长江中游不同江段四大家鱼幼鱼孵化日期和早期生长的比较研究

2008年和2009年从东洞庭湖区和长江、监利、鄂州和武穴江段采集青鱼Mylopharyngodon piceus (178尾)、草鱼Ctenopharyngodon idella (341尾)、鳙Aristichthys nobili (6尾)和鲢Hypophthalmichthys molitrix (390尾)幼鱼, 通过耳石日轮分析确定2008年和2009年这些幼鱼的孵化日期分别为5月16日至8月20日和5月14日至8月9日。与三峡建坝前相比, 四大家鱼的繁殖时间明显推迟。洞庭湖和监利江段幼鱼在仔鱼阶段形成的耳石日轮宽度较鄂州和武穴江段幼鱼同期形成的日轮宽度窄, 表明前者仔鱼阶段的生长速度小于后者, 这可能是前者离三峡大坝距离较近、受大坝影响更加显著造成的。三峡大坝下泄水温较低、营养物质较少是影响坝下四大家鱼繁殖和早期生长的重要因素。繁殖季节推迟和早期生长速度下降可能导致坝下四大家鱼早期生活史阶段死亡率增大, 从而影响其资源补充和种群数量变动。  

Hatch dates and early growth for juveniles of the four major carps from different sections of the middle Yangtze River

Juveniles of the four major carps, black carp Mylopharyngodon piceus (n=178), grass carp Ctenopharyngodon idella (n=341), bighead carp Aristichthys nobilis (n=390) and silver carp Hypophthalmichthys molitri (n=6) were collected in the East Dongting Lake and in the Yangtze River at Jianli, Ezhou and Wuxue in 2008 and 2009. Lapillar otoliths were analyzed to determine hatch dates by counting the daily increments and to reveal early growth trajectories by measuring the daily increment width. Back-calculated hatch dates of the juveniles were between May 16 and August 20 in 2008, and between May 14 and August 9 in 2009, indicating an apparent delay of spawning season compared with the pre-Three-Gorges-Dam (TGD) records. Growth rate during the larval stage was slower (revealed from narrower daily increment width in otolith formed during larval stage) for the juveniles collected in the East Dongting Lake or in the Yangtze River at Jianli (closer to the TGD) than those collected at Ezhou or Wuxue (farther from the TGD). It suggested that TGD-moderated effects on early growth and development of carps were more apparent for fishes hatched nearer to the dam. Decreased water temperature and availability of prey resource were the critical factors that affect spawning and early growth of the four major carps. The delayed spawning season and decreased early growth may induce declined survival rates of major carps in their early life history stage, which may in further affect the recruitment and population fluctuation of four major carps.