The reason: poop. When fish are concentrated so heavily in one area and fed on a schedule to maximize their growth, they produce very high levels of waste. This then falls onto the ocean floor where it breaks down and provides nutrients for algae. The algae grows, dies, decomposes, and sucks up all of the oxygen, making life very difficult on the region's fishermen. Additionally, housing fish in such close proximity led to outbreaks of disease in the fish, further tarnishing the reputation of the region's farmed fish.
According to the above article, Novia Scotia has been charged to double its seafood exports to meet global food needs. With wild fish populations being fished at full capacity, aquaculture is necessary to reach this goal. Therefore it is a very promising step to see the government stepping in to set up a trajectory to allow the industry to continue to expand, while maintaining the environmental health of the region.