). Al-though controlled propagation and augmentation of sturgeon stocks has been performed from as early as 1983, both mark-recapture and molecular genetic identification reveal more than 90% of juvenileA. sinensis within the Yangtze estuary originated from natural reproduction. Thus, it is critical that natural reproduction of the fish may be maintained in the only knowing spawning ground.Though there was an apparent interruption in sturgeon spawning between 2013 and 2015 at this only known spawning ground, we report a resumption of spawning in 2016, during which time 67 egg (or egg membrane) and 22 larval sturgeon were collected by bottom draft-net, and 454 eggs were found in the stomachs of 10 egg-preying fishes. Underwater camera observations also revealed five sites with fertilized sturgeon eggs adhering to the sub-stratum. The spawning ground was 300 m downstream of the Dajiang power plant (Gezhouba Dam). Spawning likely occurred during the early morning of November 24; estimated fecundity was less than two million; water temperature at the time of spawning was 19.7℃; and the flow and water level were 6610 m3/s and 39.7 m, respec-tively.Recent monitoring indicates the Chinese sturgeon has changed from being a "continuous" to an "accidental" spawner, and that this is likely to continue. Increased water temperature was probably the main reason for the failed spawning between 2013 and 2015, as the Three Gorges Reservoir significantly altered water temperature within the middle reach of the Yangtze River, but other factors, such as navigation, waterway regulation, over-fishing and environmental pollution, have also reduced environmental quality at the spawning ground and may have contributed to spawning failures. Without effective environmental management, the natural population of Chinese sturgeon is at high risk of extinction.In 2015 the Ministry of Agriculture issued the "Chinese Sturgeon Rescue Action Plan" to protect Chinese sturgeon, which introduced severalin-situ conservation programs, such as artificial restocking, habitat manage-ment and restoration, and the prohibition of fishing. Artificial restocking of Chinese sturgeon would also be im-proved by increasing the number and size of released individuals, thereby increasing their survival. In the middle and lower reaches of the Yangtze River, port, waterway, shipping and other construction projects should be man-aged to avoid any negative impact on the habitat and migration of Chinese sturgeon. In particular, environmental protection of Hubei Yichang Chinese Sturgeon Natural Reserve needs strengthening to maintain the hydrological conditions of spawning grounds. Additionally, improvement and rebuilding of Chinese sturgeon spawning grounds would facilitatein situ-protection of Chinese sturgeon.