Monitoring of viruses in chum salmon (Oncorhynchus keta) migrating to Korea

It is important to investigate the prevalence of salmonid pathogens because they can affect the amount of release of salmonid fry and the migration rate of adult salmonids. In this study, routine surveys were conducted for investigating virus distribution in migrating chum salmon spawners (Oncorhynchus keta) and their offsprings at the Namdae River, Yangyang, Korea, during 2006-2008. Anterior kidneys were removed from chum salmon spawner individuals, homogenized with minimal essential medium, and centrifuged to make supernatants for conducting RT-PCR. Five offspring were pooled to for conducting RT-PCR. Infectious pancreatic necrosis virus (IPNV), infectious hematopoietic necrosis virus (IHNV) and viral hemorrhagic septicemia virus (VHSV) were the target viruses for monitoring. In 2006, only spawners were investigated, and 27.5% of fish (22/80) were found to be IHNV-positive by nested PCR. In 2007, 65.6% of pooled fry (21/32) were IHNV-positive, and 9.4% (3/32) were IPNV-positive by one-step PCR. When nested PCR was conducted, 84.4% (27/32) were IHNV-positive, and 28.1% (9/32) were IPNV-positive. However, only 1.3% of spawners (1/80) were IHNV-positive by nested PCR. In 2008, 25% (8/32) of pooled fry were IHNV-positive by one-step PCR, but 59.4% (19/32) were IHNV-positive and 12.5% (4/32) were IPNV-positive by nested PCR. All of the samples tested were VHSV-negative. Although all viruses detected in this study were from chum salmon, phylogenetic analysis showed that they possibly originated from rainbow trout or clustered with the rainbow trout isolates. More extensive long-term studies are needed to clarify the origins of these viruses and their potential effects on chum salmon migration in Korea.