The sea otter has long been a symbol of the Alaska marine environment. The quest for valuable sea otter pelts in the 18th century fueled Russian colonialism in Kodiak and Alaska. Sea otters evolved initially in northern Japan and Russia and spread east to the Aleutian Islands, Mainland Alaska and as far south as the Baja California Peninsula in Mexico. Over-exploitation by Russian, American and British hunting parties reduced their numbers from an estimated 150,000-300,000 to just 1,000-2,000 animals by the early 1900s. An international hunting ban was enacted in 1911 to prevent extinction of the species. Enhydra lutris is the largest member of the weasel family. Females average 30 to 73 pounds, while males average 49 to 99 pounds. Sea otters inhabit near-shore waters, where they feed on invertebrates such as sea urchins, mollusks and crustaceans, as well as a few species of fish. It is a keystone species, controlling sea urchin populations that would otherwise damage kelp forest ecosystems. Unlike other marine mammals that are insulated by thick layers of fat, sea otters rely on their exceptionally thick fur. With nearly 1 million strands of hair per square inch, sea otter fur is the densest of any furbearer. In order to stay warm, sea otters must continually groom and clean their waterproof guard hairs in order to keep their thick under-fur dry. Fur is shed and replaced gradually rather than during a distinct molting season. Sea otters are extremely buoyant due to air trapped in their fur and to a lung capacity more than twice that of similarly sized land mammals. When feeding, sea otters typically dive for one to four minutes, and they can attain speeds of 5.6 miles per hour. Sea otters also must metabolize large volumes of food in order to maintain body temperature, consuming 25 to 38 percent of their body weight per day. Their voracious appetites have brought them into conflict with commercial fishing interests. Although sea otters get most of their water from food they consume, their large kidneys enable them to derive fresh water by drinking seawater. Births occur year-round and peak between May and June in Alaska waters. Sea otters are polygynous. Gestation lasts four months, though delayed implantation can occur. Females typically give birth to a single pup. Pup mortality is high, with estimates of just 25 percent surviving their first year. Juveniles are independent at 6 to 8 months of age. Females become sexually mature between 3 to 4 years old and have an average lifespan of 15 to 20 years. Males have a shorter lifespan of 10 to 15 years. Killer whales and sea lions are the main predators of sea otters, and bald eagles prey on pups. There are presently an estimated 70,000 sea otters in Alaska waters. Populations in the southwestern portion of Alaska have suffered recent declines, particularly in the western Aleutians. Increased predation by killer whales due to a decline in Steller sea lions and harbor seals is the prevalent theory for a loss of up to 90 percent of the sea otters in the western Aleutians. The Kodiak population is listed as stable. Sea otters in southwest Alaska waters are listed as threatened, and the U.S. Fish and Wildlife Service (Fish & Wildlife) recently designated 58,500 square miles of near-shore waters as critical sea otter habitat. "The boundary for waters listed as critical habitat generally follow the 20-meter depth contour," Doug Burn, marine mammals management biologist with Fish & Wildlife, said. Federal agencies that undertake, fund or permit activities that may affect critical habitat must consult Fish & Wildlife to ensure actions do not adversely modify or destroy critical habitat. The designation is not expected to cause fishery closures. Although sea otter hunting has been closed since 1911, Alaska Natives are allowed to hunt for subsistence purposes and to create and sell handicrafts and clothing from their pelts.