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The first scientific evidence that farm salmon are infecting large numbers of wild salmon


The first scientific evidence that large numbers of wild salmon are becoming infected with piscine reovirus (PRV) through exposure to salmon farms was published this week in the scientific journal Public Library of Science One.

The effect of exposure to farmed salmon on piscine orthoreovirus [reovirus] infection and fitness in wild Pacific salmon in British Columbia, Canada” reports that the percentage of wild salmon infected with piscine reovirus was much higher in wild salmon exposed to salmon farms, than in wild salmon not exposed to salmon farms.

Link to paper

This is the first study to address this question in BC.

Sockeye salmon swim past salmon farms Tavish CampbellJuvenile sockeye salmon swimming past salmon farms are exposed to piscine reovirus shed by the farm salmon. Photo by Tavish Campbell

Unable to sample farm fish directly from the marine pens, the team purchased 262 fresh BC farmed salmon and 35 farmed steelhead from supermarkets. Polymerase chain reaction (RT-qPCR) tests detected piscine reovirus in 95% of the farm salmon and 69% of the farmed steelhead.

Farm salmon purchased for testing Photo by A Morton (1)

262 fresh BC farmed salmon and 35 farmed steelhead from supermarkets. Photo by Sabra Woodworth

The highest percentages of wild salmon infected with PRV were found in high-density salmon farmed regions i.e. the Broughton Archipelago (45%) where First Nations are extremely concerned that salmon farms have contributed to the collapse of local wild stocks, Lois Lake (40%) where steelhead farms operate and the Discovery Islands (37%) where the Cohen Commission concluded farm salmon disease could have serious and irrevocable impact on Fraser River sockeye salmon returns. As well, 40% of returning wild adult salmon in the lower Fraser River and 76% of trout in Cultus Lake were infected.

In contrast, only 5% of wild fish on the north coast of BC and in the Skeena and Nass Rivers were infected, these regions were the furthest from salmon farms. PRV was detected in all species of Pacific salmon also and trout.

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Results are arranged in decreasing order. The “Wild” designations reflect the Region numbers in Fig 1; i.e., Wild 1 is from Region 1. Numbers of fish sampled are provided in parentheses on the horizontal axis labels. Relevant estimates and confidence limits for key differences in this figure were generated by the logistic regression modelling where the effects of potential confounding variables could be filtered out (Fig 5).

The most obvious explanation for this pattern of infection is that the highly infected farmed fish are passing the virus on to nearby migrating wild fish, and that some infected fish eventually make their way to the more remote parts of the coast. Previous research published by this team in Virology Journal reports the strain of PRV found in this study originated in Norway.

The most significant finding to British Columbians is that PRV prevalence in Fraser River salmon dropped an estimated 50% between the lower vs. the upper Fraser River.

“This suggests that salmon infected with PRV are less capable of swimming up through strong rapids in places like Hells Gate and therefore unable to reach their spawning grounds,” says co-author Dr. Rick Routledge, Simon Fraser University professor emeritus.

“When you consider that nearly 40 - 45% of salmon swimming past salmon farms are testing positive for PRV and that we found evidence that this virus could make it more difficult for salmon to swim upriver this adds up to enormous potential impact on wild salmon, whales, First Nations and British Columbians,” says lead author, Alexandra Morton.


Infectious disease is a growing problem for the global salmon farming industry. Each BC salmon farm contains up to 1.7 million Atlantic salmon. This unnaturally high density allows disease to spread easily fish to fish. While the nets keep the farm fish in, they also keep out the predators that would otherwise remove sick fish and prevent disease outbreaks. PRV causes acute infection of the salmon’s red blood cells and if the infection progresses, can damage the salmon’s heart and swimming muscles leaving the fish very weak. PRV is particularly difficult to control on the farms.

Exotic Virus

The genetics of the piscine reovirus found in BC, are so closely related to PRV in Norway, that it appears that the virus originated in Norway [1]. The majority of BC farm salmon are Mowi Norwegian stock[2]. PRV was discovered in 2010 and so Atlantic salmon eggs imported to BC 1985-2010 were not screened for PRV. The virus has spread widely through Norwegian salmon farms since 1999. See Video of Norwegian Scientists discussing piscine reovirus

Emerging Virus

Reported as harmless salmon by a Canadian government /industry researcher team[3], a more recent paper confirms PRV is a disease agent in BC and causing Hearth and Skeletal Muscle Inflammation (HSMI) in BC farm salmon as in Norway [4]. Farm salmon can infect Pacific salmon, but there is no evidence that wild salmon can infect farm salmon[5].


In 2015, Federal Court ruled that Fisheries and Ocean Canada (DFO) must test farm salmon for PRV before permitting transfer of the young fish from freshwater hatcheries into the marine pens.[6] The Minister of Fisheries has failed comply and so a second lawsuit was filed seeking a court order forcing Minister LeBlanc to test for PRV so test results can be used to comply with section 56 of the Fishery (General) Regulations which prohibits transfer of farm fish carrying a disease agent into Canadian marine waters.

Fisheries and Oceans Canada has recently initiated the process to abdicate farm salmon disease testing to the Canadian Food Inspection Agency (CFIA)[7]

Marine Harvest has provided statements to the court admitting that the industry is heavily infected with PRV and would be "severely" impacted if transfer of PRV-infected fish into ocean farms is prohibited.


Three papers co-published by a team of researchers including BC and Federal government and Marine Harvest scientists suggest PRV occurs naturally in BC [8], is spread homogeneously coastwide[3] and does not cause disease.3,[9] However, an international team, including BC federal and international scientists report PRV is causing disease in BC and thus is a disease agent.[10]

The BC lab involved in this work is under investigation by the Provincial Government for conflict of interest.[11]

A recent exposè by Canada's premier news program W5 raised questions about government research into piscine reovirus and has now issued an update revealing the discussions between government and industry that led to the longstanding position that PRV was not causing disease in BC.

I am grateful to my the co-authors.

B Roll Footage

Farm salmon sample2 - photo by Sabra Woodworth
Farm salmon purchased and tested in the ongoing research. Photo by Alexandra Morton

Farm salmon sample - photo by Sabra Woodworth (1)

Atlantic salmon head for sale in North Vancouver. Photo by Sabra Woodworth

Hand purse-seining for juvenile wild salmon near salmon farm Photo by Tavish Campbell

Hand-purse seine used to catch juvenile wild salmon near a salmon farm in the Discovery Islands. Photo by Tavish Campbell

Emaciation is a symptom of HSMI photo by Ernest Lafred

Intensive industrial salmon farming allows pathogen populations to increase. The underweight fish in the center is exhibiting underweight the characteristic of HSMI Photo by Ernest Alfred

Alexandra Morton photo by Clio Nelson (1)

Lead author, Alexandra Morton has been publishing on the impact of salmon farms on BC wild salmon and orca for the past 20 years. Her research into the spread of viruses from farm salmon began when she viewed provincial farm salmon health records as a participant of the Cohen Commission. This provincial lab is now under investigation Photo by Clio Nelson