长江常熟溆浦段中华鲟幼鱼出现时间与数量变动

Occurrence time and amount variation of juvenile Chinese sturgeon,Acipenser sinensis at Xupu,Changshu section of Yangtze River after closure of Three Gorges Dam

2002—2009年，依托长江渔业资源管理委员会办公室设立在长江常熟溆浦江段的渔业资源监测网（定置张网），对降河中华鲟似cipensersinensis）幼鱼进行了监测分析。结果表明，中华鲟幼鱼每年主要于5—7月在常熟溆浦江段出现，出现时间平均86．6d（”=8），所采获样本（n=8）全长、体长和体质量年度平均值的变化范围分别为25．2～40．1cm、23．2～33．5cm和70．7～314．8g。各年采获样本数量差异较大，从2002至2009年呈现出较明显的递减趋势，各年采获样本数量最多的旬也不一致，变化时间为5月中旬至6月下旬。从幼鲟生长来看，不同年份相同时期采获样本的规格差异较大，同一年份同一时间采获样本的规格也存在一定的差异。与三峡工程截流以前进行比较，中华鲟幼鱼在长江口出现的时间没有明显变化，但出现较高数量比例的时间由以前的6月变为5月中下旬和6月，幼鲟规格较截流前也有较大变化，推测造成这种改变的原因可能是因为人工增殖放流活动和三峡工程截流所导致的水文状况的变化。分析认为，水文状况可能是影响中华鲟幼鱼到达长江口时间的重要因素之一。

Chinese sturgeon Acipenser sinensis, a large anadromous species, mainly distributes in the continental shelf of the East China Sea and the main steam of Yangtze River. From July to August in each year, the pre-spawning individuals enter into the Yangtze estuary. Then they go against the water flow and will spawn in autumn of next year. The spawning locus is about 1 700 km away form the Yangtze estuary. After hatching, the yolk-sac larvae go downstream with the water flow and will arrive at the estuary in April and August of next year. Xupu, Changshu, a place at the Yangtze estuary, is a good site to monitor A. sinensis juveniles. So in years from 2002 to 2009, by a fixed stow net （set by the Yangtze Fishery Resources Commission for fishery monitoring） set in this site, we investigated the resources of A. sinensis juveniles. It was indicated that the emergence of the fish at Xupu, Changshu mainly occured from May to July, with the average period being 86.6 days （n=8）. The ranges of annual （n=8） average total length, body length and body weight were 25.2-40.1 era, 23.2-33.5 cm and 70.7-314.8 g, respectively. The numbers of specimens captured in each year were very different, and a decreasing trend occurred from the year 2002 to 2009. The periods （ten days as a unit） with maximum sampling number were also different, which ranged from the second ten days of May to the last ten days of June. The sizes of the juveniles were very different in the same period of different year, and the sizes of the specimens captured in the same year and same date also showed difference. Compared with the study in the four years （1987, 1990, 1991 and 1992） before the closure of the Three Gorges Dam, the time range that juveniles arrived at the Yangtze estuary showed no obvious difference（P〉0.05）. However, the occurrence proportion in each ten days during the time from May to July varied. The high occurrence proportion was mainly in June before the closure, but after the closure it also occurred in May. The size of the juveniles was also different from pre-closure. It was speculated that the difference was caused by artificial enhancement and releasing, and variation of hydrological regime. Hydrological condition was an important factor influencing the time consumption for the fish to migrate from spawning area to the Yangtze estuary