Long-term translocation and hatchery release explain population genetic structure of a recreationally fished iconic species in Japan: A review and reanalysis

Abstract

Ayu (Plecoglossus altivelis altivelis) occur widely in China, Korea, Japan, Taiwan and Vietnam. It is an important freshwater fisheries resource and popular recreational fishing species in Japan, lives for only one year and has a single breeding season. To supplement increased recreational fishing demand, huge numbers of wild-born landlocked juvenile have been translocated from Lake Biwa into most Japanese rivers for more than 50 generations. Hatchery-born juveniles have also been extensively released for more than 30 generations. Previous studies reported that landlocked and amphidromous forms of Ayu easily hybridise, but survival of landlocked larvae is very low in seawater, leading to a general consensus that the reproductive success of landlocked Ayu is very low, or even 0 in translocated rivers. However, there is limited information on the reproductive success of landlocked Ayu in translocated rivers and no study evaluated effects of translocation and hatchery release on genetic population structure in the distribution range. To provide an up-to-date review on the key management question, I performed a comprehensive literature search and analysed published microsatellite data sets collected from the distribution of this species in Japan. The analyses provide strong evidence for very high gene flow between populations, but population structure was retained in several regions, and several populations were nested. Genetic diversity was surprisingly homogeneous. Bayesian admixture analysis estimated hybrid proportion between both forms to be 37%, uncovered widespread hybridisation in Japanese rivers. Results are discussed in relation to the conservation and management of this species.