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Weather Conditions Prevent 2011 Survey of California Sea Otter Population

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California sea otter
Above: A California sea otter (Enhydra lutris nereis) swims in Monterey Bay, California. The USGS leads annual surveys of sea otters along the California coast, but heavy fog, poor visibility, and strong winds prevented completion of the 2011 survey. Photograph taken August 9, 2008, by Tania Larson, USGS. [larger version]

Distribution of sea otters in California in 2010
Above: Distribution of sea otters in California in 2010, showing spatial variation in the rate of population change over the previous 5 years. Values greater than zero (yellow, green, and blue colors) indicate increasing numbers; values less than zero (orange and red colors) indicate declining numbers. Slightly modified from figure 4 in "Spring 2010 Mainland California Sea Otter Survey Results." [larger version]
Scientists were unable to complete their 2011 survey of the California sea otter (Enhydra lutris nereis; also known as the southern sea otter) population because of heavy fog, poor visibility, and strong winds throughout the spring and summer.

The population survey has been conducted annually since the 1980s to track the recovery trend of this threatened species. The U.S. Geological Survey (USGS) leads this effort with a team of dedicated scientists and volunteers.

The population index calculated from the survey data is used by the U.S. Fish and Wildlife Service to assess the sea otter’s progress toward population recovery and determine whether the species is ready for delisting under the Endangered Species Act.

“We use two standardized methods for our visual surveys, which are telescope observations from shore and aerial observations from a small twin-engine plane,” said Tim Tinker of the USGS Western Ecological Research Center and chief scientist for the annual survey. “Although our shore-based surveys were successful, unusually heavy marine fog or high winds throughout the spring and summer repeatedly hindered our attempts to conduct aerial surveys. Both measurements are crucial, and so without the aerial observation data, we were unable to provide a reliable, standardized total count for California sea otters in 2011.”

This was the first incomplete result in more than two decades of continuous monitoring. Data from portions of the survey that were successfully completed in 2011 will still be useful in computing a population index after 2012.

Typically, the research crew conducts the annual survey in May and June, covering the entire coast from Point San Pedro in San Mateo County in the north to the Santa Barbara-Ventura County line in the south. About half of this coastline can be surveyed by using ground-based observations, but the remaining half must be counted by air because of limited coastal access.

Otters sighted by shore-based crews and aerial crews are added up to provide the “raw count” for that year. The official population index is calculated by averaging the raw count from that year and the two previous years. This precaution reduces the influence of random variation in raw counts caused by year-to-year differences in viewing conditions.

“The inability to complete the survey in 2011 introduces an unfortunate but unavoidable gap in our understanding of the population’s trajectory, with respect to recovery thresholds,” said Lilian Carswell, Southern Sea Otter Recovery Coordinator for the U.S. Fish and Wildlife Service. “But we’re grateful for the detailed survey effort by the USGS in the past three decades and in the years to come. We’ll work together with the USGS if the need arises to adjust the annual survey methods.”

In 2010, the California sea otter population index was 2,711—a decline for the second year in a row (see “California Sea Otter Numbers Drop Again,” Sound Waves, December 2010). For California sea otters to be considered for removal from the Federal List of Endangered and Threatened Wildlife, the population index has 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.

The annual California sea otter survey is a cooperative effort of the USGS Western Ecological Research Center; the California Department of Fish and Game’s Marine Wildlife Veterinary Care and Research Center; the Monterey Bay Aquarium; the University of California, Santa Cruz; and many experienced and dedicated volunteers. Assistance also comes from staff of the U.S. Fish and Wildlife Service and the U.S. Bureau of Ocean Energy Management. The 2012 survey is currently underway.

Past survey numbers are available online at


Related Sound Waves Stories
California Sea Otter Numbers Drop Again
December 2010
USGS Launches Multidisciplinary Investigation of Northeast Pacific Sea Otter Populations and Nearshore Ecosystems
March 2010
2009 Spring Survey Shows Drop in California Sea Otter Numbers
September 2009
California Sea Otters: Population Recovery Continues at Slower Rate
September 2008
California Sea Otters - 2007 Survey Count Reaches New High
September 2007
California Sea-Otter Numbers Dip Again in 2006, But Overall Population Trend Remains Up
August 2006

Related Web Sites
Sea Otter Studies at WERC

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cover story:
Gas Hydrates and Climate Warming

Real-Time Mapping of Methane Concentrations

Coastal and Marine Geoscience Data System

Exploring Geophysical Data Using GeoMapApp and Virtual Ocean

Weather Prevents Survey of California Sea Otter Population

Exhibit Will Celebrate Collaboration Between Artists and Scientists

Antarctic Science and Arts

USGS Scientists Selected as Fellows of the American Geophysical Union

Staff Chinese Scientist Visiting USGS Pacific Coastal and Marine Science Center

Belgian Volunteer Assists Staff in Everglades National Park


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