The dynamics of <i>Salmincola californiensis</i> infecting reservoir-rearing juvenile Chinook Salmon

ABSTRACT Objective We evaluated the infection dynamics of Salmincola californiensis in populations of wild juvenile Chinook Salmon Oncorhynchus tshawytscha rearing in reservoirs by addressing (1) prevalence and intensity of infection, (2) spatial and temporal patterns in relation to reservoir conditions, and (3) the effects on host survival in wild populations. Methods We analyzed historical (2012–2016) and contemporary (2020) field data from three Willamette River basin reservoirs in Oregon. We employed mixed-effects hurdle models to evaluate relationships between infection metrics and environmental variables and used full likelihood methods to estimate parasite-associated mortality in reservoir-dwelling Chinook Salmon populations. Host abandonment by copepodids also was experimentally evaluated to correct field estimates of infection intensity. Results Infection prevalence and intensity were positively related to fish age, water temperature, and reservoir surface elevation while being negatively related to reservoir inflows. Prevalence increased fivefold for each 11-week period, with yearlings having 33 times higher infection prevalence than subyearlings. Evidence of parasite-associated mortality was detected both during reservoir residency and passage through dams, with estimated lethal thresholds of three to eight adult parasites per fish. Significantly lower infection rates were observed in fish sampled below dams compared with those within reservoirs. Conclusion Environmental conditions influenced by reservoir operations significantly affect S. californiensis infection dynamics in juvenile Chinook Salmon. The relationships between increased infection rates and reduced water flow, higher temperatures, and elevated reservoir levels suggest potential management strategies through altered reservoir operations. Our findings demonstrating parasite-associated mortality in wild populations highlight the ecological importance of this host–parasite interaction and its relevance to salmonid conservation in regulated river systems.