When sea otters want to rest, they wrap a piece of kelp around their body to hold themselves steady among the rolling waves. Likewise, California’s sea otter numbers are holding steady against the many forces pushing against their population recovery, according to the latest field survey led by federal, state, aquarium, and university scientists.
Biologists with the U.S. Fish and Wildlife Service (Service) work alongside conservation partners to conserve and protect the southern sea otter - Enhydra lutris nereis - a federally listed Threatened species found in California. Scientists with the Service’s sister agency, U.S. Geological Survey (USGS), calculate a population index each year through an annual range-wide field survey to inform and guide conservation and management of the species.
For 2014, USGS reports the population index as 2,944 (data available online). It’s a negligible bump from the 2013 report of 2,939.
For southern sea otters to be considered for removal from the “Threatened” species list, the population index would have to exceed 3,090 for three consecutive years, according to the Service’s Southern Sea Otter Recovery Plan.
“Through this partnership, we are able to use sound science to understand how sea otters are responding to threats, which tend to be different in different parts of their range, and may also change from year to year,” said Lilian Carswell, the Service’s Southern Sea Otter Recovery Coordinator.
“For an animal so few in number, sea otter population trends can be influenced by many local and range-wide factors,” says biologist Tim Tinker with the USGS Western Ecological Research Center, who leads California sea otter research with co-investigators from the California Department of Fish and Wildlife’s Office of Spill Prevention and Response, the Monterey Bay Aquarium, University of California, Santa Cruz, and other institutions. “We are seeing elevated mortality suggestive of food resource limitation in some parts of the range, and increasing mortality from white shark attacks in others. But our federal, state, aquarium, and university sea otter research alliance is making progress in understanding how all these trends relate to environmental factors along the California coast.”
California sea otters feed and live in the “nearshore marine ecosystem” — the stretch of ocean that hugs a shoreline—making them a good indicator species for detecting pollutants and pathogens washed down from coastlands. And along many parts of Pacific North America, sea otters play critical roles in natural food webs, keeping important ecosystems like kelp forests and seagrass beds in balance.
Studying sea otters, then, not only helps us understand their population recovery, but it also uncovers clues on the health and rhythm of nearshore marine ecosystems — the same waters many people swim, fish, and make a living in every day.
Mothers and Pups
Sea otters were presumed extinct in California after the 19th Century fur trade years but were rediscovered in the 1930s by the public, when as few as 50 animals were documented persisting in nearshore areas off the coast of Big Sur.
In that context, the present state of sea otters in California isn’t terrible. But population recovery has been slow. Part of the reason is the extreme, edge-of-the-knife life history of sea otters.
Not equipped with blubber like whales and seals, sea otters must rely on their fur coat and their super-high metabolic rate to stay warm. “The average adult sea otter has to actively hunt and eat 20 to 30 percent of its body mass in food each day just to meet its energy requirements,” says Tinker.
Research published this year by UC Santa Cruz, USGS, and Monterey Bay Aquarium addressed the question of how this energy demand changes for a nursing mother sea otter, who has to eat not only to keep herself alive but also to produce milk for her pup. Researchers calculated that a mother sea otter has to nearly double her energy intake to keep herself and her pup fed — she has to find enough groceries and eat for two, so to speak, for six months straight.
Indeed, researchers have documented a high mortality rate in prime-breeding-age female sea otters in California from what they call “end-lactation syndrome” — where females are so underweight and energetically stressed after raising a pup that they are more vulnerable to life-threatening diseases and infections.
The new research offers an explanation for the high mortality in breeding-age females in the center part of the sea otter’s California range (Seaside to Cayucos) — which is likely at or near “carrying capacity,” the maximum population size that can be sustained over the long term by the resources available in the area.
"These fundamentally high energy demands are likely the underlying reason why we see so much mortality among prime-age females in the middle of their geographic range, where the density of the sea otter population is highest and resources are limited,” explains study lead author Nicole Thometz of UC Santa Cruz. “In the center of the range, they appear to be up against their physiological limits.”
But give even more credit to these sea otter moms: the 2014 population index also reported a record high for pups.
“The ratio of dependent pups to adult otters remains high overall, and together with data from tagging studies, that tells us that reproductive rates are within the normal range and not limiting recovery,” says USGS biologist Brian Hatfield, who coordinates the annual population survey.
Le Morte d’Otter
Of course, it remains to be seen whether more pups will translate into overall population growth.
Hatfield also coordinates a yearly review of sea otter strandings — the number of dead, sick or injured sea otters recovered along California’s coast each year, including those reported by the public to researchers. In 2013, scientists came across a total of 340 stranded sea otters—a slight drop from 2012 but still remarkably high. However, Hatfield points out that the stranding number accounts only for sea otters that people find, and past research indicates that possibly fewer than 50% of sea otters that die in the wild wash ashore.
