California’s sea otter population could triple by recolonizing San Francisco Bay

Study highlights importance of estuaries as prime habitat for the endangered southern sea otter

sea otter
Sea otters are thriving in Elkhorn Slough. (Photos by Ron Eby)
sea otter in eelgrass bed
Eelgrass beds in Elkhorn Slough benefit from the presence of sea otters.

The picture of sea otters frolicking among kelp beds and rocky shoals has become an iconic image of the California coastline. But it may be drawing attention away from the value of other habitat that could truly help the endangered species in its recovery—estuaries.

In fact, a new study released today concludes that California could more than triple its population of southern sea otters, from an estimated 3,000 to nearly 10,000, by repopulating the largest estuary on the coast—the San Francisco Bay.

It would essentially end up lifting the sea otter out of its endangered species status,” said Brent Hughes, assistant professor of biology at Sonoma State and lead author of the study published in PeerJ: the Journal of Life and Environmental Sciences. “For the conservation of the sea otter, this would be huge.”

Hughes did pioneering research on the ecological role of sea otters in Elkhorn Slough as a graduate student at UC Santa Cruz, where he worked with Tim Tinker and Kerstin Wasson, both coauthors of the new paper and adjunct professors of ecology and evolutionary biology at UCSC.

Elkhorn Slough’s protected habitats provide a special haven for sea otters, as well as remarkable opportunities for studying them,” said Wasson, who is research coordinator for the Elkhorn Slough National Estuarine Research Reserve. “Sea otters were associated with kelp forests only because the surviving southern sea otters off of Big Sur were in that habitat; if you go back in history, you’d be as likely to think of otters as salt marsh and seagrass animals, and indeed, there used to be more in San Francisco Bay than the entire southern population size now.”

Recovery plans

According to Hughes and his coauthors, current southern sea otter recovery plans have not included estuaries as target habitats. This “may be an artifact of where the surviving population persisted,” they concluded in their paper, titled “Species recovery and recolonization of past habitats: Lessons for science and conservation from sea otters in estuaries.”

The dogma, widely reinforced in both the scientific and popular media, is that sea otters do best in saltwater kelp forests,” said Brian Silliman, Rachel Carson associate professor of marine conservation biology at Duke University’s Nicholas School of the Environment and a partner in the study. “But this is based on studies and observations made while these populations were in sharp decline. Now that they are rebounding, they’re surprising us by demonstrating how adaptable and cosmopolitan they really are.”

"This really changes our view of the ecological role of sea otters,” said Tinker. "They are not only keystone marine mammals of the outer coast, they are also important apex predators in estuarine habitats."

Remnant population

The southern sea otter was widely believed to be extinct due to the expansive fur trade of the 18th and 19th centuries, which reduced the global population from between 150,000 and 300,000 to roughly 2,000. Then in 1914 a remnant population of about 50 southern sea otters was found along the rugged Big Sur shoreline. Thanks to conservation efforts, the population has since grown to more than 3,000, but their numbers are still far below their historic numbers and range.

While conservation efforts have focused on protecting otters in these rocky coastal habitats, evidence shows that southern sea otters were once abundant in California estuaries, including in San Francisco Bay. Early accounts by Spanish explorers noted otter populations as far south as San Jose and as far north as Richardson Bay. “Sea otters probably numbered in the thousands in this estuary prior to being driven to local extinction by over-hunting,” the researchers noted.

The only estuary in California that is currently home to a distinct and self-sustaining population of sea otters is Elkhorn Slough at Moss Landing in Monterey Bay. Overall, southern sea otters have only recolonized about 13 percent of their historic range, according to the study.

Hughes said one of the reasons otter habitat has remained fragmented and why sea otters have not been able to migrate north and reestablish residency in San Francisco Bay is the presence of great white sharks near the Golden Gate. “We call it ‘the gauntlet,’” said Hughes. “Otters really can’t get past the gauntlet.”

But if otter populations were established inside San Francisco Bay and out of the range of great whites, they would become the top predator and would likely thrive.

Using existing studies and modeling of sea otter growth, the researchers concluded that the San Francisco Bay could support about 6,600 sea otters, more than twice the current estimated population of 3,000. “It would change the game in terms of how we look at sea otter conservation,” Hughes said. 

Along with Sonoma State University, this study was a result of extensive collaboration between researchers and managers from the U.S. Fish and Wildlife Service, the Western Ecological Research Center of the U.S. Geological Survey, the National Park Service, Elkhorn Slough National Estuarine Research Reserve, the Monterey Bay Aquarium, Hakai Institute, San Francisco State University, San Diego State University, California Polytechnic State University, UC Davis, UC Santa Cruz, and Duke University.