Juvenile semi-wild fish have a higher metabolic rate than farmed fish

Fish from commercially farmed stocks are often released into the natural environment to supplement wild populations. This practice is often applied to salmonid fish as they are an essential fishery resource and also used for recreational angling. However, farmed fish tend to show lower survival rates after release than wild fish. For this reason, the release of semi-wild fish is increasingly used in Japan; these fish are generated using female fish from domesticated stocks and male fish of wild origin. The survival rate of released semi-wild fish is higher than that of farmed fish, but the reason for this is unknown. This study compared the metabolism and swimming performance of semi-wild and farmed masu salmon (Oncorynchus masou). The analyses showed that resting metabolic rate (RMR), maximum metabolic rate (MMR) and swimming speeds that minimize energy costs of travel (optimal swimming speed) were higher in semi-wild fish than in farmed fish. Critical swimming speed did not differ significantly between the two groups of fish. Semi-wild fish with high RMR may have a social status advantage over farmed fish because a previous study reported that SMR, which is the value closest to basal metabolism significantly affects feeding motivation. This means that individuals with higher social status may be more motivated to feed. As RMR is proportional to food requirements, then release programs should be planned taking food resources at the release site into consideration.