Susceptibility of Chinook salmon, Oncorhynchus tshawytscha (Walbaum), and rainbow trout, Salmo gairdneri Richardson, to infection with Vibrio anguillarum following sublethal copper exposure*

Chinook salmon, Oncorhynchus tshawytscha (Walbaum), and rainbow trout, Salmo gairdneri Richardson, were used to determine if sublethal copper exposure would increase their susceptibility to Vibrio anguillarum infection. Fish were pretreated with copper at fractional levels of the 96 h copper LC 50 before exposure to the pathogen. Mortality by vibriosis was greater among fish exposed to 9% of the copper LC 50 for 96 h than unexposed fish. Peak susceptibility to vibriosis depended in part on the interaction of exposure time and copper concentration. The higher copper concentrations produced peak susceptibility to infection in shorter time periods. After the peak of susceptibility, sensitivity to infection declined to near control levels in those fish where exposure was continued. Rainbow trout stressed by copper required about 50% fewer pathogens to induce a fatal infection than non‐copper exposed fish.