Hakai Magazine

sea otter and pup
A sea otter and her pup float in California’s Monterey Bay. Otters have made a huge recovery in the century after the fur trade nearly wiped them out. Now, conservationists are considering reintroducing them to estuaries. Photo by Suzi Eszterhas/Minden Pictures

We’ve Been Systematically Underestimating Sea Otters’ Historical Habitat

Estuaries once harbored thousands of otters, and now conservationists want to extend recovery efforts to California’s largest estuary: San Francisco Bay.

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by Isobel Whitcomb

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Before the fur trade wiped out the majority of California’s sea otters, thousands inhabited the west coast’s largest estuary—San Francisco Bay. Though the otters there went extinct by the mid-1800s, other populations managed to survive by taking refuge along the state’s rugged coastline. In isolated pockets, those survivors found safe havens, says Tim Tinker, a wildlife biologist at the University of California, Santa Cruz. Hundreds of years later, conservationists helping the sea otters recover focused their efforts on where they found those remnant populations, overlooking the estuaries they once called home.

“That unconscious bias has affected everything. It’s affected where we do research and where we think otters will be,” says Tinker. Now, those recovery efforts are being reevaluated.

It is well known that otters perform an important role in coastal kelp forests by keeping herbivorous sea urchins in check. According to a new study coauthored by Tinker, they have an equally important job in estuaries. The finding suggests that reintroducing sea otters to estuaries could benefit those ecosystems.

Skeletal evidence unearthed by archaeologists beginning in the early 1900s had given scientists an inkling that sea otters once inhabited estuaries, but the connection was firmly established in the 1980s when the animals began living in Elkhorn Slough, a four-square-kilometer estuary that meanders inland from Monterey Bay. In the mid-1990s, conservationists at the Monterey Bay Aquarium began introducing rehabilitated otters to the area, helping the small population grow. By 2015, otters numbered more than 100.

Until recently, however, scientists considered Elkhorn Slough’s burgeoning population something of an anomaly.

That changed in 2013, when Brent Hughes, a biologist at Sonoma State University in California, noticed the estuary’s eelgrass, an endangered seagrass, was bouncing back from the brink of extinction. The otters, Hughes discovered, were causing a trophic cascade that benefited the eelgrass: by feasting on crabs, otters were preventing the crustaceans from overeating microorganisms that keep algae blooms at bay. Without these oxygen-depleting blooms, the seagrasses could recover. Three decades after the otters’ return, the slough’s eelgrass has increased sixfold.

“That was the aha moment,” Hughes says. “That’s when we realized, okay, these animals really should be in these estuaries.”

In the new study, which Hughes led, the researchers imagined if the sea otter conservation success seen in Elkhorn Slough were transposed to other estuaries. San Francisco Bay, they found, could potentially support 6,000 otters—more than double the population of sea otters currently ranging from California to British Columbia.

Some experts, however, have qualms about expanding recovery efforts to these ecosystems—even if they do represent the sea otters’ true historical range.

“The whole business of moving otters into estuaries is fairly controversial,” says James Estes, an ecologist at the University of California, Santa Cruz, who was not involved in the study. There’s a big difference between Elkhorn Slough and San Francisco Bay, he says.

The bay has changed dramatically since it last harbored otters more than a century ago, says Karl Mayer, the sea otter field response coordinator at Monterey Bay Aquarium. It’s traversed daily by large tankers and commuter ferries, and lined by two crude oil refineries, making it more dangerous for sea otters.

Compounding those threats, old landfills leach heavy metals and DDT into the bay, which could be particularly dangerous for otters, Mayer adds. That’s because shellfish, otters’ primary food, are filter feeders that can absorb high concentrations of toxins from the water.

Because of their predilection for shellfish, sea otters could also disrupt shellfish harvesting. That might cause some hard feelings among local fishers, Mayer says. But even without help from humans, otters could make their way back to San Francisco Bay, he adds.

But Tinker says reintroduction is worth exploring—to the bay and to estuaries all along California. “In the future, estuaries will play a major role in recovery. If you asked us 20 years ago, none of us would have said this.”

Correction: A previous version of this article implied there was an active shellfish industry in San Francisco Bay.