well, we know the virus travels in sewage. and, we know that lots of countries dump their sewage into the water. if the salmon was caught not far from a dumping site, including perhaps in british columbia, then it's not entirely insane to think the virus may have entered the salmon's gills, or otherwise been stored in it's bodies, even just via osmosis, as a consequence of living near a dumping site; it's another question to speculate as to whether the salmon may have been actively infected by the virus.

however, it's easy enough to argue that somebody may have sneezed over the cutting board, as well.

so, i would hope that other possible explanations are being explored.

https://www.cbc.ca/news/health/coronavirus-outbreak-china-who-1.5612838