The University of Washington did a study several years ago in which they found that 100% of wild caught salmon had parasites, while farmed salmon had virtually none. Note that in Japan salmon is not traditionally eaten as sashimi, most Japanese find it too oily. You'll see it at kaiten-zushi type of places and it's usually farmed Atlantic salmon that's why it looks like the salmon nigiri in N. America. I am told that in Hokkaido (known for its salmon), it is usually served cooked, if served raw it is heavily salted.