Southern sea otters (Enhydra lutris nereis) are on the federal threatened species list and their population is struggling to recover. These furry predators remain an icon of the California coast, so when word spreads about a record year for sea otter deaths, it quickly captures the public’s attention.

This past weekend, USGS Western Ecological Research Center (WERC) researchers prepared a preliminary summary of sea otter deaths (called “strandings”) observed in 2010 and shared it with research colleagues and local stakeholders (data PDF available here). A record number of strandings was observed in 2010, with 304 recovered otter carcasses.

But much of this data is preliminary.

“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, the lead scientist for sea otter research at WERC. Those findings are expected to be submitted for review and publication later this year.

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

• 304 strandings represent about 11% of the current population estimate, an increase from the 8-10% reported in the recent decade.
• 32% of recovered carcasses were of adult and subadult females -- potential mothers -- an increase from pre-2008 figures of 27%.
• Carcass necropsies revealed diseases from a number of chemical and biological factors, including toxins from inland lakes.
• 22% of recovered carcasses showed signs of fatal shark attack, up from pre-2000 figures of 10%. There is evidence that shark attacks were an important factor in otter deaths in 2010.

Based in Santa Cruz, California, Tim Tinker succeeded Jim Estes as the USGS sea otter projects leader. Image credit: Brian Hatfield/USGS.

“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 Southern Sea Otter Stranding Network was implemented by the California Department of Fish and Game (CDFG) in 1968 and is currently overseen by USGS with support from CDFG. The purpose of this network is to verify all reports of stranded sea otters in California, and recover the carcasses whenever possible. 

This network is comprised primarily of USGS-WERC; CDFG Marine Wildlife Veterinary Care and Research Center (which also conducts the necropsies); Monterey Bay Aquarium’s Sea Otter Research and Conservation program (SORAC); California Academy of Sciences; The Marine Mammal Center (TMMC) and the Santa Barbara Museum of Natural History. 

There are also other official projects on the Southern sea otter population.

As part of the federal Southern sea otter recovery and management plan, USGS will once again conduct its annual sea otter population survey along the California coast in spring 2011, now in its 29th year and with results typically announced in early summer.

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 locations, 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 of these different locations, USGS hopes to discern the health of our nearshore ecosystems and any potential implications for our natural and economic resources.

-- Ben Young Landis

For more WERC sea otter research information, visit http://www.werc.usgs.gov/seaottercount.

Top: California's sea otter population estimates have dipped for two straight years, according to the annual USGS survey. Image credit: Tania Larson/USGS.