The Impact of Farmed Salmon on Wild Populations
How sea cage salmon farming threatens species survival
More farmed salmon equals fewer wild salmon.
In fact, according to a 2024 review of factors contributing to the long-term decline of Atlantic salmon in the Conne River in Newfoundland, Canada the sea cages in which salmon are farmed are the single biggest threat to wild salmon — even greater than climate change.1
Sea cages can be so devastating because they are often located along critical migration paths for wild Atlantic salmon smolts.2,3 As those wild smolts migrate from freshwater rivers to the ocean, they move through waters contaminated by sea cages, where parasitic sea lice thrive.4 The lice are attracted by high-density conditions in the sea cages, which allow the sea lice to breed exponentially faster.5 These lice cause severe stress, tissue damage and even death to the wild smolts migrating near them. They can literally eat the smolts alive.6
Additionally, farmed salmon that escape and interbreed with wild Atlantic salmon weaken genetic diversity, producing offspring unfit for survival in the wild.7
Studies show that wild salmon near sea cages experience higher mortality rates and faster declines than wild salmon in other areas. That impact is directly linked to the size and number of nearby salmon farms.8, 9, 10
While eliminating climate change is an intractable problem, regulating or even eliminating sea cages is within our grasp.
In-text Citations:
- J. Brian Dempson, Travis E. Van Leeuwen, Ian R. Bradbury, Sarah J. Lehnert, David Coté, Frédéric Cyr, Christina Pretty & Nicholas I. Kelly (18 Apr 2024): A Review of Factors Potentially Contributing to the Long-Term Decline of Atlantic Salmon in the Conne River, Newfoundland, Canada, Reviews in Fisheries Science & Aquaculture, DOI: 10.1080/23308249.2024.2341023
- Bjerck, H.B., Urke, H.A., Haugen, T.O. et al. Depth use of wild Atlantic salmon post-smolts migrating through fjords. Anim Biotelemetry 12, 34 (2024). https://doi.org/10.1186/s40317-024-00390-1
- Frazer L. 2009. Sea-cage aquaculture, sea lice, and declines of wild fish. Conservation Biology, 23: 599– 607.
- Gargan, Paddy & Tully, Oliver & Poole, Russell. (2007). Relationship Between Sea Lice Infestation, Sea Lice Production and Sea Trout Survival in Ireland, 1992‐2001. 10.1002/9780470995495.ch10.
- Helland, I.P., Finstad, B., Uglem, I., Diserud, O.H., Foldvik, A., Hanssen, F., Bjørn, P.A., et al. 2012. What determines salmon lice infections on wild salmonids? Statistical calculations of data from the national salmon lice surveillance program 2004–2010 (in Norwegian). NINA Report, 891. 51pp.
- Krosek, M., J. Ford, A. Morton, S. Lele, R.A. Myers, and M. Lewis. 2007. Declining wild salmon populations in relation to parasites from farm salmon. Science 318, 1772-1775.
- Wagner G. N., McKinley R. S., Bjørn P. A., Finstad B. 2003. Physiological impact of sea lice on swimming performance of Atlantic salmon. Journal of Fish Biology, 62: 1000–1009.
- Gargan, P. G., Forde, G., Hazon, N., F., R. D. J. & Todd, C. D. 2012. Evidence for sea lice-induced marine mortality of Atlantic salmon (Salmo salar) in western Ireland from experimental releases of ranched smolts treated with emamectin benzoate. Canadian Journal of Fish and Aquatic Sciences, 69: 343-353
- Krkošek, M., Revie, C.W., Gargan, P.G., Skilbrei, O.T., Finstad, B., and Todd, C.D. 2013. Impact of parasites on salmon recruitment in the Northeast Atlantic Ocean. Proceedings of the Royal Society B: Biological Sciences 280: 20122359.
- Thorstad, E.B. et al. 2015. Effects of salmon lice Lepeophtheirus salmonis on wild sea trout Salmo trutta — a literature review. Aquaculture Environment Interactions 7: 91–113.
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