Our surrogate-raised sea otters are helping restore a wetland

Otter 501 meanders through the tidal creeks near Yampah Island in Elkhorn Slough with a dozing pup on her chest. She massages the pup’s rump and blows air into its fur as she makes her way toward a main channel to feed.

To an observer, 501 might look like any other sea otter going about her business. But she’s thriving in the wild today because of a rather remarkable program at Monterey Bay Aquarium.

According to surprising new research, the same can be said of the majority of Elkhorn Slough’s otters.

A rough start

An Aquarium worker feeds Otter 501 after her rescue from a beach in Morro Bay. Photo © Sea Studios Foundation.

When she was just three days old, 501 became separated from her mother. She washed ashore near Morro Bay, helpless and hungry. Left alone, she probably would have had only hours to live. Thankfully, rescuers found 501 on the beach and brought her to Monterey Bay Aquarium. Our Sea Otter Program is the only facility in the world that rescues, rears and releases stranded southern sea otter pups like 501.

After raising 501 on formula for about eight weeks, our caregivers introduced her to Toola, one of the Aquarium’s resident sea otters and an experienced surrogate otter mom. Toola offered 501 maternal care and taught her to find and eat live prey. About eight months later, we released her into Elkhorn Slough—a coastal wetland about 20 miles north of the Aquarium.

Today,  501 is one of dozens of sea otters in the slough that are descended from our surrogate-rearing program.

Historically, the program faced criticism for helping individual distressed pups survive without contributing to recovery of the wild population. Recent research suggests our work has had significant positive impacts—both on the wild otter population in Elkhorn Slough and on the health of that ecosystem as a whole.

Toola the sea otter, acting as a surrogate mom to an orphaned pup.

Parenting a pup

In its early years, our Sea Otter Program team struggled to raise stranded otter pups and successfully return them to the wild.

“Our staff and volunteers had to hand-rear these little furballs for months,” says Andy Johnson, the Aquarium’s Sea Otter Program Manager. “And we had to take the pups out for long swims in Monterey Bay so they could learn diving and foraging skills.”

Those human-reared pups, for the most part, didn’t do well in the wild. They struggled to find food, climbed onto kayaks and seemed recklessly comfortable around humans. Andy and his colleagues began to wonder whether all their hard work was doing the wild otter population any good.

Toola’s acceptance of her first surrogate pup in 2001 was a turning point. The fact that she treated the pup as her own was a big victory, since otters aren’t known to adopt orphaned pups in the wild. After she reared the pup and taught him to dive for food, the Sea Otter Program team released him into the wild.

“We realized, ‘Wow, he’s really making it out there,’” says Animal Care Coordinator Karl Mayer. “He was raised in tanks behind the scenes at the Aquarium, but he was doing it—he was making the transition to life in the wild.”

In the 15 years since, our Sea Otter Program has continued to pair non-releasable female sea otters from our Sea Otter Exhibit with rescued pups. Once a surrogate otter mom forms a bond with her foster pup, she teaches it the skills it needs to survive in the wild, like grooming and foraging. When a pup is ready, our biologists release it into Elkhorn Slough—California’s largest tract of tidal salt marsh outside San Francisco Bay.

Otter 501 in the wild with her first successful pup. Photo ©Elkhorn Slough National Estuarine Research Reserve.

A forever home

Aquarium scientists diligently keep track of these surrogate-reared animals for years, collecting data on how long they live and how many offspring they have. What they’ve learned is encouraging.

“These young animals survive as well as wild-reared otters, and they retain their wariness of people,” says Karl. “Even better, the females mature, give birth and raise pups. Otter 501 is just about to wean her fifth pup!”

By 2016, this dataset offered a unique opportunity to ask a very important question: Has the Aquarium’s sea otter surrogacy program made a significant contribution to the slough’s otter population?

To answer this question, Karl and Andy partnered with U.S. Geological Service scientist Tim Tinker. Using the Aquarium’s data on the wild survival and reproductive success of surrogate-reared otters, along with the USGS’s annual otter population counts, Tim built a model that predicted the number of otters in Elkhorn Slough between 2001 and 2015, both with the Aquarium’s otter surrogacy program and without.

An otter mom munches a clam in Monterey Bay while her pup works on its feeding technique.

“The Aquarium wanted to know how much of the observed population growth in the slough was directly attributable to the release of surrogate-reared animals,” says Tim. Andy adds: “In other words, if our surrogacy program had not happened at all, how many otters would we have in Elkhorn Slough?”

The result: Almost 60% of the 140 or so sea otters living in Elkhorn Slough today are there as a result of the Aquarium’s sea otter surrogacy program.

Considering the southern sea otter population as a whole—which, according to the latest USGS census, is about 3,200—the release of a few animals each year through the Aquarium’s surrogacy program doesn’t make a big difference. But this new research shows it has a huge impact on local and regional populations, like the otter population in Elkhorn Slough.

A sea hare feeds on algae growing on seagrass. By eating the predators of these beneficial grazers, otters help keep Elkhorn Slough’s seagrass beds healthy.

A wetland makeover

In kelp forests, sea otters are well-known keystone predators. They keep sea urchin populations in check, which allows kelp to flourish and provides habitat for a host of other species.

Research by Tim and fellow University of California, Santa Cruz scientist Brent Hughes suggests otters can help restore the health and biodiversity of estuaries like Elkhorn Slough, too.

The slough’s food web is a little more complex than in the kelp forests. Otters eat crabs, which eat invertebrate grazers, which clean algae off seagrass and allow it to thrive. Large, healthy seagrass beds provide habitat for numerous fish and invertebrate species in the slough.

In the years since the Aquarium began releasing otters into Elkhorn Slough, the seagrass beds in the slough have rapidly expanded—evidence of the power of sea otters to restore wetland ecosystems.

More than half the wild otters at Elkhorn Slough today are there as a result of the Aquarium’s Sea Otter Program.

Andy and his colleagues are now exploring the potential benefits of surrogate-reared otter releases in other areas that were part of the otters’ historical range before the fur trade decimated their population. Perhaps the team could help seed reproductive sea otter populations that could restore degraded estuaries all along the California coast.

As for otter 501 and her kin, they just have to keep doing what otters do: eating voraciously, raising pups, and keeping their ecosystems healthy.

–Diana LaScala-Gruenewald

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Learn more about Monterey Bay Aquarium’s Sea Otter Program.