What’s more is that I hear even the Kosher fish markets just buy salmon from regular goyishe sources (as is mutar as stated above), slap a hechsher on it and mark up the price.
Do you really think Ossie’s, Freund’s, etc. buy their fish in supermarkets? That’s ridiculous.
They go to the fish market, buy whole fish and clean and cut it in their own facilities.
If you want to do that and save money, you could. Maybe you should do your own fishing and save even more.