Open-net salmon farming: the impact of escapes
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18 November 2021
Farmed salmon kept in open-net cages can escape into the wild both through repeated ‘trickle’ losses that see relatively small numbers escape over a prolonged period and through large-scale one-off events. In either case, the impact can be significant.
Why do escapes present a problem for wild fish?
Selective breeding over 50 years of salmon farming has created significant physical and genetic differences between wild and farmed Atlantic salmon1.
Wild salmon are highly adapted to their local environments and show strong differentiation between different populations. Most farmed salmon have been bred from limited stock for a high growth rate, high fat content and delayed sexual maturation along with other commercially driven traits.
A recent study found 44 genetic markers of domestication in farmed Atlantic salmon relating to commercially important characteristics after just 12 generations of selective breeding1. These genetic changes separate the farmed salmon not only from wild salmon generally but also from the populations they were originally sourced from.
Farmed salmon and wild salmon are very different fish.
The impact of escapes
1. Farmed salmon have a very low rate of survival in the wild
Farmed salmon have been changed so much by domestication that they are not well adapted to the environment they escape into. In fact, they are less than half as likely to survive in the wild compared to wild Atlantic salmon. The likelihood of survival for farmed salmon lowers even more the further through development they are at the point of escape.
2. Volume of escapes
While official statistics suggest otherwise, scientists estimate that accounting for under-reporting the number of salmon escaping from farms globally has outnumbered the total wild populations in some years. Despite being poorly adapted to survival outside of farms, so many farmed salmon escape that they still cause significant problems for populations of wild salmon.
In 2020 the number of salmon reported to have escaped from Scottish salmon farms was 204,901 4 ,5. The total number of wild salmon caught in Scotland in 2020 was 45,366 and most of the latter figure were outside the main salmon farming areas 4 ,5.
3. Escaped farmed salmon hybridise with wild Atlantic salmon and reduce their fitness
Because of the number of farmed salmon that escape each year, some survive to breed with wild salmon producing hybrids. These hybrids have a lower genetic fitness than wild salmon but still compete for food sources. 23.3% of sites surveyed in Scotland show evidence of hybridisation between wild and farmed salmon, and evidence of hybridisation was concentrated in areas with freshwater and marine salmon farming6. 23.3% is thought to be an underestimate of the true extent of genetic mixing.
Reducing the impact of escapes
The only solution (to prevent escapes and thus potential breeding with wild fish) is to move all salmon farming in Scotland into closed containment systems which put physical barriers between farmed and wild fish.
Populations of wild salmon in smaller river systems are particularly vulnerable to insidious and frequent introgression – potentially triggering an extinction vortex in these populations. The proposed expansion of Atlantic salmon farming without significant changes to farming practices (and the elimination of escapes) poses a serious and escalating threat to the genetic integrity of many wild salmon populations, especially those close to areas of salmon farm production (both marine and freshwater).
In Norway escaped farmed salmon and sea lice have been identified as posing the largest threats to the viability of wild salmon populations.
1.Gutierrez, A.P., Yáñez, J.M. and Davidson, W.S. (2016). Evidence of recent signatures of selection Gutierrez, A.P., Yáñez, J.M. and Davidson, W.S. (2016). Evidence of recent signatures of selection during domestication in an Atlantic salmon population. Marine Genomics, 26, pp.41–50.
2. Skaala, Ø., Besnier, F., Borgstrøm, R., Barlaup, B., Sørvik, A.G., Normann, E., Østebø, B.I., Hansen, M.M. and Glover, K.A. (2019). An extensive common‐garden study with domesticated and wild Atlantic salmon in the wild reveals impact on smolt production and shifts in fitness traits. Evolutionary Applications, 12(5), pp.1001–1016.
3. Skaala, Ø., Glover, Kevin A., Barlaup, Bjørn T., Svåsand, T., Besnier, F., Hansen, Michael M. and Borgstrøm, R. (2012). Performance of farmed, hybrid, and wild Atlantic salmon (Salmo salar) families in a natural river environment. Canadian Journal of Fisheries and Aquatic Sciences, 69(12), pp.1994–2006 and Bradbury, I., Burgetz, I., Coulson, M., Verspoor, E., Gilbey, J., Lehnert, S., Kess, T., Cross, T., Vasemägi, A., Solberg, M., Fleming, I. and McGinnity, P. (2020). Beyond hybridization: the genetic impacts of nonreproductive ecological interactions of salmon aquaculture on wild populations. Aquaculture Environment Interactions, 12, pp.429–445.
4. Murno, L.A. (2021). Scottish Fish Farm Production Survey 2020. Marine Scotland Science.
5. Thorstad, E.B., Fleming, I.A., McGinnity, P., Soto, D., Wennevik, V. & Whoriskey, F. 2008. Incidence and impacts of escaped farmed Atlantic salmon Salmo salar in nature. NINA Special Report 36. 110 pp.
6. Gilbey, J., Sampayo, J., Cauwellier, E., Malcolm, I., Millidine, K., Jackson, F. and Morris, D.J. (2021). A national assessment of the influence of farmed salmon escapes on the genetic integrity of wild Scottish Atlantic salmon populations. Marine Scotland Science.
7. Norwegian Scientific Advisory Committee for Atlantic Salmon. Status of wild Atlantic salmon in Norway 2020. Linked here.