Atlantic salmon has been exposed to directional selection for aquaculture-related traits for >12 generations. Consequently, farmed salmon deviate from wild conspecifics in multiple traits , but especially growth rate under aquaculture conditions ( increased in farmed) and survival in the wild ( reduced in farmed) . Not surprisingly, interbreeding between escaped salmon and wild populations has resulted in alterations of fitness-related traits in naturally produced introgressed individuals in the wild , including reduced survival in fresh water which may result in reduced production of wild salmon. However, the long-term consequences of introgression are not fully known and there is a knowledge gap related to what extent natural selection can reconstruct the original characteristics of the wild salmon populations.
In order to elucidate the impact of natural selection on the long-term consequences of introgression we conducted a study where domesticated salmon was planted out as eyed-eggs in a river with a fish trap for smolt recapture . Surviving individuals were recaptured as age 3-5 year old- smolts and thereafter reared under aquaculture conditions until reaching maturation. These individuals were then used as brood stock and crossed with individuals of identical genetic background that had been held for the entire generation under aquaculture conditions and thus not exposed to natural selection in the wild. The offspring of these unique crosses were thereafter compared in a pedigree-controlled common-garden growth study, in order to quantify the reversal of the genetic gain achieved through directional breeding for the main trait targeted: increased growth. Here we document natural-selection induced loss of growth capacity in offspring of farmed salmon, thus documenting the first evidence of de-domestication in fish.