Japan is surrounded by sea and therefore Japanese eat a lot of amount of seafood and a lot of types of seafood. On the other hand, Japanese do not eat so many freshwater fish as Central European do. It is quite hard to get trout at supermarket in Japan.
Usually European pan fry or deep fry the fish, but Japanese style is much various. The most common way is grilled or eat raw. Then, stewed is very popular too. Deep fried is also available but not considered as traditional style of cuisine.
Tuna is a relatively large saltwater fish. Europeans know tuna as canned tuna but do not eat so much the fresh ones as Japanese do. Japanese of course eat the canned tuna but most common way to eat is either sashimi or sushi.
The best tuna is caught by fishermen from Ohma harbor in Aomori prefecture in northern Honshu. Then, brought to Tsukiji Fish market in Tokyo which waits for the auction. The final price can reach more than 1 million euros per tuna!