Some Factors in Susceptibility of Juvenile Rainbow Trout and Chinook Salmon to Chondrococcus columnaris
Susceptibility of juveniles of chinook salmon (Oncorhynchus tshawytscha) and rainbow trout (Salmo gairdneri) to Chondrococcus columnaris disease appeared to be influenced by age offish, crowding, and water temperature. Rainbow trout of about 1 g and chinook salmon of about 3 g were less susceptible than smaller fish, and tests with chinook salmon suggested that age was more important than weight. When chinook salmon averaging 4.3 g were exposed from July to October to the pathogen in river water, mortality ranged from 1.0% in a trough with 50 fish to 12% in one with 900. Mortality of rainbow trout averaging 2.4 g that were exposed to the pathogen in trough river water averaged 4.7–22% after 4 weeks, at temperatures from 17.2 to 17.8 C; the survivors, however, were generally resistant to subsequent infection, suggesting development of an immune response. Times to 100% mortality of sibling chinook salmon averaging 3.9 g that were exposed for 25 min to about 2.5 × 105 organisms per liter of sterile river water were about 19 hr at 10 C and 8 hr at 22 C. Survival of rainbow trout was higher when they were held at 2.2 degrees C below than when held at 2.2 degrees C above seasonal river temperatures.