The following is a case of fish disease that occurred mid-September 1998 in Canada. The site manager of an Atlantic salmon (Salmo salar) fish farm called a fish veterinarian to report his concerns. These concerns included gradually increasing mortalities in three of his eight seawater fish pens over the past two weeks. Normally, he expected to have an average of 10 mortalities per day per pen, but the number had increased to an average of 40. He said that the affected fish were lethargic, anorexic, and had a dark body color. The site manager was also worried, because a fish farm one mile away was reportedly having disease problems. Because this histoiy was vague and not indicative of any specific disease, a visit to the farm was scheduled.

The fish farm was located on a southern bay of Newfoundland. This is a center for salmonid aquaculture, because it has proper water temperatures and good tidal movement to flush away organic matter produced by the fish. The farm itself had eight polar circle pens with nets extending to 20 feet in depth. Each pen included 10-15 thousand fish, which had been hatched in a single freshwater hatchery in the spring on 1997. They were transferred to this saltwater site in the spring of 1998 for growout to a weight of 5-8 pounds. This population had not suffered any serious outbreaks of disease in that time and had not been exposed to antibiotic treatment.