A Field Investigation of Marine Anemia in Farmed Salmon in British Columbia
Stephen, Robert Craig
This investigation was designed to describe the medical ecology of marine anemia as it occurred in salmon populations in British Columbia in order to develop and evaluate causal and intervention hypotheses. The historical case definition of marine anemia was evaluated by conducting an observer variation trial. A new working case definition was developed by reviewing features of previously diagnosed cases and applying mathematical decision rules. Retrospective and prospective observational methods were used to describe the changing spatial, temporal, and host distributions of the disease. Behavioural observations of marked moribund fish and mortality surveys were employed to evaluate methods of sampling for marine anemia in seapens. Finally, a descriptive epidemiological study was conducted to describe the impact of marine anemia in British Columbia, to identify potential causal factors, and to suggest possible avenues for control. Marine anemia was described as an endemic disease of farmed chinook salmon inBritish Columbia. The broad demographic characteristics of the disease and its limited impact suggested that the introduction of a new pathogen was probably not the "cause" of the emergence of marine anemia. Historical problems with identifying the disease in individuals and populations suggested that marine anemia is more likely a newly described disease than a truly new disease. The reliance on surface moribund fish as indicators of the prevalence and impact of marine anemia in seafarms potentially biased previous descriptions of the disease. When an explicit case definition, applicable to all members of the seapen population was used, the disease could not be associated with excessive mortality rates. The diagnosis was, however, associated with a variety of factors capable of stimulating an excessive immune response. Future control efforts should be directed towards decreasing the overall level of infectious disease in seapens, as well as social and physiological stressors inherent in current techniques of seapen farming. Our results emphasized the importance of studying a disease in a population and ecological context before intervention and causal inferences can be accepted.