Size-dependent susceptibility to infectious salmon anemia virus (ISAV) in Atlantic salmon (Salmo salar L.) of farm, hybrid and wild parentage

A total of 388 Atlantic salmon juveniles of farm, hybrid and wild parentage were challenged in mixed group replicate tanks by intraperitoneal injection of a cell-cultured isolate of the infectious salmon anemia virus (ISAV). The first mortalities were observed 21 days post-challenge and the experiment was terminated after 38 days. No significant differences in timing of mortality or total mortality were observed between experimental groups (mortality across replicates: farm = 92%, hybrid = 88%, wild = 87%). The length frequency distributions of all experimental groups were distinctly bimodal, permitting fish to be putatively characterized as smolts (> 12 cm total length) and non-smolts (< 12 cm total length). Within all experimental groups, fish < 12 cm displayed lower mortality and delayed timing of mortality compared to fish > 12 cm. When the presumptive non-smolts of all experimental groups were removed from the data set, no effect of fish size on either mortality or timing of mortality were observed, suggesting that the process of smoltification may influence susceptibility to ISAV.