> Fish immune systems sole reason for being is to stop parasites from being present in the meat while the fish is alive.

Ah, good news for you then! Fish immune system most definitely does NOT stop parasites. Every (and I mean it, every) salmon you've ever eaten had some parts of parasites in them.

That's also why you absolutely should NOT eat fresh-caught salmon without thoroughly cooking it. Industrially-caught salmon is always frozen, and it kills parasites.

These fish grew large enough for humans to eye as food, because parasites were effectively limited by the immune system from devouring the entire fish. It's not perfectly effective, but it doesn't need to be.