by Ben Young Landis 

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Southern sea otters (Enhydra lutris nereis) are on the federal threatened-species list, and their population growth has been puzzlingly sluggish and inconsistent. These furry predators remain an icon of the California coast, and so when word spreads about a record year for sea otter deaths, it quickly captures the public's attention.

In late January, researchers at the U.S. Geological Survey (USGS) Western Ecological Research Center (WERC) prepared a preliminary summary of sea otter deaths (called "strandings") observed in 2010, which showed a record number of 304 recovered otter carcasses. The data were shared with research colleagues and local stakeholders.

"The USGS and its partners are currently analyzing the 2010 stranding data in the context of 25 years of sea otter observations in California," says Tim Tinker, lead scientist for sea otter research at WERC. The researchers expect to submit their findings for review and publication later this year.

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USGS scientists normally prefer to carefully assess and compare new data before educating the public about overall results and implications. The 2010 preliminary data, however, do offer some hints about stranding trends:

• 304 strandings represent about 11 percent of the current population estimate, an increase from the 8 to 10 percent reported in the recent decade.
• 32 percent of recovered carcasses were of adult and subadult females—potential mothers—an increase from pre-2008 figures of 27 percent.
• Carcass necropsies revealed diseases from several chemical and biological factors, including toxins from inland lakes. (See related news release from the University of California, Santa Cruz.)
• 22 percent of recovered carcasses showed signs of fatal shark attack, up from pre-2000 figures of 10 percent.

"Our past research indicates that only about half of sea otters that die in the wild are ever recovered, so a single year's numbers can't be considered an accurate or unbiased indicator of population mortality," says Tinker. "Nonetheless, the number of dead sea otters observed—relative to population estimates—has been elevated in recent years, so we're trying to discover why."

The California Sea Otter Stranding Network, which was implemented by the California Department of Fish and Game (CDFG) in 1968, is currently overseen by the USGS with support from the CDFG. The purpose of this network is to verify all reports of stranded sea otters in California and to recover the carcasses whenever possible. The network is composed primarily of the USGS WERC; the CDFG Marine Wildlife Veterinary Care and Research Center, which also conducts necropsies; the Monterey Bay Aquarium's Sea Otter Research and Conservation program (SORAC); the California Academy of Sciences; the Marine Mammal Center; and the Santa Barbara Museum of Natural History. Additional agencies and organizations also contribute stranding data and conduct studies of the southern sea otter population.

As part of the federal southern sea otter recovery and management plan, the USGS will once again conduct its annual sea otter population survey along the California coast in spring 2011. An ongoing USGS-led study of radio-tagged sea otters in Monterey and Big Sur is comparing the health, behavior, diet, and survival rates of sea otters at these sites, to investigate the impacts of human stressors on sea otter populations. Also, USGS scientists in California, Washington, and Alaska are collaborating with state, federal and Canadian colleagues to study sea otters throughout the Pacific coast of North America. By comparing the coastal environments and sea otter health at these different locations, the USGS hopes to discern the health of our nearshore ecosystems and any potential implications for our natural and economic resources.