Comparison of the Susceptibility of Four Rainbow Trout Strains to Cold-Water Disease

Susceptibility to cold-water disease was compared among four strains of Rainbow Trout Oncorhynchus mykiss : Arlee strain from Ennis National Fish Hatchery, Montana (AL-EN), the Arlee strain from Jocko River Hatchery, Montana (AL-JR), a cold-water disease-resistant strain (WV), and the Harrison–Hofer strain (HH). Bacterial challenges were either by bath or intraperitoneal injection (50 μL of 0.65 optical density). Each strain was exposed at 75 d after hatch to either the CSF 259-93 (Idaho) or 09–104 isolate (Utah) of Flavobacterium psychrophilum. Injection controls received a phosphate-buffered saline (PBS) solution and bath controls were exposed to uninoculated sterile broth (tryptone yeast extract salts) mixed 1:1 with hatchery well water. For injected fish, the WV had significantly lower mortality (20.0–36.7%) than HH and AL-EN (76.7–96.7%) but did not significantly differ from AL-JR (46.7–56.7%). Injected fish had significantly higher mortality than bath-exposed fish. For bath-exposed fish, the WV had significantly lower mortality (0%) than the HH (10.0–26.7%), but both Arlee strains had intermediate mortality values (0–13.3%) that did not significantly differ from either the HH or WV strain. There were no significant differences between the two bacterial isolates, indicating similar virulence and similar resistance response of WV to another novel isolate of F. psychrophilum.