Farm salmon escape event: levels of farm/wild hybridisation

Discussion

A total of 5,281 wild-caught salmon were examined for the presence of F1 hybrids, which may have arisen from the escape event that occurred from the Carradale North fish farm during Storm Ellen in August 2020. Taking into consideration all results, there is no evidence that the fish that escaped during this event bred with wild fish in the areas sampled in the months immediately after the escape, during the 2020 spawning season.

A single F1 fish was observed in Scotland in 2020. This fish could not have been a result of any hybridisation resulting from the Carradale farm escape as it is from a cohort of fish produced from spawning from the year before the escape. No F1 fish were identified in 2021, the year any impacts would have initially been expected to be observed. The numbers of farm/wild hybrid fish identified in both years are similar to those previously observed in these Scottish areas (see Figure 19 in Gilbey et al., 2021). This indicates that while there is an overall presence of farm introgression in these areas in both years, there was no immediate increase as a result of the Carradale escape event.

A single fry from the English samples was identified as a possible hybrid F1 fish, and no other hybrids were identified. Analysis of the English samples was based on comparing levels of introgression between age classes. Parr, which could not have been influenced by the escape, were compared to fry, which had the potential to be impacted. Care must be taken when interpreting such a comparison, as it is well known that there are behavioural, physiological and mortality differences between hybrid fish and wild conspecifics between juvenile age classes (e.g. Skaala et al., 2012; Solberg et al., 2015; Glover et al., 2017) and so, a fry vs. parr comparison is not a like-for-like one. It is difficult, then, to come to firm conclusions based on a single individual and the comparisons available. The picture may become clearer with a more detailed survey of both parr and fry to determine true levels of ‘background’ introgression in these English areas.

Immediately following the escape event, large numbers of escaped farm fish, purportedly escapees from the event, were observed in rivers relatively near to the farm (Fisheries Management Scotland, pers. comm.; Burns et al., 2021). However, this did not translate into an increase in the detection of F1 hybrids in these areas, despite it being known that escaped farm fish can enter rivers and breed with wild individuals wherever such interactions occur (e.g. Glover et al., 2017; Gilbey et al., 2018; Wringe et al., 2018; Diserud et al., 2020; Glover et al., 2020; Gilbey et al., 2021).

Successful spawning relies on the simultaneous fecundity of both parental fish. Salmon in aquaculture have, and continue to be, selected for delayed sexual maturation (Iversen et al., 2016; Rivera et al., 2021). The escape event occurred in August 2020, with natural spawning in the areas of investigation following just a few weeks later, in the early autumn. Gametogenesis takes significant resource investment and takes a number of weeks to complete (Thorpe, 1994; Fleming, 1996; Mobley et al., 2021). The lack of an impact of the escaped fish may thus be a result of them not becoming sexually mature after escaping so close to this spawning period.

There is evidence that adult farm escapees may not necessarily use the same stretches of river for spawning as wild fish (Økland et al., 1995; Thorstad et al., 1998; Moe et al., 2016). In addition, there may also be differences in the timing of spawning compared to wild fish (Webb et al., 1991; Saegrov et al., 1997; Fleming et al., 2000; Moe et al., 2016). Thus, a likely lack of maturation, together with a mismatch in “time and space” (Glover et al., 2017) may all have contributed to a mismatch of spawning relative to the wild salmon, thereby reducing the potential for introgression and resulting in the low numbers of F1 hybrids detected.

The absence of an immediate observed impact on levels of hybrid F1 fish being produced does not equate to the absence of negative impacts on wild fish stocks from such large-scale escape events. Even if no immediate introgressive impact is seen in the local area, there may still, in the future, be hybridisation in either the area of the escape, or further afield. The migratory tendency of farm escapees is disrupted relative to wild fish (Hansen, 2006; Madhun et al., 2017 and references within these). While the incidence of escaped farmed salmon in rivers is strongly correlated with the density of production in an area (Fiske et al., 2006; Green et al., 2012; Mahlum et al., 2021), it is also the case that immature escapees may migrate long distances and then return to either the area of release or rivers far from their escape location (e.g. Hansen and Youngson, 2010; Glover et al., 2017; Mahlum et al., 2021). Thus, the introgressive impacts of an escape event may not be immediate and/or local but may spread across both time and space. However, it would be extremely difficult to disentangle these potential impacts from other reported escape events and/or unreported losses in areas where these issues could be a factor.

Together with genetic effects, escaped farm fish may also negatively impact wild populations through non-reproductive ecological interactions (Bradbury et al., 2020 and references therein). Mechanisms such as pathogen transfer, competitive interactions, and ecological disturbance can all result in loss of productivity in wild populations and cause indirect genetic changes through the disruption of localised selective landscapes. Again, such outcomes from a specific escape event would be very difficult to disentangle from the ongoing farm/wild interactions occurring in the areas under investigation here.

The escape event on August 20^th, 2020, at Mowi’s Carradale North fish farm resulted in an escape of tens of thousands of fish with thousands of them estimated to have entered rivers in the surrounding area. There was no immediate significant genetic impact in the spawning season immediately following the escape, probably due to the lack of maturity in these fish. This does not mean that there may not be ongoing impacts distributed over space and time following the event; however these would be extremely difficult to quantify and disentangle from other sources of introgression over the area in question.

The results indicate that immediately following the Carradale escape, hybridisation with wild salmon was very limited in this specific case. In other such large-scale escapes, immediate impacts have been seen (e.g. Wringe et al., 2018). 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.