Farm salmon escape event: levels of farm/wild hybridisation

Executive summary

A genetic survey was undertaken in 2020 and 2021 of salmon obtained from rivers in areas of Scotland and England local to an escape event that occurred from the Carradale North fish farm during Storm Ellen in August 2020. Taking into consideration all results, there is no indication that this escape event resulted in significant interbreeding of escaped farm fish with wild stocks in the 2020 spawning season in the months immediately after the escape.

On August 20^th 2020, Mowi’s Carradale North fish farm shifted position after its seabed anchors became dislodged during Storm Ellen. This damage resulted in a reported 48,834 farmed Atlantic salmon escaping into the wild. There were reports that large numbers of farmed origin salmon were present in the areas surrounding the escape location in the weeks immediately after the escape event, with a minimum estimate of 3,000 fish entering rivers. This was of concern to local fisheries managers, as escaped farm fish have the potential to breed with wild individuals, resulting in hybrids that are less fit than their wild counterparts, potentially leading to a significant detrimental impact on wild populations.

Genetic material was obtained from wild salmon fry captured during surveys undertaken in 2020 and 2021 to examine if any hybridisation between the escaped fish and wild stocks had occurred. Using similar genetic methods to those previously employed to estimate levels of hybridisation (introgression) in wild Scottish salmon populations, the samples were examined to identify the presence of signatures of first generation (F1) hybrid fish (from crosses of wild and farm individuals). In Scotland, the prevalence of any existing F1 fish was determined in the 2020 cohort (2,358 samples), which could not have been influenced by the escape event. These levels were then compared to those in the 2021 cohort (2,586 samples), which could potentially be impacted by hybridisation from escapees from the event. For samples collected from rivers in Cumbria in the North of England during 2021, prevalence’s of F1 fish in fry (279 fish) were determined and compared to parr (58 fish), as no fry samples were available from 2020. The proportions of F1 fish were then compared between these age classes, with parr acting as a background estimate with which to compare the 2021 fry cohort.

In the samples obtained from Scotland, a single F1 fish was observed in the 2020 cohort and none in the 2021 cohort. In the samples from England, a single F1 fish was identified in the 2021 fry. Considering the complete lack of fish identified as F1 in the Scottish 2021 fry, and only a single individual in the English 2021 fry, there was, overall, no evidence of substantial hybridisation occurring in the 2021 spawning season in both countries due to the escape event. This is likely a result of the farmed fish being immature and therefore unable to breed with wild individuals in the year of the escape.

It should be noted that the lack of immediate impact on levels of hybridisation in the wild stocks under investigation does not mean that such a large escape event resulted in no negative impacts on wild fish stocks. Even if no immediate introgressive impact was seen in the local area, there may still be hybridisation occurring in either the area of the escape, or further afield, as it is known that immature fish escaping may migrate long distances and return to either the area of escape and/or rivers far from their escape location at a later time. Thus, the impacts of any escape event may not be immediate and/or local but may spread across both time and space. Relating such impacts to a single escape incident would be extremely difficult.

The results of this investigation indicate that immediately following the Carradale escape, hybridisation with wild salmon was very limited in this specific case. However, in other such large-scale escapes, immediate impacts have been seen. Thus, it is evident that each such event should be considered based on the particular situation pertaining at the time in regards to factors such as numbers, timing, wild stocks, and, of particular importance, the maturation status of the escapees. As such, there is a continuing requirement to strengthen both practical (on site) and regulatory regimes to prevent escapes occurring.