Fish disease detected at RSPCA-approved salmon farm

Research shows the overwhelming majority of infectious salmon anaemia cases occur in farmed fish in sea water.

A disease has been detected at a salmon farm in the Highland region.

Infectious salmon anaemia (ISA) was found among the fish population at Loch Duart’s Lochmaddy salmon farm in Sutherland, an Scottish SPCA-approved site.

The disease is a viral disease of Atlantic salmon. Research shows the overwhelming majority of cases occur in farmed fish in sea water.

A salmon disease was reported at Loch Duart's Lochmaddy salmon farm in Sutherland (pic: Andrew Milligan)

It was reported at the site by Marine Scotland officials.

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ISA exists in two forms: a pathogenic and a non-pathogenic form.

A Scottish Government spokesperson said the pathogenic form, referred to as HPR-deleted, which is listed in Scottish fish disease control legislation, requires statutory control.

However, it was the non-pathogenic form, referred to as HPR-0, which was present at the site.

The spokesperson said: “Results from on-site sampling at Loch Duart’s Lochmaddy site and subsequent laboratory investigation found the Infectious Salmon Anaemia Virus (ISA) was the benign form which does not cause mortality in farmed salmon stocks.

“On that basis, no further action was taken, or was appropriate, under fish health legislation.”

The disease was first reported in Norway in 1984, but has since been reported in Canada, the USA, the Faroe Islands, Ireland and Scotland.

Both outbreaks of ISA in Scotland in 1998-99 and 2008-09 were successfully eradicated.

Atlantic salmon is the only susceptible species known to develop clinical disease, but ISA virus can replicate in rainbow trout and sea trout.

The Fish Health Inspectorate (FHI) undertakes risk based surveillance for aquatic animal disease within fish and shellfish and proactivity publishes the results and reports of its inspections.

2022 figures showed salmon deaths on fish farms in Scotland nearly doubled compared to the year before. Campaigners blamed the surge in deaths on overcrowding, while growing levels of disease, parasites and jellyfish blooms were also cited as reasons.

The FHI data showed that nearly 15m salmon mortalities were reported by farms in Scotland from January to November 2022, compared with 8.58m in all of 2021 and 5.81m in 2020.

Campaigners, however, have said more fish are dying than mentioned in published data.

FreeSalmon and Costal Communities Network Scotland claim there have been almost twice as many fish deaths between 2018 and 2023 as those published by FHI.