Off the hook

As Vince McDonagh reports, a study by Norway’s Institute of Marine Research has added to the evidence that farmed salmon are not spreading diseases to their wild counterparts.

Marine researchers in Norway now believe there is little risk of farmed salmon infecting their migrating wild cousins with viral and bacterial diseases.

The Norwegian Institute of Marine Research (IMR) says this was shown by an investigation last year which monitored viruses and bacteria in migrating post smolt wild salmon and sea trout.

The study’s results should help to counter some of the claims frequently made against salmon farmers.

Those sections of the media which are anti-salmon farming frequently carry reports about diseases spreading to the wild population from farmed stock.

This is in spite of an earlier Canadian Department of Fisheries and Oceans report concluding that there is no, or minimal, connection between net-pen Atlantic salmon farming and the spread of disease in wild Pacific salmon.

The Norwegian Institute does concede that infectious disease in Atlantic salmon farming in Norway is a serious problem which has an impact on welfare of infected salmon.

It also says that the problem leads to substantial financial losses, something which was borne out in recent financial reports from some of the main salmon farming companies.

Each year, IMR checks whether wild fish have been infected with four selected viruses that also affect farmed fish. Bacteria that cause serious disease in salmonids were also added to the check list.

Abdullah S. Madhun, IMR researcher

Institute researcher Abdullah S. Madhun says that some 138 fish were examined and only three tested positive for salmonid alphavirus, more commonly known as SAV.

A further six tested positive for heart inflammation known as PRV-1 and 10 for piscine myocarditis virus or PMCV.

No fish tested positive for the bacterium Renibacterium salmoninarum, he added.

While all of these viruses can cause diseases in the fish, the levels of virus that were found are characterised as “very low”.

The findings of viruses are uncertain since there were indications of very low levels. For that reason, it can be a false positive result, says Madhun.

The investigations were carried out on 50 post-smolt salmon caught in a large fjord in Rogaland known as the Boknafjorden, 50 post-smolts from the Hardangerfjord in Vestland county and 38 sea trout taken outside Hitra in Trondelag.

Madhun says: “These are farming-intensive areas where the farmed fish have different disease profiles. If wild fish are affected by the farmed fish in terms of infection, then we expect to find such infection in wild fish caught in these areas.”

The Institute of Marine Research also investigated the prevalence of infectious salmon anaemia virus (ISA), salmonid alphavirus (PD virus), and bacterial kidney disease (BKD) infections in wild post-smolt Atlantic salmon and sea trout caught in 2023 in three aquaculture production areas.

Salmon kidney showing infection with Renibacterium salmoninarum

The fish were collected as part of the national monitoring programme for salmon lice. The post-smolts were taken in outer parts of Boknafjord and Hardangerfjord by trawling during May and June.

The Institute believes these findings complement and confirm its previously reported data and may indicate that the prevalence of pathogen infections in wild salmon post-smolt and sea trout is not significantly affected by the occurrence of these infections in fish farming.

In 2023 the researchers also investigated whether the wild fish were infected by the bacterium Renibacterium salmoninarum, which can cause the notifiable and serious more commonly known as bacterial kidney disease (BKD).

“Before 2022, the last known detection of this bacterium was from 2019. Then it was found in wild-caught broodstock for cultivation in Western Norway, but from December 2022, BKD has been detected or suspected at 14 breeding facilities in Norway,” says Madhun.

This increase in outbreaks meant that a test for this virus in wild fish was also introduced last year in Norway.

“The sea trout that was tested was caught in an area that had nine BKD outbreaks last year, but the mail smolt was from areas without BKD detection. None of the fish, neither sea trout nor post smolt, were infected with this bacteria,” he says.

The Institute says that the findings in this year’s report complement and support previous monitoring results.

Madhun concludes: “The results of the monitoring programme indicate that there is a very low incidence of infection in migrating wild salmon and sea trout.”

The study also suggests that there is a low incidence of infection from farmed fish to wild fish.

“It appears that the prevalence of infections in wild fish, caused by the examined viruses and bacteria, is not significantly affected by the occurrence of these infections in fish farming. The results may also indicate that it is unlikely that wild fish act as a reservoir for the investigated viruses and bacteria,” says Madhun.

This monitoring is carried out annually on behalf of the Norwegian Food Safety Authority. The samples are taken from fish that are collected as part of the monitoring programme for salmon lice on wild salmon (NALO).

Photo: Dave Thompson/USFWS