1Graduate School of Agricultural and Life Sciences, the University of Tokyo, Bunkyo, Tokyo 113-8657, 2Freshwater Fisheries Research Division, National Research Institute of Fisheries Science, Fisheries Research Agency, Nikko, Tochigi 321-1661, Japan
The reproductive cycle of smallmouth bass Micropterus dolomieu naturalized in Lake Akimoto at Fukushima Prefecture was investigated to obtain biological information for developing techniques to exterminate this species in Japan. Samples were captured monthly from April to December 2002. In male testes, spermatozoa or spermatids were observed from April to July and from September to December. In August, males showed low gonadosomatic index (GSI ) and the testes were occupied by spermatocytes. In females, oocytes with early or late perinucleolus and yolk stages were observed from April to June and from October to December. From July to September, females showed low GSI and a decrease in the number of oocytes with yolk stages. In both males and females, plasma testosterone and 17,20β-dihydroxy-4-pregnen-3-one levels reached peaks in May. In males, 11-ketotestosterone reached a peak in May. In females, estradiol-17β showed high levels in April. From these results, the reproductive period of smallmouth bass can be considered to be from spring to early summer. However, they have gonads keeping a certain degree of maturity from autumn to early winter.