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California Sea Otter Numbers Drop Again

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After a decade of steady recovery, the southern sea otter—a Federally listed threatened species—is in decline for the second year in a row, according to the latest population survey by U.S. Geological Survey (USGS) researchers.

"We have seen a decrease in sea otter numbers throughout most of their range, particularly in those areas where most of their reproduction occurs, while pup counts have dropped to 2003 levels," said Tim Tinker of the USGS Western Ecological Research Center and lead scientist for the annual survey. "A number of human and natural factors may be influencing this trend, and we are working to better understand what those are."

Sea otter off Monterey, California. Tim Tinker.
Above left: Sea otter off Monterey, California. Photograph by Joe Tomoleoni, University of California, Santa Cruz. [larger version]

Above right:
USGS research wildlife biologist Tim Tinker, shown here on the Alaska Peninsula, is the lead scientist on the annual sea otter survey in California. He is also a professor at the University of California, Santa Cruz. Photograph by Brian Hatfield, USGS. [larger version]

Population estimates are calculated as 3-year averages of annual survey results, which compensate for variations in observation conditions and give scientists a more reliable picture of abundance trends. This year's estimate—averaging counts from 2010, 2009, and 2008—is 2,711 otters. This represents a 3.6-percent drop in the overall population and an 11-percent drop in the number of otter pups in comparison with 2009 estimates.

Number of southern sea otters counted during spring surveys, plotted as 3-year running averages.
Above: Number of southern sea otters counted during spring surveys, plotted as 3-year running averages. (Example: Values for 2010 are the averages of the 2008, 2009, and 2010 counts.) Southern sea otters are a threatened population on the Endangered Species list; for them to be considered for delisting, the 3-year running averages of total counts (blue-green dots) would have to exceed 3,090 for 3 consecutive years. [larger version]

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

Researchers also saw the sea otters' geographic distribution shrink this year, with fewer animals at the northern and southern ends of their range, which now extends from Pigeon Point to Gaviota State Park in California.

Current distribution of sea otters in California
Above: Current distribution of sea otters in California, showing spatial variation in the rate of population change over the past 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. [larger version]

"Movements and clumping of male otters away from the range edges may partly explain this change," said USGS biologist Brian Hatfield, who organizes the annual otter survey. "And the low pup count might be linked to heavy storms and reduced kelp cover this past winter, since we also saw a record number of pups and juveniles that got stranded ashore."

As for the overall population decline, all indications point to elevated mortality.

"Our data suggest that breeding-age females are dying in higher-than-usual numbers from multiple causes, including infectious disease, toxin exposure, heart failure, malnutrition, and shark attacks," Tinker said.

Sea otters are active predators that rely on nearshore coastal waters. As a result, they are constantly exposed to many stressors, such as chemicals and pathogens from coastal water pollution, ingestion of toxin-contaminated prey, and reduced food abundance. Chronic exposure to multiple stressors could make otters more susceptible to illness and injury and lead to a greater chance of death.

"Recovery will clearly depend on our understanding the factors contributing to slow population growth in recent years and the current downturn," said Lilian Carswell, Southern Sea Otter Recovery Coordinator for the U.S. Fish and Wildlife Service. "Research is fundamental to recovery efforts because we need to understand which stressors are most strongly affecting survival rates in order to develop targeted measures to address them."

To help untangle this complex puzzle of stressor effects and interactions, Tinker is leading a study to compare sea otter health between areas with different types and severity of stressors, to find out which area has healthier otters and why. This study—funded in part by the California Coastal Conservancy and the U.S. Fish and Wildlife Service, with additional support from the University of California, Santa Cruz; the California Department of Fish and Game; and the Monterey Bay Aquarium—is described on a USGS Web page at http://www.werc.usgs.gov/seaottercount (scroll down to "Comparing Sea Otter Populations").

"Remember, sea otter health can tell us a lot about the health of the coastal waters that humans also enjoy," Tinker said. "So, we're eager to learn more."

The survey was conducted during the month of May, from Point San Pedro in San Mateo County down to the Santa Barbara/Ventura County line. The survey has been conducted annually since 1985 and 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; 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 Bureau of Ocean Energy Management, Regulation and Enforcement.

Details about the spring survey, including graphs, maps, and data summaries, are posted online at http://www.werc.usgs.gov/seaottercount.

Related Sound Waves Stories
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
Food Choices and Location Influence California Sea Otter Exposure to Disease
March 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 the Western Ecological Research Center

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Sea Otter Numbers Drop

Prehistoric Tsunamis and Great Earthquakes

Seafloor Mapping in Coastal MA

ResearchUnlocking Model Data via Web Services

Outreach Antarctic Science and the Cultural Arts

Staff Internship in Everglades National Park

Mendenhall Fellow to Study Sediment Fluxes

New Mendenhall Fellow in Woods Hole

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