From the website of Alaska department of Fish and Game:Plains bison (Bison bison bison) are the smaller of two subspecies of American bison found in North America, and they are an introduced, rather than native, species in Alaska.
In 1928, 23 plains bison were moved from the National Bison Range in Montana to the Delta River area in Alaska’s Interior. The herd grew to 400 animals over the next two decades. Today four herds totaling about 900 animals range freely in the state. The largest herd is located near Delta Junction, and smaller herds have been established by translocation from the Delta herd to the Farewell, Chitina River, and Copper River areas.