The southern, or California, sea otter (Enhydra lutris nereis) is listed as a threatened species under the federal Endangered Species Act in 1977 and is also protected under California state law and by the Marine Mammal Protection Act. 

Sea otters historically ranged across the Pacific Rim from Baja California to northern Japan, and the California sea otter population at one time was estimated between 16,000-20,000. The fur trade of the 18^th and 19^th centuries caused sea otter populations to collapse, and by the early 1900s, the southern sea otter was believed to be extinct. In 1938, the public learned that a small remnant population of otters had been discovered living off the Big Sur coast. The population has slowly expanded, and since 1995, has fluctuated around 2,000 animals.

Sea otters are what scientists refer to as "keystone species," a term that refers to their functional role within their ecosystem. The feeding habits of the sea otter have a top-down effect on the marine ecosystem, meaning that their actions affect organisms lower in the food chain. Sea otters are also indicators of the health of that ecosystem. Sea otters eat shellfish which, unfortunately, put the otter at odds with the commercial shellfish industry. By limiting the numbers of shellfish that feed on kelp, the sea otter promotes the development of kelp forests. Kelp beds, in turn, provide habitat for fish and other invertebrate species. 

In 1986, Congress passed a law that would allow for sea otters to be "translocated" to another area, to create this second breeding colony. From 1987-1990, otters were captured from the "parent range" north of Point Conception and translocated to San Nicolas Island, one of the Channel Islands, located 60 miles off the California coast. But the translocations were stopped when it became clear that sea otters were not remaining at San Nicolas Island at anywhere near the numbers expected. Since 1990, fewer than 25 sea otters have stayed at the island, even though 140 sea otters were moved to the island.

At the same time the translocations were going on, and as part of the same law, an otter-free "management zone" was created, to lessen the worries of shellfish fishermen that translocated sea otters would hurt their business. This zone was located in the area off the California coast from Point Conception southward to the U.S.-Mexican border. And, from 1987 until 1993, otters found straying from the parent range into the management zone were captured and taken back to the parent range. This management program ended, however, when the Fish and Wildlife Service observed that a high percentage of the sea otters died shortly after they were returned to the parent range.

Sea otter populations also began to change. From 1995 -1999, sea otter numbers declined four out of the five years. The spring 2000 count was up slightly but scientists do not know if this is a trend that will continue. Reasons for the population decline are not entirely clear, but appear to be related to disease, entrapment in fisheries gear, potential food limitations and habitat degradation.