Kudoa thyrsites is a myxozoan parasite of marine fish with a global distribution. In British Columbia (BC), Canada, severe infections are associated with an economically significant degradation of fillet quality in farmed Atlantic salmon. Exposures to naturally occurring actinospores at a coastal research laboratory were used to test the hypothesis that the prevalence and severity of K. thyrsites infections acquired by exposure of Atlantic salmon to seawater (SW) of various depths are not different. In Expt 1, fish were exposed to SW from 1, 7 or 13 m below the surface. Following exposure to deeper-sourced SW, the prevalence of K. thyrsites, determined from microscopic examination of muscle histology sections, was greater in all 4 trials and the severity of infection was greater in 2 trials. In Expt 2, infections were compared over time among salmon held in tanks supplied with deep-sourced SW (raw or UV-irradiated) or in a surface net-pen. The infection was observed in 35 of 40 fish sampled between 3 and 6 mo after tank exposure to raw SW. Coincidentally, the parasite was observed in 4 of 40 fish maintained in the net-pen. No consistent association of the parasite infection was observed with temperature; however, reduced salinity and solar radiation were not ruled out as factors which may reduce the risk of infection from surface SW. These findings require verification at commercial aquaculture sites in BC, as they will inform considerations related to farm siting and net-pen configuration.
Keywords: Atlantic salmon; Depth; Infection; Kudoa thyrsites; Myxozoa.