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.