Amemasu is a natural trout that Hokkaido should be proud of.
They can live in headwaters, mountain streams, main streams, brackish water, and the sea. During the summer, it is possible to catch American trout swimming upstream from the lake. You can enjoy fly fishing using dry flies using techniques known only to local anglers and local guides. You'll never forget the sight of a 50cm long wildfish jumping out to the surface of the water.
One of my guest (who is over 70 yeas old) has fish over 20inches Amemasu in stream by Dryfly every year.
From late autumn to spring, the brackish water near the river mouth is the habitat of American trout. This is also where you can catch large fish. These fish are called kunja in Russia. The biggest fish I ever caught was 90 cm, and it was in the Tokachi River a few years ago in the spring. The average size is about 60cm, and they are generally caught by throwing a streamer with a two-handed rod, spey casting, and wading.
More deatails is this page.
From late autumn to spring, Hokkaido is in season for wild huge Amemasu trout.