Marine aquaculture of Atlantic salmon (Salmo salar) is a relatively new industry where breeding programmes have led to rapid genetic change in the captive populations that were established alongside conspecific wild individuals. Throughout its 50-year history, the farming of Atlantic salmon has been associated with escapees. Studies have shown that escapees may enter rivers and spawn successfully which may lead to farmed-to-wild genetic introgression and maladaptation in wild populations. Introgressed wild populations are less productive – they produce fewer out-migrating smolt and have higher mortality at sea. Genetically introgressed wild salmon show changes in important traits like growth rates, ages of smoltification and maturation, and reduced homing behaviour.
In this Editor's Choice article, Diserud et al. combined two large-scale datasets from decades of monitoring escaped farmed salmon and introgression in Norwegian rivers to model how anthropogenic, environmental, and population factors influence the proportion of escapees and level of introgression in Norwegian rivers. Previously, scales from more than 470,000 wild-caught salmon have been analysed to determine their origin (farm escapee or wild) according to fish scale growth patterns, while the proportion of wild origin has been estimated for more than 50,000 individuals with a wild growth scale pattern confirming that they were hatched in the wild. Of the 239 Norwegian wild salmon populations genetically analysed, two-thirds showed evidence of genetic change caused by interbreeding between escaped farmed and wild salmon.
Diserud et al. also assembled and tested a large set of potential predictor variables that may be associated with the occurrence of escaped farmed salmon and their introgression into wild salmon in Norway. They found that genetic introgression is primarily determined by the proportions of escaped farmed salmon in rivers, and those proportions are primarily determined by farming intensity and wild population size. This suggests that as long as salmon aquaculture is based on technologies where non-sterile fish can escape, all anadromous wild Atlantic salmon populations are at risk. Large marine protected areas without salmon aquaculture may slow down the rate of introgression by increasing the distance between intensive aquaculture and wild populations, but they do not stop it. Finally, the study shows that genetic introgression is only to a minor extent a function of local environmental factors and salmon population characteristics.
Read the full paper, Natural and anthropogenic drivers of escaped farmed salmon occurrence and introgression into wild Norwegian Atlantic salmon populations, in ICES Journal of Marine Science.
Editor's Choice articles are always free to read in ICES Journal of Marine Science.
farmed salmon together with wild Atlantic salmon in a Norwegian river. Photo: Rune