Much like terrestrial animal farms, fish farms are incubators for disease.
When diseases and parasites from wild populations get into the farmed ones, they can run riot. The high density and low genetic diversity of the farmed fish, and the fact that they stay put in the net pens makes it easy for parasites to quickly spread through the population.
LINK (via: Hakai Magazine)
Here is the story about the lice outbreak in New Brunswick referenced in the article.