MURORAN, Hokkaido—Visitors from around Japan and overseas continue to flow into the municipal Muroran Aquarium here for a look at Satsuki, a seal that bangs on a plastic pail to beg for food.

The spawning season for ito, often called “phantom fish,” is drawing to a close in the Kushirogawa river water system in eastern Hokkaido. Ito are also known as Sakhalin taimen. It is one ...