Infectious hematopoietic necrosis is generally considered a disease of salmonid alevins, fry, and early juveniles. It has been suggested that this disease kills only young salmonids and that susceptibility decreases with increased size and age of the fish. To determine if decreased susceptibility occurs with increasing size, we exposed four different sizes of rainbow trout Oncorhynchus mykiss (mean weight, 0.2–13.1 g) and kokanee (lacustrine sockeye salmon) Oncorhynchus nerka (0.2–7.2 g) to two waterborne strains of infectious hematopoietic necrosis virus. Both species were exposed to four concentrations of a type‐1 virus strain, which was isolated from fish in Oregon, and a type‐2 virus strain obtained from fish in Idaho. Neither strain of virus was consistently more virulent over all exposure concentrations for kokanee at the 0.2‐g fish size, At all other sizes, kokanee were consistently more susceptible to the Oregon strain than to the Idaho strain (sign test, P < 0.002). Conversely, all sizes of rainbow trout were consistently more susceptible to the Idaho strain than to the Oregon strain (sign test, P < 0.001). The results showed that there are differences in virulence for type‐1 and ‐2 strains of virus and that these strains have the ability to produce disease in kokanee and rainbow trout as large as 7.2 g and 13.1 g, respectively.

Juvenile steelhead trout (Salmo gairdneri), coho salmon (Oncorhynchus kisutch), and spring chinook salmon (O. tshawytscha) were infected by intraperitoneal or intramuscular injection with Aeromonas salmonicida or A. hydrophila at seven temperatures from 3.9 to 20.5 °C. At 3.9 and 6.7 °C, mortality in fish infected with A. salmonicida varied from 2 to 26% among the three salmonid species. At 20.5 °C 93–100% of these animals died within 2 or 3 days; at 6.7 °C or lower the fish survived for 12–23 days. Growth of A. salmonicida in vitro was influenced by temperature in a manner very similar to its influence on the in vivo infection. Comparable experiments with A. hydrophila gave results much like those with A. salmonicida, though some differences were noted. At a temperature of 20.5 °C percent mortality ranged from 64 to 100%. At 9.4 °C or below no deaths attributed to A. hydrophila occurred. Fatally infected fish died more rapidly at the higher temperatures. Key words: Aeromonas salmonicida, A. hydrophila, water temperature, furunculosis, motile aeromonas septicemia, coho salmon, chinook salmon, steelhead trout

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