Viruses are particular concerns because of their capacity for rapid evolution. In farmed salmon, viruses have a constant supply of a new host to infect, so there's no negative fitness consequence and the virus evolves to become more virulent. It is a different situation for wild salmon, where densities are not as high. We do have to worry about having large captured populations of fish and the potential for rapid evolution, which has been demonstrated in many parts of the world, including with PRV in Norway.
In the SSHI, we've amassed strong evidence that PRV is a risk to wild salmon, particularly in Chinook and coho salmon. That risk does need to be managed. I can go into details on what our evidence is, if you are interested.