Experimental tests of genetic divergence of farmed from wild Atlantic salmon due to domestication

The genetic response of Atlantic salmon to culture is important in predicting the success of these fish in nature and their impacts on wild populations through competition and interbreeding. We compared a seventh-generation strain of farmed Atlantic salmon from Sunndalsøra, Norway, with its principal founder population from the wild, the River Namsen. The fish were reared from eggs in a common environment and assessed for the extent of genetic divergence in several fitness-related traits. Morphology had diverged, as farmed juveniles showed more robust bodies and smaller rayed fins than the wild juveniles. Ecologically important aspects of behaviour also differed. Farmed juveniles were more aggressive in a tank environment typical of culture facilities, while wild juveniles dominated in a stream-like environment. Farmed juveniles were also more risk-prone, reappearing from cover sooner after a simulated predator attack. It was not surprising that growth performance was higher in farmed than wild juveniles, as the former had been subjected to intentional selection for this trait. Correlated responses to this selection may also explain the higher rate of smolting and lower rate of male parr maturity in the farmed than the wild salmon. Competition with wild juveniles, however, negatively affected the growth of the farmed juveniles, particularly under semi-natural conditions. Our results indicate that farming of Atlantic salmon generates rapid genetic change, as a result of both intentional and unintentional selection in culture, that alters important fitness-related traits.