Widespread genetic introgression of escaped farmed Atlantic salmon in wild salmon populations

Abstract

Farmed Atlantic salmon (Salmo salar) escape from net pens and enter rivers to spawn, potentially resulting in genetic introgression and reduced fitness of wild salmon. Here, we quantify genetic introgression of farmed to wild salmon, using molecular genetic markers, in populations from 147 salmon rivers, representing three-quarters of the total wild salmon spawning population in Norway. For 109 rivers with adult modern samples and sample sizes of 20 or more, the average level of farmed genetic introgression was 6.4% (median = 2.3%), with a range between 0.0% and 42.2%. Fifty-one of these rivers showed significant farmed genetic introgression when compared with historical reference samples. We observed a highly significant correlation between estimated farmed introgression and average proportion of escaped farmed salmon. We quantify levels of introgression as unweighted averages or weighted by population sizes, to compare geographical regions and to compare levels of introgression in rivers and fjords designated as locations deserving a high level of protection. We found a generally lower level of introgression in National Salmon Rivers and National Salmon Fjords subjected to formal protection by parliament. We conclude that farmed to wild genetic introgression is high in a large proportion of Norwegian salmon rivers, with the highest levels found in the most intensive areas of salmon farming. The extensive genetic introgression documented here poses a serious challenge to the management of farmed and wild Atlantic salmon in Norway and, in all likelihood, in other regions where farmed-salmon escape events occur with regularity

Environmental problems and regulation in the aquaculture industry. Insights from Norway

“…In the case of escaped farmed salmon, it is reported that successful inbreeding between escaped farmed and native salmon reduces the fitness and productivity (McGinnity et al 2003), dilutes the genetic gene pools (Fjørtoft et al 2017;McGinnity et al 2004;Roberge et al 2008), and threatens the survival of the native salmon offspring (Hindar et al 2006). Karlsson et al (2016) found statistically significant introgression in half of the wild populations studied and levels of introgression above 10% in 27 of 109 rivers represented by adult samples.…”

The major threats to Atlantic salmon in Norway

“…interspecific hybridization, outbreeding due to immigration, inbreeding, and genetic drift due to reduced population size). The major impact on Norwegian Atlantic salmon populations is introgression (Allendorf et al, 2001) from escaped farmed Atlantic salmon (Glover et al, , 2013Karlsson et al, 2016).…”

“…introgression of farmed Atlantic salmon is documented (Glover et al, , 2013Karlsson et al, 2016) and >10% (Ryman et al, 1995;Karlsson et al, 2016). We use the term "lost in nature", because several Norwegian populations are conserved in the National Gene Banks (i.e.…”

“…The number of critically endangered or lost populations in nature was quantified based on the Norwegian Environment Agency database (Norwegian Environment Agency, 2016b), where lost populations (no longer self-reproducing) are listed. Critically endangered populations in terms of genetic integrity was added to these based on published studies Glover et al, 2012Glover et al, , 2013Karlsson et al, 2016). The criteria ranged from no affected populations to more than 20 affected populations.…”

“…The proportion of genes from farmed Atlantic salmon in the wild populations varied from 2 to 47%. Karlsson et al (2016) recently documented that 51 of 109 Norwegian Atlantic salmon populations showed significant genetic introgression from farmed salmon (using diagnostic SNP marker developed by Karlsson et al, 2014). The mean introgression level in all rivers was 6.4%, and 27 populations (25%) had introgression levels above 10% (maximum 42%).…”