Identification and quantification of farmed red sea bream escapees from a large aquaculture area in Japan using microsatellite DNA markers

A large number of farmed red sea bream (Pagrus major) and their gametes have escaped and dispersed into the wild from net-cage aquaculture since the advent of the technology in the mid-20th century. These escapees have likely been recruited into wild populations, affecting their genetic diversity. In this study, we used assignment methods with microsatellite markers to assess the frequency of escapee farmed red sea bream within the wild populations in a major aquaculture production region (UWJ) and in two regions with relatively lower production (IWG and SIJ) in Ehime Prefecture, Japan. The frequency of escapees in UWJ ranged from 14.1% to 30.2%, as shown by three methods of assignment. Escapees were frequently identified among specimens caught by angling (ranging from 26.7% to 46.7%), suggesting that they stay in the same area even after escape or are less cautious than wild individuals. No escapee was found in IWG and SIJ, areas with less extensive aquaculture. Some wild-caught fish appeared to have hybridized with farmed fish, as indicated by the threshold of membership coefficients obtained by simulation. These results clearly indicate that large numbers of farmed individuals have escaped from UWJ, a major aquaculture area, since the inception of red sea bream farming in Japan. Genetic reshuffle has possibly occurred following hybridization of wild and farmed individuals.