Estimation of disease-induced mortalities in outmigrant salmon associated with environmental stressors in the Columbia River Basin

Infectious disease within outmigrant juvenile salmon in the Columbia River Basin is modulated, in part, by stressors that influence host-susceptibility. Through the application of a dose(stressor)-structured population dynamic model, we show that chemical (both in the river and estuary) and in-river (dams and predation) stressors influence host-susceptibility, increasing the mean force of infection (defined as the per capita acquisition rate of infection) by a factor of 2.2 (4.8-fold upper 95%) and 1.6 (2.4-fold upper 95%), respectively. Using Listonella anguillarum as a model pathogen, non-chemical in-river and chemical stressors contribute equally to a cumulative incidence of delayed disease-induced mortalities in Chinook salmon that range from 3 to 18% for estuary residence times of 30 to 120 days, respectively. Mitigation of environmental stressors that increase host-susceptibility could represent a significant component in future management strategies to recover listed stocks.