The effects of changing demographics on the distribution of marine anemia in farmed salmon in British Columbia.

The changing geographic distribution of marine anemia (plasmacytoid leukemia) was compared with the evolving demographics of the chinook farming industry in British Columbia to explore the hypothesis that the disease had spread throughout the province between 1987 and 1992. Through retrospective and prospective methods, it was shown that the apparent spread of the disease was likely an artifact, resulting from changes in the distribution of fish farms throughout the province and corresponding changes in the intensity of regional disease surveillance. When viewed over a 5-year period, there were no statistically significant differences in the prevalence of the disease amongst fish sampled from each of the major salmon farming regions of British Columbia. By increasing the intensity of surveillance for the disease in apparently negative regions or chinook farms, one could routinely find cases of the disease. The results suggest that marine anemia is an endemic problem for farmed chinook salmon in British Columbia and is not a spreading epidemic.