Look at the additives on the average can of fish- Salt, Soybean Oil, "Vegetable Broth(pretty ambiguous term)" , and…. Phosphates. All horrible for cat consumption. Cats have been eating fish for untold thousands of years. It's not the fish, it's what we've been putting into fish for the last century or so.
Canned Salmon is typically the purest canned fish from a processing and packaging standpoint. The problem there is that canned Salmon commonly comes from farmed fish, so although additives aren't mechanically introduced into the product, engineered fish food with hormones is. I've seen some of the Salmon farm pens in Alaska. They feed them highly processed pellets with God knows what in them. Any ideal fish for human and animal consumption should be free range/open ocean. Same with any animal protein.
Then there's the mercury issue. Some species and habitats will see more mercury levels than others. Fresh and brackish water fish caught in inland waters are at the highest risk.
