Health monitoring of wild fish

The Norwegian Veterinary Institute monitors the occurrence and distribution of pathogens in wild salmonids.

The overarching goal of the health monitoring program for wild anadromous salmonids is to examine the occurrence and sources of pathogenic agents in wild salmonids. The National Veterinary Institute is responsible for the freshwater phase of the program while the Institute of Marine Research is responsible for the marine phase.


Piscine orthoreovirus-3 (PRV-3) is present in marine farmed rainbow trout and in wild sea trout in Norway.

In 2021, the health monitoring programme for wild salmonids investigated the presence of PRV-3 in wild freshwater resident wild brown trout, and in freshwater based inland aquaculture of rainbow trout and brown trout in Norway.

PRV-3 was detected in brown trout in a stock enhancement hatchery in the County of Innlandet and in wild brown trout from four lakes. Two of these lakes were stocked with brown trout from the PRV-3 positive hatchery. Two of the PRV-3 positive wild brown trout were lacking the adipose fin, indicating that they were released from the hatchery. The results indicate that PRV-3 is present in freshwater resident brown trout, but also that the occurrence can be associated with release from stock enhancement hatcheries. Accordingly, the study suggest that virus can be spread by stocking of infected fish in the wild. Sequencing of PRV-3 from the PRV-3 positive stock enhancement hatchery and the lakes could provide stronger evidence of this virus spread. Histopathological investigations would provide information regarding the health effects of PRV-3 infection in brown trout.

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