The Genetic Threat of Domesticated Salmon to Wild Populations

History has long warned us about the unintended consequences of tampering with nature. When domesticated salmon bred for life in captivity escape their sea cages and breed with their wild cousins, the result is weaker, less resilient hybrids that threaten the future of wild salmon populations.^{1, 2}

Ocean farms are in the business of farming salmon for profit. Producers seek to increase salmon production while decreasing the resources needed to raise farmed salmon.^{3, 4}

They populate sea cages with smolts that have been artificially bred, hatched and grown in captivity, hatchery environments. These young salmon have been selectively bred over several generations, making them much different  from wild Atlantic salmon. The intensive artificial selection process results in faster growing fish (egg to adult in 36 months for farmed salmon compared to about 60 months for wild salmon to reach the same size).^{5, 6} 

Even under normal conditions, farmed salmon can escape from sea cages due to human error (i.e., mishandling fish during health treatments and harvest). But when storms, predators, boats and equipment failure compromise the cages, tens of thousands of fish can escape at once.^{7, 8}

Researchers have observed escaped salmon across regions where salmon farming occurs, and in some areas, the number of escaped farmed salmon actually surpasses or is close to the total wild salmon population. A notable example is the Magaguadavic River in New Brunswick, Canada. In this river, located in one of the world's most intensive salmon farming regions, studies have shown that farmed salmon have contributed to more than 80% of the run in 7 of 15 years in which counts have been made. This indicates that, in those years, farmed salmon significantly outnumbered the wild salmon population in the river.⁹

Similarly, in Norway, research has highlighted the extensive genetic mixing between farmed and wild salmon. A study analyzing wild salmon populations across 147 rivers found that in some locations, up to 42% of the genetic material in wild salmon originated from farmed salmon. This high level of genetic introgression suggests a substantial presence of escaped farmed salmon within these wild populations.¹⁰

These escapees are poorly adapted to the natural environment, susceptible to predation and often carry diseases with them.^{11, 12} Additionally, when farmed salmon breed with wild Atlantic salmon, the resulting hybrids are genetically weaker and less suited to survival in the wild.^{13, 14}

This process is known as genetic introgression — the movement of genes from one species into the gene pool of another.^{15, 16}

Studies have shown that genetic introgression leads to reduced genetic diversity and resilience in wild salmon populations, whose numbers have already dropped 75 percent over the past 50-plus years.^{17, 9}

Introgression causes irreversible damage in wild gene pools. And there are no remedies or countermeasures available.^{6, 17}

Large-scale studies highlight the significant extent of this interbreeding.^3,4 For example, a comprehensive genetic analysis of 147 salmon rivers—covering three-quarters of the wild Atlantic salmon spawning population in Norway—found that farmed genetic introgression occurred in 109 rivers, with an average level of 6.4% and a maximum of 42.2%. Fifty-one rivers showed significant introgression compared to historical samples, with the highest levels found in regions of intensive salmon farming.⁴ This extensive interbreeding poses a serious threat to the genetic integrity and fitness of wild populations, not only in Norway but likely in other regions where escape events are common.

Over time, this genetic mixing threatens the viability and recovery of wild salmon populations by reducing their ability to thrive in their natural habitat.^{13, 17}

Studies also show a decrease in wild salmon reproductivity due to the introduction of farmed genes. This brings the wild Atlantic salmon population one step closer to extinction.^{14, 5}