For the sea otter carcasses that do get reported, a subset is sent to the State of California’s Marine Wildlife Veterinary Care and Research Center, where scientists conduct necropsies to determine the primary causes of death and to identify other factors that may have contributed to the death of each animal.
These necropsies are how scientists learned about the increase in bite wounds from great white sharks — apparently the result of “exploratory bites” that do not lead to consumption of the otter by the shark, but that nonetheless often result in the death of the otter. This cause of death has been common at the north end of the range (from Pigeon Point to Seaside) for some time, but in recent years it has become most intense in the southern end of the range (from Cayucos to Gaviota) — to the point where the adult population now has a declining trend.
Not great news for the dense patch of sea otters stuck in the central range, trying to expand northward and southward.
In addition to end-lactation syndrome and shark bites, the state’s “CSI: Sea Otter” laboratory has revealed many other causes of death for sea otters across California. Over the years, California state veterinarians have found sea otters poisoned by the microbial toxin microcystin — which can flow into the sea after blooming in warm lakes with an overabundance of nutrients — as well as sea otters infected with protozoan parasites that can cause fatal brain infections, including Toxoplasma gondii , which is transmitted in the feces of wild and domestic cats.
Other known sources of California sea otter mortality include bacterial infections, heart disease, mating trauma from aggressive male sea otters, boat strikes, and gunshots. Even today, the U.S. Fish and Wildlife Service has an unsolved case from 2013 of three sea otters found near Monterey, California, where gunshot was determined to be the cause of death.
Matters of Otter Importance
That’s a lot of different factors determining the survival of a creature central to the survival of giant kelp forests — the underwater jungles teeming with marine life that stretch from California to Alaska.
“Sea otters are considered a keystone species of the kelp ecosystem because they are such voracious predators, and they are uniquely capable of limiting herbivorous invertebrates like sea urchins that, if left unchecked, can decimate kelp beds and the fish habitat they provide,” says USGS biologist Tim Tinker, who with his mentor Jim Estes coauthored a 1998 study that described this epic circle of life in Alaskan waters.
The alliance of USGS, U.S. Fish and Wildlife Service, state, aquarium, and university researchers are continuing to study the ecological and environmental roles that sea otters can play, not only in kelp forests of the outer coast, but in protected tidal estuaries as well.
Over at the Elkhorn Slough National Estuarine Research Reserve (owned and operated by the California Department of Fish and Wildlife in partnership with NOAA), alliance researchers have two ongoing studies. One study has revealed that sea otters help restore seagrass beds by keeping crab populations in balance, which in turn lets sea slugs thrive and munch on algae that would otherwise smother the seagrasses. Another study is investigating how sea otters utilize tidal wetland habitats — where they may benefit from the rich food sources of estuaries and the ease of resting in warm shallows and sun-bathed marshes—helping us understand how sea otters might one day recolonize and balance similar habitats found elsewhere in California.
Further down the coast in Big Sur, a recent analysis led by USGS geneticist Liz Bowen reported that sea otters living near a 2008 wildfire showed a physiological response to chemicals generated by fire ash — uncovering another pathway through which land and sea environments can interact.
As for the sea otter numbers in California, USGS and alliance researchers are currently analyzing more than a decade of research to tease out the driving factors tempering their population recovery and spread — a recovery, as new findings continue to show, that should benefit California’s nearshore marine ecosystem.
“We already knew that sea otters played a vital role in coastal ecosystems, but the exciting discoveries of the last several years suggest that we have really only begun to understand the far-reaching effects of this top predator,” says U.S. Fish and Wildlife Service biologist Lilian Carswell. “If sea otters can recolonize new areas of their historic range, we are almost certain to see an upswing in population growth. That will be good for sea otter recovery, good for the nearshore environment, good for all of us—because we all benefit from the services that intact ecosystems provide.”
How Do Researchers Count Sea Otters?
Appreciating Sea Otters, Safely
Sea otters can be found in nearshore areas along the California coastline, including areas of high human activity, such as harbors. Like any other wild animals—especially carnivores—they should be appreciated from a distance. Here are some tips for watching sea otters, suggested by the Monterey Bay Aquarium and the U.S. Fish and Wildlife Service.
From September 21-27, 2014, research and education institutions in California and around the U.S. are hosting sea otter science events as part of the annual Sea Otter Awareness Week.
Data and figures from the annual survey are available on the USGS project website. Sea otter biologists from California research institutions are available for media requests during #SeaOtterWeek. Please contact the following media offices: