Gray whales are divided into two discrete populations, one on either side of the North Pacific Ocean. The eastern population migrates along the West Coast of North America between winter calving grounds along Baja California, Mexico, and summer feeding grounds in the Bering and Chukchi Seas between Alaska and Russia. The eastern population has made perhaps the most complete recovery of any large whale population depleted by commercial whaling. The annual migration of some gray whales back and forth between calving and breeding grounds can exceed 10,000 miles, making it the longest annual migration of any mammal. The western population occurs along the Asian coast, where it migrates between summer feeding grounds off Sakhalin Island, Russia (about 500 miles north of the Japanese island of Hokkaido), and winter calving grounds at an unknown location suspected to be in the South China Sea. In contrast to the eastern population it remains one of the most critically endangered populations of any large whale numbers perhaps by 100 animals.
Commercial whaling severely depleted both populations between the mid-1800s and early 1900s. As a result, gray whales were protected under a ban on commercial hunting adopted by the League of Nations in the mid-1930s. This ban, that also covered right whales, was the first international agreement to protect a whale species from commercial whaling operations. The ban on commercial gray whale catches has been carried forward since the late 1940s by the International Whaling Commission.