There was actually a very sensible rule followed in China and Japan about eating raw fish. You only ate sea fish raw, while freshwater fish were always cooked.

There's a good rationale behind this. Freshwater fish tend to carry parasites that can cross the species boundary between fish and human. Whereas, sea fish do not tend to carry such parasites. You can understand the reason for that based on natural selection. Its an exceedingly rare event for a sea fish to be eaten by a land mammal (at least it was on the scale of evolutionary time before modern times), so most parasites of sea fish have not evolved a mechanism to exist in land mammal bodies. Whereas fresh water fish are often eaten by land mammals, and so many strains of parasites have adapted to make a smooth transfer from fish to land mammal body.

Its very sensible NOT to eat raw Salmon. Although they end up in the sea, they have two phases of life in bodies of fresh water where they can pick up parasites that can transfer to a human host.