Recovery Slows for California's Sea Otters, 2012 Survey Shows
The southern sea otter population continues its pattern of tepid recovery, according to the latest population survey led by scientists from the U.S. Geological Survey (USGS), the California Department of Fish and Game (CDFG) Office of Spill Prevention and Response, and the Monterey Bay Aquarium.
Since the 1980s, USGS scientists have calculated an averaged population index each year for the southern sea otter, a federally listed threatened species in California (see http://ecos.fws.gov/speciesProfile/profile/speciesProfile.action?spcode=A0A7). The population index is calculated as the average of total sea otter counts from three consecutive annual surveys. Sometimes called the “three-year running average,” the population index helps to compensate for year-to-year variability in observation conditions and gives scientists a more reliable picture of sea otter abundance trends.
For the 2012 report, USGS listed the population index as 2,792. (Note: the 2012 population index is the average of only two years’ counts, 2010 and 2012, because—for the first time in more than two decades of monitoring—poor weather conditions prevented completion of the 2011 survey; see Sound Waves article at http://soundwaves.usgs.gov/2012/06/fieldwork.html). The 2012 survey survey data, released last August, are available online at http://www.werc.usgs.gov/seaottercount.
“Just as the polar bear has become symbolic of protecting the Arctic, so is the status of the sea otter emblematic of the health of the central California coast,” said USGS Director Marcia McNutt. “These annual surveys and the associated studies to understand the drivers for population changes are critical factors in ensuring the continuing survival of not just the sea otter, but the entire complex ecosystem for which this icon is integral.”
After nearly a decade of slow recovery in California sea otter numbers, the population index indicated a stall in 2008—and a decline to 2,711 in 2010. Even though the 2012 numbers are a comparative increase from those 2010 figures, the longer term trend suggests that the population recovery may be at a plateau.
“We saw a similar plateau in the late 1990s, before sea otter numbers began to rise again in the early 2000s,” said Tim Tinker, a biologist with the USGS Western Ecological Research Center (http://www.werc.usgs.gov/) who supervises the annual survey. “Recent shifts in mortality causes have brought to light additional explanations for the cessation of growth we’ve seen over the last few years.”
USGS scientists updated their database of sea otter strandings—the number of dead, sick, or injured sea otters recovered along California’s coast each year—in August 2012. The numbers are available at http://www.werc.usgs.gov/seaotterstranding.
In 2011, scientists from the California Department of Fish and Game (CDFG), the USGS, the Monterey Bay Aquarium, and other institutions came across a total of 335 stranded sea otters—a record high. Efforts are made to recover and examine each reported sea otter carcass, and a subset of fresh carcasses are sent to the CDFG Marine Wildlife Veterinary Care and Research Center (http://www.dfg.ca.gov/ospr/Science/marine-wildlife-vetcare/), where veterinarians conduct necropsies to determine the primary causes of death and to identify factors that may have contributed to the death of each animal.
“We saw an increase in death due to white shark ‘tasting’ bites,” said Melissa Miller, the necropsy veterinarian at the CDFG Marine Wildlife Veterinary Care and Research Center. “We are working closely with our collaborators to understand what could be driving this new trend. The usual causes of deaths were also evident: harmful algal toxins, parasites and infectious diseases, mating trauma, emaciation, bacterial infections, heart disease, and boat strikes round out the list.”
According to Tinker, the continued lack of population growth in the center of the geographic range—where sea otter densities are highest—adds evidence that sea otter populations may be approaching equilibrium abundance in these long-established areas. “The population density of sea otters is ultimately limited by their prey resources, although reduced food abundance may act in concert with other factors, such as infectious diseases,” said Tinker. (For more information, read an interview with Tinker, titled “What’s the Future for California Sea Otter Populations,” at http://www.werc.usgs.gov/outreach.aspx?RecordID=150.)
“With natural factors like the shark bites and food limitation, there’s little we can or should do,” said Lilian Carswell, Southern Sea Otter Recovery Coordinator for the U.S. Fish and Wildlife Service. “But to cope with non-natural factors, the population recovery at the very least will depend on sea otters expanding into new areas that can support sustained populations.”
Sea otters are considered a keystone species of the kelp ecosystem, preying on herbivorous invertebrates, such as sea urchins, that can decimate kelp beds, and consequently fish habitat, if left unchecked. Scientists also study sea otters as an indicator of nearshore ecosystem health (see http://on.doi.gov/nearshore), since sea otters feed and live near the coast and often are the first predators exposed to pollutants and pathogens washed down from coastlands, such as the microbial toxin microcystin (http://news.ucsc.edu/2010/09/otter-toxin.html).
The annual population index is calculated from visual surveys conducted along the California coastline by researchers, students, and volunteers from the USGS (http://www.werc.usgs.gov/seaottercount), the CDFG Office of Spill Prevention and Response (http://www.dfg.ca.gov/ospr/), the Monterey Bay Aquarium (http://www.montereybayaquarium.org/cr/sorac.aspx), the Santa Barbara Zoo (http://www.sbzoo.org/), the University of California, Santa Cruz (http://ims.ucsc.edu/lml.html), the U.S. Fish and Wildlife Service (http://www.fws.gov/ventura/species_information/so_sea_otter/), and the U.S. Bureau of Ocean Energy Management (http://www.boem.gov/). The surveys are coordinated by USGS scientist Brian Hatfield for the Fish and Wildlife Service, which is the agency responsible for managing the southern sea otter’s recovery.
Surveys are conducted via telescope observations from shore and via low-flying aircraft, typically from April through June. In 2012, the surveyed coastline extended from Point San Pedro in San Mateo County south to Rincon Point near the Santa Barbara-Ventura County line.
in this issue:
Recovery Slows for California's Sea Otters