I've got a great list of seafood in Otaru from the website of Otaru Fisheries Cooperative Association.
Local people in Otaru can get fresh seafood at fish markets easily.
When I go to the fish market, staff of each store lets me know about fish in season and the best way to cook them.
For visitors from overseas, it might be difficult to take raw seafood to their countries, but there are some restaurants in each fish market and also many kinds of seafood is processed to be stored for long period.
I translated the list in English, so have fun seafood in season in OTARU!!