Farm salmon escape event: levels of farm/wild hybridisation

Background

On August 20^th 2020, Mowi’s Carradale North fish farm shifted position after its seabed anchors became dislodged during Storm Ellen. The farm comprised ten circular net pens containing a total of 550,700 Atlantic salmon (Salmo salar), with an average weight of approximately 4.2 kg each. Four pens were damaged, two of which experienced torn netting. Mowi reported that this damage resulted in 48,834 salmon escaping into the wild (Anon, 2020). In the weeks immediately after the escape event, there were reports that large numbers of farmed origin salmon were observed in rivers in the areas near to the escape location. It was estimated that a minimum of 3,000 farmed fish entered Scottish rivers (Burns et al., 2021).

The potential presence of such large numbers of farmed fish entering Scottish rivers raised concerns for the health of the wild populations they could interact with. Interbreeding between escaped farmed salmon and wild conspecifics, and the resulting introgression of genetic material from farm stocks into the wild, brings risks to the diversity, genetic integrity, fitness, and viability of wild salmon populations (Naylor et al., 2005; Glover et al., 2020). In Scotland, significant introgression has previously been found in areas of both marine and freshwater farm production with a focus along parts of the Scottish west coast and western Isles (Gilbey et al., 2021). To examine whether the 2020 Carradale escape event resulted in detectable levels of introgression, a genetic survey was undertaken.