Disease
Open net-pen salmon farming began on British Columbia’s coast in the 1980s.
In 1991, the industry expanded into the primary Fraser River sockeye migration route.
Immediately, the Fraser River sockeye runs migrating past the farms began to decline.
The one exception was the Harrison sockeye. These salmon migrate to sea along a southerly route that avoids the salmon farms. While other Fraser runs declined, the Harrison sockeye instead increased in numbers over the same time period. (Morton and Routledge 2016).
This is a widely reflected trend (Ford & Myers 2008). The only wild salmon populations thriving are those not exposed to salmon farms.
Viruses – The single greatest threat from salmon farms in the ocean
Wild salmon populations stay healthy because predators eat the weaker, slower fish. This means that sick fish vanish before they infect other fish around them.
Open net-pen fish farms on the other hand, keep predators out, leaving ailing fish to die slowly, shedding disease over a much longer time.
Farm conditions suited to the spread of disease
- Fish farms can crowd in more than a million fish – allowing disease to spread easily from fish to fish, building up to astronomical numbers.
- Crowding allows viruses to reproduce quickly. This is dangerous because it increases mutations that can greatly multiply in virulence.
- Most BC salmon farms raise Atlantic salmon – wild Pacific salmon may have no immunity to diseases of this imported species.
- Disease is nature’s response to overcrowding so farmers resort to drugs. While antibiotics can keep farm salmon alive until they reach market size, they will in many cases remain contagious. Drugs may keep the farm fish alive but wild fish get no such protection.
- Wild salmon migrating past fish farms encounter a blizzard of larval sea lice, viruses and bacteria. As they breathe, contaminated water passes over their gills and brings biological contaminants into direct contact with their bloodstream.
- Salmon pathogens are not simply drifting particles. In many cases, they have evolved to seek out and infect the fish, acting like ‘salmon smart bombs’.
- Salmon farms are located in narrow passages so wild salmon are exposed to farms at the two most vulnerable stages in their lives – as juveniles leaving the rivers and as adults preparing for courtship and spawning.