Southern Sea Otter
Southern sea otters (Enhydra lutris nereis), also known as California sea otters, were listed as threatened in 1977 under the Endangered Species Act. Reduced range and population size, vulnerability to oil spills, and oil spill risk from coastal tanker traffic were the primary reasons for listing. As a consequence of their threatened status, southern sea otters are also recognized as depleted under the Marine Mammal Protection Act. Under state law, southern sea otters are “fully protected” mammals.
Ranging from San Mateo County in the north to Santa Barbara County in the south, southern sea otters live in the nearshore waters along the mainland coastline of California. A small population of sea otters lives at San Nicolas Island as a result of translocation efforts initiated in 1987. Sea otter counts are conducted annually by the U.S. Geological Survey. The latest census and stranding information is available at www.werc.usgs.gov/seaottercount.
Southern sea otters are among the smallest of marine mammals and may live for 15-20 years in the wild. They feed on a variety of benthic (bottom-dwelling) invertebrates—including sea urchins, abalone, crabs, clams, marine snails, marine worms, sea stars, sand dollars, squid, and octopus—and sometimes use tools to break open their food. Individual animals tend to specialize on a subset of the overall population diet. Most adult female sea otters give birth to one pup each year. Birth peaks occur in the spring and fall, but pups may be born at any time of year. Male sea otters typically aggregate at the northern and southern limits of the range, or in sandy embayments within the range, in winter and early spring, when some males that have maintained breeding territories in the predominantly female center of the range abandon their territories and join other males in these areas.
Sea otters depend on clean, water-resistant fur—up to 650,000 hairs per square inch—for insulation against cold ocean water. Due to their small body size and lack of blubber, sea otters have to produce a high level of internal heat to stay warm. To satisfy their high energy requirements, sea otters spend much of their time foraging for food and eat an average of 25% of their body weight each day.
Historically, sea otters ranged along the North Pacific rim from the northern Japanese islands to mid-Baja California, Mexico. Southern sea otters occupied the southern portion of this range, probably approximately to Oregon, or possibly as far north as Prince William Sound in Alaska. The California population prior to exploitation is thought to have numbered about 16,000 animals. During the 18th and 19th centuries, sea otters were hunted for their luxurious pelts, and by the early 1900s, the species was believed to be extinct in California. Southern sea otters are descended from a small colony that survived along the Big Sur coast and became generally known to the public in 1938. The southern sea otter population has grown slowly since that time, but it has exhibited high levels of mortality in recent years.
The sea otter population along the mainland coast of California has increased, but much more slowly than other recovering sea otter populations, which have grown at rates of up to 17-20% annually. Southern sea otters along the mainland have never increased more than 5-6% per year.
High mortality (rather than low fecundity) appears to be responsible for the slow overall growth and periods of decline in southern sea otters. Causes of death determined from recovered carcasses include white shark attacks, infectious disease (such as encephalitis caused by the protozoal parasites Toxoplasma gondii and Sarcocystis neurona), acanthocephalan worms, bacterial and viral infections, domoic acid toxicity, microcystin poisoning, and cardiac lesions. However, it is not known whether the causes of death in recovered carcasses represent an unbiased picture of mortality in the population as a whole. Food limitation, nutritional deficiencies, and exposure to chemical contaminants are also likely influencing patterns of mortality. Determining what factors are driving mortality and identifying the mechanisms and pathways by which sea otters are being affected is critical to the development of effective management actions. The final revised recovery plan [1.8mb] for the federally threatened southern sea otter is available here.
Allowing natural range expansion is key to recovery, according to the recovery plan. The translocation program allowed the reintroduction of sea otters to San Nicolas Island but also required the removal of sea otters from a “no-otter” management zone. An extensive review of the translocation program was completed in December, 2012, resulting in termination of the program.
If you have any questions regarding the southern sea otter, please contact:
Lilian Carswell - Southern Sea Otter Recovery & Marine Conservation Coordinator
By phone: (805) 612-2793
2493 Portola Road, Suite B
Ventura, CA 93003