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Salmon, aquaculture, and the spread of Infectious Salmon Anemia

Coho salmon - public domain image

Coho salmon - public domain image

In 2008, a deadly virus decimated Chilean aquaculture facilities, causing $2 billion in damage and crippling an industry. This week, preliminary reports suggest that this same disease may have infected wild salmon in the north Pacific. The internet has been blowing up with news reports of Infectious Salmon Anemia (ISA) detected in wild salmon populations. Reports range from balanced - Deadly Fish Farm Virus Found in Wild Pacific Salmon – to hyperbolic - B.C.’s salmon feedlots need to be closed – but all hinge on the fact that ISA, a lethal salmon-infecting virus previously resigned to aquaculture facilities, has been detected in wild salmon populations in British Columbia. This has the potential to be a very big deal. ISA is 90% lethal and mortality occurs in 10 days or less. The virus is waterborn, but can also be transmitted through handling with contaminated equipment. There is no treatment once a fish is infected.

Before I go on, a couple points need to be clarified:

  1. ISA does not infect humans, though as it threatens a fishery and a major agricultural industry, it most certainly affects humans.
  2. ISA was isolated from 2 wild sockeye salmon. It has not been confirmed from independent test yet, although one statement indicates that the current infection is from a non-infectious strain of ISA (which raises some interesting questions about who currently knows what about this outbreak).

What is ISA?

ISA is caused by an Isavirus in the Orthomyxoviridae family of RNA viruses. This family also includes the influenza viruses. The virus originates from a non-lethal strain found in Atlantic salmon. It made the jump into aquaculture facilities, where it mutated into a lethal strain. The first reports of ISA began in Norwegian fish farms in the mid-1980′s. Since then, ISA has appeared in several aquaculture facilities around the world. While it predominantly affects Atlantic Salmon (the most common farm raised salmon, even in Pacific aquaculture facilities), it is also known to infect Atlantic herring, Atlantic cod, Sockeye Salmon, Coho salmon, rainbow trout, brown trout, and arctic char.

Fish farms, especially off-shore floating pens, are breeding ground for diseases. Concentrated livestock makes vector transmission much more likely, and factory farmed fish, bred or engineered for rapid growth, tend to have weakened immune systems, making them more susceptible to disease than their wild counterparts. Off-shore pens and fish escapes provide a mechanism for introduction of disease vectors into wild populations. Whether or not the disease takes hold in wild populations depends on the biology, physiology, and behavior of the fish. It may be that a disease which wreaks havoc on farmed fish may not be able to take hold outside of the farm.

ISA thrives at 15 degrees C, which the waters around British Columbia reach in the summer. If ISA really has jumped into wild salmon populations, it may not become epidemic until the waters warm.

The future of Salmon

Beyond being an incredibly profitable industry that supports many communities on the Pacific coast, salmon fulfill vital ecosystem processes both offshore, where wild adult salmon mature, and inshore, when they return to their natal rivers to reproduce. An ISA epidemic in wild populations would have major repercussions for the entire north Pacific marine ecosystem and could dramatically destabilize west coast fisheries.

For now, we’re waiting on independent confirmation that ISA has made the leap from farm-raised to wild salmon, and that it is the lethal strain. Currently, fish farms in British Columbia have not reported any incidents of ISA, so if the case is confirmed, it is unlikely that the source of the outbreak was from those farms. It’s possible that low levels of ISA have always been present, undetected in fish stocks, or that the virus was transported from other fish farms through shipping. Also possible, though less likely, is that the newly opened northwest passage has allowed infect Atlantic salmon to migrate through the arctic to the Pacific ocean. Were that the case, this would be only the beginning of large-scale ecologic changes that will accompany the warming north.

If this truly is an outbreak of the lethal strain, this may be the beginning of the end for open-ocean salmon farms.


Deep-sea biologist, population/conservation geneticist, backyard farm advocate. The deep sea is Earth's last great wilderness.


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