Why? Because the fish farmers use too much water, introduce invasive species, and endanger the native fish and wildlife. They and the deer ranches have a PROVEN track record. ALL BAD!
If you eat, you are involved in aquaculture. And it would be better if you researched a few things for yourself rather than parroting somebody like David Suzuki.
The track record for fish farms is that they produce over 60% of the world catch and provide sustainable, healthy, high quality protein for a majority of the planet. And, yes just like any farm, factory, mine, or plant, some create more problems than others.
But forsaking a multi-hundred billion dollar market, hundreds of thousands of jobs, just to have some political borders the fish have never recognized anyhow, is just plain crazy. No fish farmer I know has done anything harmful to their environment and none has done anything other than provide a good quality food source for their populace. Surely less damaging than any cattle ranch, row crop farm, or wild harvest operation I know of.
Water is reused over and over at fish farms, not consumed, waste water is usually recycled for irrigation, concentrated wastes are made into fertilizers. Processing wastes are rendered back to fish food. Safe, clean, efficient, effective, sustainable, and profitable.