Pollock is fine, they're mostly caught along the Alaska water regions. They're cheap because they're easy to catch. They hover around together and most big commercial fishing boat can catch a few tons in one scoop lol
FYI, fillet of fish from Mickey d's is 100% pollock.
Re: the earlier points about farmed fish: my understanding is that farm-raised fish in general is much less nutritious than wild-caught products. I think they tend to use feeds that aren't so great at replicating the fish's natural diet.