Abstract:

A preliminary analysis on the diel and distributional abundance patterns of fish eggs and larvae was carried based on ichthyoplankton samples collected at Luzhou,Luohuang,Yidu and Wuxue sections of main stream of the Yangtze River during 2007 and 2009 respectively.It turned out that there was no clear diel difference in drifting densities of fish eggs and larvae at Luzhou and Luohuang sections.However,there were distinct diel differences in the drifting densities of eggs and larvae at Yidu section.There were significant differences 〖WTBX〗(n=38，p〖WTBZ〗=000) between the drifting density of fish eggs in 06〖DK〗∶00 (mean density was 1302 ind. /100m3) and in 18〖DK〗∶00 (mean density was 328 ind./100m3),while the highest drifting density of larvae was found in 22〖DK〗∶00.And a significant diel difference was found in drifting densities of fish larvae at Wuxue section 〖WTBX〗(n=62，p=000)〖WTBZ〗,the average density at 08〖DK〗∶00（8621 ind./100m3） was clearly greater than at 18〖DK〗∶00（4485 ind./100m3）.In Yidu location,the drifting densities of different species showed speciesspecific difference. The eggs of 〖WTBX〗Hemiculter bleekeri〖WTBZ〗 (Warpachowski),〖WTBX〗Ctenopharyngodon idellus〖WTBZ〗 and 〖WTBX〗Xenocypris argentea〖WTBZ〗 were mainly collected in daytime,〖WTBX〗Aristichthys nobilis〖WTBZ〗 and 〖WTBX〗Hypophthalmichthys molitrix 〖WTBZ〗were mainly collected in the night,while 〖WTBX〗Parabotia fasciata〖WTBZ〗 and 〖WTBX〗Parabramis pekinensis〖WTBZ〗 were collected all daylong. Larvae of most fishes were collected in the night,while the species from Pseudolaubuca were mostly collected in daytime.According to the capture time and the development stages of fish eggs,the peak spawning period of the four Chinese major carps were during nighttime.The spawning time of 〖WTBX〗Hemiculter bleekeri〖WTBZ〗 was concentrated in first half of the night,〖WTBX〗Xenocypris argentea〖WTBZ〗 and 〖WTBX〗Culter alburnus〖WTBZ〗 were concentrated in midnight,while 〖WTBX〗Parabotia fasciata〖WTBZ〗 and 〖WTBX〗Parabramis pekinensis〖WTBZ〗 could be spawning all daylong.Speciesspecific diel abundance patterns and their causes must be considered when designing effective sampling programs for fish embryos and larvae