Sea otters’ perilous path to recovery

For more than 30 years, the Monterey Bay Aquarium has been a key contributor to sea otter recovery in California. Aquarium researchers and policy experts have advanced scientific knowledge, promoted improved management and raised public awareness of the contributions sea otters make to healthy coastal ecosystems. A new research paper in the journal Ecography draws on three decades of Aquarium research to establish a link between sparse kelp cover along the California coast and a recent rise in sea otter mortalities from white shark bites. The finding illuminates a new challenge for everyone working toward sea otter recovery: Will sea otters be able to run the gauntlet of white sharks and expand back into their historical range without human assistance? Conservation Research staffer Athena Copenhaver explores the challenge.

Senior research biologist Teri Nicholson fans out her left hand, tapping each finger as she recites a brief list of unusual names: Jiggs, Goldie, Hailey, Milkdud . . .

Exhibit sea otters like Rosa play a key role behind the scenes, as surrogate mothers rearing stranded otter pups.

They might sound as though they belong to beloved pets, but Teri is actually recalling the stranded southern sea otter pups taken in by Monterey Bay Aquarium back in 1984.

Although Teri and her colleagues didn’t know it at the time, these first four orphaned pups became foundational data points in a pioneering sea otter study that spans the lifetime of the Aquarium.

The study, recently published in Ecography, uses information collected from 725 live-stranded sea otters between 1984 and 2015 to illuminate the critical relationship between a healthy kelp canopy, sea otter population recovery, and sea otter deaths from white shark bites.

Sea otter pups, rescued and raised by the Aquarium, are contributing to recovery of the wild population. Photo © Sea Studios Foundation

“By rescuing and rehabilitating stranded animals, we can observe symptoms and determine possible reasons the animal might have stranded,” explains Teri. “And, that means we can look for patterns in threats otters face over time.”

The Aquarium sea otter research team works closely with government agencies and partner institutions to collect deceased otters, too. Sea otter carcasses recovered on beaches are valuable to researchers because findings from a necropsy—an animal autopsy—can help piece together not only the story of each individual sea otter’s life, but also the larger issues facing the population as a whole.

“Plus,” says Teri, “if the carcasses are fresh, pathologists and vets – even years later – can examine the tissues to find clues about ocean conditions at that time.”

By the early 2000s—about the time the Aquarium began refining its innovative sea otter surrogacy program—Teri realized she and her colleagues had amassed a significant dataset on live-stranded otters.

“We looked at all the data and were blown away,” she says.

What the study revealed

The team determined that human-related threats to sea otters had dramatically declined over time and today account for less than five percent of all strandings.

By the end of the 19th century, fur traders had killed most of the estimated 150,000 to 300,000 sea otters around the Pacific Rim. Photo courtesy University of British Columbia

“One hundred and fifty years ago that would not have been the case,” explains sea otter research coordinator Michelle Staedler, who co-authored the study. “In the 18^th and 19^th centuries, the greatest threat to sea otters was being hunted by humans for the fur trade.”

Before commercial hunting began, population estimates for sea otters around the Pacific Rim ranged from 150,000 to 300,000 animals. At the start of the 20^th century, only a few thousand survived. In California, they were thought to be extinct until a tiny colony was discovered in the 1930s along a remote section of the Big Sur coast.

But what about other threats, like predators or food scarcity?

Food availability limits further growth of the sea otter population within its current range. Photo by Neil Fisher

Within their core range in California, where sea otter population growth is largely limited by food availability, more than 63 percent of all strandings are strongly related to crowding. The researchers expected this, because although the sea otter population has increased, its geographic range has not expanded to match.

Ten years ago, however, researchers could never have suspected that a new and different kind of challenge would someday threaten sea otters.

A new and growing threat

“White shark bites are causing the majority of problems at the edges of their range,” says Michelle. “We are starting to see more strandings related to shark bites within the range center as well.”

White shark bites are causing the majority of problems at the edges of the otters’ range. Photo courtesy CSU Long Beach Shark Lab

Aquarium researchers determined that the absence of significant kelp canopy coverage at the peripheries of the sea otter range, especially in waters north of Santa Cruz and south of Point Conception, can inhibit sea otters’ ability to reproduce and survive.

Without sufficient kelp canopy cover, sea otters, especially reproductive females and their pups, can be left vulnerable to shark bites—a multi-faceted problem that might also reflect a shift in the predatory behavior of sharks from open ocean to nearshore coastal waters.

So how much kelp is enough? According to the study’s findings, nearshore habitat with at least 10 percent kelp canopy cover provides sea otters with significant protection—but more would be better.

The kelp canopy is shrinking

Kelp cover along the Central Coast has diminished in recent years—a transformation that can be linked most significantly to a 2013 outbreak of sea star wasting disease. The epidemic caused urchin-eating sea stars to wither and die. Absent their main predators, sea urchin numbers skyrocketed. Left unchecked, urchins consume large quantities of kelp unabated, destroying once-vibrant kelp forests, leaving behind extensive “urchin barrens”.

Sea urchin populations exploded after a die-off of sea stars. That led to a rise in “urchin barrens”, where kelp is devoured by urchins faster than it can grow. Photo courtesy Stanford University Woods Institute for the Environment

“Otters can only eat so many urchins,” says Teri. “And within large urchin barrens, many of the urchins may become undernourished and therefore unappetizing to the otters.

“So while we know sea otters can enhance kelp growth by eating urchins, they cannot make kelp grow in habitat that’s unsuitable for its survival,” says Teri.

In the wake of these findings, new questions arise: Will sea otters be able to expand back into their historical range without human assistance? And, if they do move beyond the limits of their current range, will there be enough kelp to give them protective cover from inquisitive—and hungry—sharks?

Ecosystems need sea otters

“Right now, sea otters occupy less than 25 percent of their historical range,” explains Teri. “That range used to extend from Newport, Oregon, to Punta Abreojos in Baja California. But they are a long way from Oregon and Baja now.”

Healthy populations of sea otters contribute to coastal ecosystem resilience, in kelp forests and in estuaries like Elkhorn Slough, where they help eelgrass to thrive. Photo by Ron Eby.

Because sea otters are known to be powerful ecosystem engineers—in kelp forests and in coastal wetlands—part of the Aquarium’s conservation strategy for building resilient coastal ecosystems has focused on sea otter recovery. This includes, eventually, facilitating their expansion back into their historical territory. Because of the Aquarium’s surrogacy program, the only one of its kind to rear and release orphaned sea otters pups, researchers can physically place animals where they might be most needed. In recent years, the wetlands of Elkhorn Slough have benefited from this approach, to the point where roughly 60 percent of all sea otters currently in the slough are graduates from the Aquarium’s surrogacy program or descendants of those animals.

Teri folds her hands together and smiles as she reflects on her nearly 25 years of sea otter research.

“For years people have been looking at us asking, ‘What good are you guys doing with these stranded animals?’” she says. “Finally we can say that this work can benefit the sea otter population by helping reestablish these animals in local or regional areas. We now have insights, data and a program that can provide a solid scientific foundation to inform new thinking about how we might reintroduce sea otters into areas where they haven’t roamed for more than a century.

“It’s fascinating work.”

—Athena Copenhaver