Don Staniford, who runs the Scottish Salmon Watch website, has been campaigning against salmon farming in Scotland for 25 years.
On January 10 he posted on Twitter: 'Infectious Salmon Anaemia reported at Landcatch's Ormsary hatchery in November...'
Landcatch - part of Dutch-based Hendrix Genetics - however, told the Argyllshire Advertiser that Mr Staniford's claims omit some important details.
Infectious Salmon Anaemia (ISA) is a disease of Atlantic salmon, first reported in Norway in 1984.
It is a notifiable disease and fish farms are routinely inspected for clinical signs in fish, the symptoms of which include severe anaemia, fluid build-up in the body cavity, internal haemorrhage and darkening of the liver.
But there are different strains of the virus - ISAv (HPR0), which is non-pathogenic (does not cause disease in other fish), and the more serious pathogenic ISAv (HPR-deleted) strain.
Scottish Government records show that there have been two HPR-deleted outbreaks in Scotland - in 1998-99 and 2008-09 - both successfully eradicated.
Mr Staniford made his claims after receiving a reply from Scottish Government department Marine Scotland to a Freedom of Information request.
In its response, Marine Scotland confirmed that, among others in Scotland, one case of ISA was found on November 1, 2021 at Ormsary Broodstock Unit, operated by Landcatch.
Routine movement restrictions were placed on the unit before being revoked when the presence of ISA HPR0 was confirmed.
Jarl van den Berg, general manager for Hendrix Genetics UK salmon operations, said: 'There has been no ISA (HPR-deleted) found in Ormsary. What has been found at Ormsary is the non-pathogenic HPR0 strain, and is of no concern or subject to any measures.
'The pathogenic strain can be a huge problem, as seen in Norway. That is why Marine Scotland is active in monitoring to protect the Scottish waters.
'In the process of identifying the strain the site was shut down and after five days it was clear that it was the non-pathogenic HPR0 strain and all measures were lifted.'
He added: 'As an industry in Scotland we are clear of ISA and have been for many years and will continue to be so thanks to the efforts of institutions like Marine Scotland.'
Mr Staniford also suggested that ISA might have been imported via infected salmon eggs (ova) from Iceland.
Mr van den Berg responded: 'The import of eggs has nothing to do with it. It is common for Scotland to import eggs from neighbouring regions and these are all tested for a plethora of diseases.'
'Scottish Salmon Watch campaigns to close down the welfare and disease nightmare of salmon farming in Scotland,' said Mr Staniford.
'Salmon farming is a cancer on the coast and the thousands of cages littering Scotland must be closed down to safeguard wild fish and the tourist economy.'
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