The process of genetic introgression – the movement of information from one gene pool to another through hybridization and backcrossing – of escaped farmed Atlantic salmon in wild salmon is a major threat to the future viability of wild salmon populations. Today, farmed Atlantic salmon outnumber their wild counterparts hundredfold, and although only a small proportion of the farmed fish escape, the number is large enough to make up a large proportion of the amount of spawning salmon in many rivers.
Important commercial traits in farmed Atlantic salmon, such as growth rate, have been genetically changed through selective breeding. These traits are however not favoured in nature, and offspring from interbreeding with wild salmon have lower survival and fitness rates. Farmed salmon also have lower genetic variation than wild salmon. Consequently, ongoing escapes and hybridization with wild salmon is expected to reduce the viability and adaptability of wild salmon populations. Until now, knowledge of the magnitude of genetic introgression in wild salmon populations has been scarce.
In this study, the authors show that half of the wild salmon populations in Norway are genetically changed from genetic introgression of farmed salmon. For this, 147 wild salmon populations representing three quarters of the wild spawning salmon in Norway were surveyed, including more than 20 000 fish that had hatched in the wild. The average level of farmed genetic introgression was 6.4%, with as much as 40% in some populations.
With widespread genetic introgression one of the most important threats to wild salmon populations, the results of this study provide a way forward to better manage the remaining wild populations of Atlantic salmon. The investigation also serves as an example for how genetic introgression can be monitored in other species, from domesticated to wild populations on a large scale.
An escaped farmed salmon and wild salmon spawning in a Norwegian river. Photo: Rune Muladal