Sushi is not the only way to eat fish in Japan. Then, may be sushi is the most unpopular way to eat fish in Japan. It does not mean that Japanese dislike sushi, but sushi is not a daily food which you expect to eat.
Except eating as raw, Japanese cook fish as fried, deep fried, grilled and simmered. Then, may be simmered fish is the most traditional way to eat fish in Japan. Simmered fish is simmered with a sauce either soy sauce or miso bean paste. Then, same type of fish is not simmered in both ways but usually either soy sauce or miso.
For example, mackerel is simmered in miso but not in soy sauce. Mackerel which Japanese say BLUE TYPE OF FISH is better to simmered in miso flavor due to mackerel and other fish with blue skins smells more fishy than other fish which has other color of skin.
On the other hand, red snapper and sea bream which are not relatively blue skin are simmered with sauce including soy sauce. Other ingredients for the sauce are sugar, mirin sweet rice wine and water.
Simmered fish can be eaten nationwide in Japan but not offered in izakaya style pub restaurant. Then, better to eat along seashore than inland places.