The Canadian Food Inspection Agency says secondary tests have confirmed that an infectious virus was present in fish at farm operated by Mainstream Canada, off Vancouver Island.
The agency said all the fish have been destroyed and the farm in Dixon Bay, near Tofino, will remain under quarantine until all pens, cages and equipment have been cleaned and disinfected.
Infectious haematopoietic necrosis (IHN) spreads rapidly and can kill up to 100 per cent of a population that become infected.
The virus was first suspected at Dixon Bay following routine testing on May 14.
The virus, which attacks a fish's blood, usually kills the animal within a week of exposure.
The CFIA said the IHN virus does not pose a risk to human health and is found in wild fish in the Pacific. The agency also said the last confirmed case of IHN detected in B.C. farmed fish occurred in 2003.
Fish farms on alert
A quarantine was imposed earlier at another Mainstream Canada fish farm at Bawden Point, north of Tofino in Herbert Inlet, also over fears the virus might be present.
A third quarantine had been imposed on a fish farm on the Sunshine Coast operated by Grieg Seafood. Later testing for the IHN virus at that location came back negative.
The CFIA said that in June it will begin posting reports for all confirmed cases of federally reportable aquatic animal diseases on its website, on a monthly basis.
In addition to screening for IHN, the Canadian Food Inspection Agency conducts surveillance testing for infectious salmon anaemia and infectious pancreatic necrosis.