The Sea Otter population thought to have once been 150,000 to 300,000, occurring along the North Pacific from northern Japan to the central Baja Peninsula in Mexico. Its abundance was greatly reduced by human exploitation. Between 1751 and 1911 the distribution was reduced to 13 known remnant populations: two in the Kuril Islands and Kamchatka; one in the Commander Islands; a total of 10 in the following areas: Aleutian Islands (2)and along the Alaska Peninsula (3); Kodiak Island (1), Prince William Sound (1), the Queen Charlotte Islands (1), central California (1), and San Benito Islands (1). However, the Queen Charlotte, Canada and San Benito Island, Mexico remnant Sea Otter populations have died out and likely did not contribute to the recolonization of the species following near extirpation (Kenyon 1969).
Sea Otters currently have established populations in parts of the Russian east coast, Alaska, British Columbia, Washington, and California, and there have been reports of single-animal observations in Mexico and Japan. Population estimates made between 2004 and 2007 give a worldwide total of approximately 106,822 Sea Otters. In Russia, population monitoring has increase for Sea Otters throughout the range and number are about 19,000 are in the Kurils, 2,000 to 3,500 on Kamchatka and another 5,000–5,500 on the Commander Islands. The Sea Otter population in Alaska was estimated at between 100,000 and 125,000 individuals in 1973. By 2006, however, the estimated population had fallen to an estimated 73,000 animals in Alaska. A massive decline in Sea Otter population in the Aleutian Islands accounts for most of the change; the cause of this decline is not known, although orca predation was suspected. The Sea Otter population in Prince William Sound was also hit hard by the Exxon Valdez oil spill, which killed thousands of Sea Otters in 1989. Along the North American coast south of Alaska, the Sea Otter's range is discontinuous. Between 1969 and 1972, 89 Sea Otters were flown or shipped from Alaska to the west coast of Vancouver Island, British Columbia. They established a healthy population, estimated to be over 3,000 as of 2004, and their range is now from Tofino to Cape Scott. In 1989, a separate colony was discovered in the central British Columbia coast. It is not known if this colony, which had a size of about 300 animals in 2004, was founded by transplanted otters or by survivors of the fur trade. In 1969 and 1970, 59 Sea Otters were translocated from Amchitka Island to Washington State. Annual surveys between 2000 and 2004 have recorded between 504 and 743 individuals, and their range is in the Olympic Peninsula from just south of Destruction Island to Pillar Point. California has over 3,000 Sea Otters, descendants of approximately 50 individuals discovered in 1938. The spring 2009 Sea Otter survey counted 2,654 Sea Otters in the central California coast, which is suggestive of a population that is stable or slightly declining; counts are down from an estimated pre-fur trade population of 16,000 (USGS unpublished data). California's Sea Otters are the descendants of a single colony of about 50 southern Sea Otters discovered near Big Sur in 1938; their principal range is now from just south of San Francisco to Santa Barbara County. In the late 1980s, the US Fish and Wildlife Service relocated about 140 California Sea Otters to San Nicolas Island in southern California for establishing a reserve population should the mainland is struck by an oil spill. The San Nicholas population initially shrank as the animals migrated back to the mainland, as of 2005, only 30 Sea Otters remained at San Nicholas, thriving on the abundant prey around the island.