Before commercial hunting began in the mid-1700s, an estimated 150,000 to 300,000 sea otters occurred in coastal waters throughout the rim of the North Pacific Ocean from northern Japan to Baja California, Mexico. In 1911, hunting was prohibited under the terms of an international treaty for the protection of North Pacific fur seals and sea otters signed by the United States, Japan, Great Britain (for Canada), and Russia. By then, only a few thousand otters remained. The survivors were scattered among small colonies in remote areas of Russia, Alaska, British Columbia, and central California.
Since the prohibition on commercial hunting in 1911, sea otters have recolonized or have been reintroduced into much of their historic range. By the time the Marine Mammal Protection Act was enacted in 1972, the California population had grown from as few as 50 to more than 1,000 individuals (an average annual growth rate of about 5 percent) and had recolonized more than 200 mi (370 km) of the California coast. Remnant groups in Alaska grew even more rapidly and, in the late 1960s and early 1970s, several hundred otters were moved from Amchitka Island and Prince William Sound to try to reestablish populations in southeastern Alaska and the outer coasts of Washington and Oregon. In 1995, the Fish and Wildlife Service estimated that there were approximately 100,000 sea otters in Alaska, more than 2,300 in California, and more than 300 in Washington, and that all the populations were growing. Subsequently, however, both the California and southwestern Alaska populations were found to have declined, the latter by as much as 90 percent in some areas.