California Sea Otter Surveys and Research

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WERC collaborates with other research scientists to conduct annual population surveys of the southern sea otter -- a federally listed threatened species. In coordination with the California Department of Fish and Game and other institutions, ongoing surveys and research continues to inform the southern sea otter recovery plan for the U.S. Fish and Wildlife Service, and contributes to our understanding of sea otters and nearshore ecosystem health from California to Alaska.

Sea otter research by scientists at the Santa Cruz Field Station of the USGS Western Ecological Research Center (WERC) focuses on this smallest marine mammal’s population biology and its role as a keystone species in the nearshore marine community.

WERC scientists conducting long-term research in California and Alaska seek to answer complex ecological questions. How do sea otters impact near-shore marine food webs? Why is California’s threatened sea otter population growing so slowly? What are the consequences of the rapid sea otter decline in the Aleutian Islands? We work with state, federal, and local partners throughout the species’ range, utilitizing tools and expertise from ecology, chemistry, geography and other disciplines.

WERC sea otters

Sea otters (Public domain.)


A standardized method to survey sea otters was developed by WERC scientists and put into use in 1982 in California. Survey procedures involve counting animals during the "spring survey" -- generally beginning in April and wrapping in June, depending on weather conditions. 

Two-person teams use binoculars and spotting scopes to count individuals from accessible stretches of coastline and from fixed-wing aircraft in the remaining areas. The counts made from shore are plotted on maps and then entered into a spatial database. The aerial counts are entered directly into a geographic information system-linked database in the aircraft. The survey records the total otter numbers, the number of dependent pups, and the number of independents (adults and subadults), observed.

WERC surveying sea otters

Two-person teams use binoculars and spotting scopes to count individuals (Public domain.)

These data, in conjunction with findings from several more in-depth studies provide the necessary information to assess female reproductive rates and changes in reproductive success of the California sea otter population through time.

The surveys, conducted cooperatively by scientists of the U.S. Geological Survey, California Department of Fish and Wildlife, U.S. Fish and Wildlife Service and Monterey Bay Aquarium with the help of experienced volunteers, cover about 375 miles of California coast, from Half Moon Bay south to Santa Barbara. The information gathered will be used by federal and state wildlife agencies in making decisions about the management of this sea mammal.

Spring survey results are used as an indicator of the population trend of California sea otters. No single year's survey result is indicative of a population change, however. Factors that can influence the count include viewing conditions, abundance and species composition of surface canopy kelp, observer experience, and distribution and movements of the animals.

To reduce the influence of anomalously high or low counts during any particular year, three-year running averages -- commonly called a "population index" -- of the survey results are also used to assess whether the population is growing or declining. 

This population index is what is reported to the U.S. Fish and Wildlife Service. For southern sea otters to be considered for removal from threatened species listing, the population index would have to exceed 3,090 for three consecutive years, according to the threshold established under the Southern Sea Otter Recovery Plan by the U.S. Fish and Wildlife Service. 

Browse the menu, click on the "Data and tools" tab, "News" tab, and "Related" tab to learn about sea otter biology and the annual California sea otter population survey. Click on the links below to explore other sea otter research projects conducted by WERC scientists and partners.