Concerns over the future survival of the species led to the development of national regulations on polar bear hunting, beginning in the mid-1950s. The Soviet Union banned all hunting in 1956. Canada began imposing hunting quotas in 1968. Norway passed a series of increasingly strict regulations from 1965 to 1973, and has completely banned hunting since then. The United States began regulating hunting in 1971 and adopted the Marine Mammal Protection Act in 1972. In 1973, the International Agreement on the Conservation of Polar Bears was signed by all five nations whose territory is inhabited by polar bears: Canada, Denmark, Norway, the Soviet Union, and the United States. Member countries agreed to place restrictions on recreational and commercial hunting, ban hunting from aircraft and icebreakers, and conduct further research. The treaty allows hunting "by local people using traditional methods". Norway is the only country of the five in which all harvest of polar bears is banned. The agreement was a rare case of international cooperation during the Cold War. Biologist Ian Stirling commented, "For many years, the conservation of polar bears was the only subject in the entire Arctic that nations from both sides of the Iron Curtain could agree upon sufficiently to sign an agreement. Such was the intensity of human fascination with this magnificent predator, the only marine bear."