Report on the Occurrence of Infectious Hematopoietic Necrosis Disease in Out-Migrating Sockeye Salmon Fry at Pinkut Creek Spawning Channel in the Spring of 2021

Infectious hematopoietic necrosis virus (IHNV) occurs naturally in wild and enhanced stocks of Sockeye Salmon, where it can result in severe infectious hematopoietic necrosis (IHN) disease. We report the occurrence of an IHN epizootic in Sockeye Salmon fry out-migrating from Pinkut Creek Spawning Channel. IHN disease was suspected during fry enumeration where upwards of 10% of early out-migrating fry exhibited external hemorrhaging. Analyses confirmed high load IHNV infections particularly at the beginning of the out-migration period. Clinically diseased fry became less apparent over the course of the out-migration period with concomitant declines in IHNV prevalence and load. Despite the decreasing progression of IHN disease over the migration period, the occurrence of the viral outbreak at Pinkut Creek Spawning Channel corresponded with a marked decrease in the egg-to-fry survival rate and in the total number of fry produced for brood year 2020. At Fulton River Spawning Channel, a nearby Babine Lake companion facility, IHNV was detected in out-migrating fry, however no signs of IHN disease or decreases in survival were noted and IHNV prevalence remained less than 10%. Whether IHNV infections lead to disease is dependent upon a myriad of environmental, virus and host factors that often interact in unforeseen ways. Herein we highlight factors potentially contributing to the IHNV outbreak at Pinkut Creek Spawning Channel and make operational and research recommendations to mitigate against the occurrence of IHN disease and to better understand the triggers responsible for such outbreaks in Sockeye Salmon populations.