Slow night, this is worrying for the future of our trout fishery. The fish farms would likely want to import brood stock, trout specially breed to grow fast under farmed conditions. These new fish, would sooner or later bring new diseases with them, and any escape fish would introduce genes better surveying in a farm then the wild environment. The quality of our wild trout fishery could be decimated. Our wild trout have been isolated for many generations, so their immune systems to same of the diseases not already present here would not be the best.
Even if no new diseases are introduced, the large concentrations of fish in a small area would cause problems.. In the wild diseases and parasites are normally at low levels and natural predation normally deals with the sick individuals minimizing further spread, in a crowded pen there is no natural predation, and the fish are in regular contact so its the perfect environment for a epidemic to begin. This high level of diseased farmed fish can easily spread into the surrounding population.
This video was linked to on another forum, I strongly suggest watching it. The video is mainly on Salmon, but the same argument applies for any sort of fish farming.