[LeifCarrotson]

Fish immune systems sole reason for being is to stop parasites from being present in the meat while the fish is alive. They're literally swimming through a soup of arthropods, plankton, algae, bacteria, and viruses that would love nothing more to turn their meat into more of themselves. There's always a bigger fish that is trying to eat them, yes, but the smaller critters want to eat as well!

Freezing doesn't kill the parasites, it slows the clock that started ticking when the fish was killed. It's not pasteurization, like what's done to canned tuna. It just slows the clock when you refrigerate or freeze the fish, but does not reset it to zero. And of course, if you're eating fresh fish that was healthy when it was killed, there's no need for an intermediate freezing or pasteurizing step.

This situation is different because the "clock" starts when the cell cultures are removed from the donor salmon. The whole blob/tank/plate/catalyzing surface (I'm not sure what the design is, I wish they had more documentation) on which the product grows for the whole time that the product is growing is vulnerable to a single bacterium that would grow out of control, like an immunocompromised human might be killed by an ordinary illness that most people would shrug off in 24 hours.

[dekhn]

Freezing (properly) is widely considered (by scientific establishment) to kill most parasites, not just slow them down.

[mapt]

When biologists talk about parasites, they're talking about numerous organisms from multiple kingdoms in one of the widest ecological niches.

When the FDA talks about freezing killing parasites in fish, they're talking specifically about anisakis worms - https://en.wikipedia.org/wiki/Anisakis

[cyberax]

> 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.

[mapt]

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.