You make a good point Rob28 open sea cage fish farms do have...

You make a good point Rob28 open sea cage fish farms do have some serious enviromental concerns. From the research I have done there are 4 main areas of concern with these type of operations.
Waste between 25-50% of the fish food ends up as feces which invariable ends up accumulating on the ocean floor smothering the sea bed up to 50 metres from the cages causing oxygen depletion

Escapes when caged farmed fish do escape and it does happen they can infect the wild fish population with any disease or parasites they could be carrying which happens in these type of semi controlled enviroments

Disease/Parasites drugs used to control these conditions end up finding there way into the marine enviroment

Chemicals from anti biotecs and paints used on cages to prevent marine growth end up in the enviroment and no doubt the caged fish.

In short one could say that open sea caged fish lots are intensive feedlot operations that dump drug laced waste into the open sea.
So CAQ being a land based controlled enviroment has a lot going for it without all the vageries of farming in the open sea