San Francisco Bay could act as a haven for California’s threatened southern sea otters and could potentially triple the existing population, according to a new study led by Sonoma State University.

Historical evidence including archaeological finds supports that the bay once hosted a large population of otters prior to the mid-19th century, the study states. While the bay and surrounding areas have undergone significant changes during the past 150 years, the study’s lead author, Brent Hughes, said their findings show otters could still flourish if reestablished.

“Just because of the massive size of San Francisco Bay and its existing habitat, it could still support a thriving sea otter population,” said Hughes, an assistant professor of biology at Sonoma State University. “However, there are a bunch of risks and we highlight that in the paper. So it’s not like you can throw sea otters into the bay right now and it wouldn’t be without any risk to the sea otter.”

The new study, published in “PeerJ: the Journal of Life and Environmental Sciences” in December, found that San Francisco Bay could support about 6,600 southern otters. Today, there are about 3,000 otters found in central and southern California waters. Other estuaries such as those in Drakes Estero, Tomales Bay, Bodega Bay and Humboldt Bay could provide further habitat, the study states.

Establishing a new population of otters has proven to be successful in some regions in the past, but also controversial. Otters can provide several ecological benefits to local estuaries and could help the restoration of eelgrass beds that are vital to marine animals and migratory birds, researchers say. But otters also eat substantial amounts of shellfish each day to sustain their weight and compete with fishermen for their catch.

“To maintain their core temperature, they have to eat 25% of their weight every day to stay alive,” Hughes said. “That’s a lot.”

Hog Island Oyster Co. vice president Terry Sawyer said from his experience handling otters while working with the Monterey Bay Aquarium, he knows they are persistent and strong enough to tear through the mesh bags that aquaculturists use to grow oysters and other shellfish in Tomales Bay.

“They’re fascinating, high-maintenance animals and they’re incredibly powerful,” he said. “And if they really want to get at something, they’ll get it.”

Great white ‘gauntlet’

California’s and Baja California once had as many as 20,000 southern sea otter in the years before fur traders began methodically killing them for their valuable pelts beginning in the mid-19th century.

Once thought extinct, a small group of otters survived with their progeny making up the existing population. The otters are listed as a threatened species under the Endangered Species Act.

While an otter is occasionally spotted in and around San Francisco Bay, the bulk of the state’s population is located in waters south of Half Moon Bay or north in Washington and Alaska.

Michael Harris, a California Department of Fish and Wildlife senior environmental scientist who has studied sea otters for nearly 30 years, said otters have had difficulty expanding beyond their range for various reasons, but great white sharks are high on the list.

The state has been monitoring sea otters for more than 50 years and typically finds between 460 to 490 dead sea otters each year. Of the cases where trauma is determined as the cause of death, shark bites account for about half of them, Harris said.

To expand further north, otters would need to pass through a “gauntlet” known as the Red Triangle, Hughes said. This triangle-shaped patch of ocean stretching between Big Sur and Bodega Bay has one of the highest concentrations of great white sharks.

Young great white sharks are usually the biting culprits, mistaking the otters floating on the surface for more blubbery prey such as seals or sea lions, Hughes said. After taking an experimental bite and finding a mouthful of fur instead of blubber, the shark usually lets go and moves on.

While superficial, these bites can often be fatal have a significant impact on limiting the population size as well as the range of the otters, Harris said.

By establishing new populations in estuaries, conservation agencies could expedite what could potentially take decades or even centuries if left to natural processes, Hughes said.

Risks and rewards

San Francisco Bay and the society around it has fundamentally changed since otters once thrived there.

Prior to European settlers, the otters in the bay were hunted by Native American tribes, but the waters themselves were relatively undisturbed, Hughes said. Today, the otters would face modern dangers of heavy vessel and boat traffic, hazardous waste, sewage spills, oil spills and a significantly larger human presence.

Otters can be highly susceptible to pathogens and human interaction has been shown to have a detrimental impact on otters, especially pregnant females that constantly live on the “metabolic edge,” Harris said.

Extensive study would be required before any formal reestablishment proposal moves ahead, Hughes and Harris said.

Still, there are regions in the bay, especially in the North Bay, where the otters would be less exposed to these hazards, Hughes said.

Sea otters are what’s known as a keystone species in that an ecosystem would become drastically changed if they were no longer present. In the case of otters, their eating habits help to control the population of crabs, sea urchins and other invertebrates.

The textbook example of this is otters controlling sea urchin populations, which in turns reduces the urchins’ consumption of kelp forests. The decimation of North Coast kelp forests by urchins in recent years following significant losses of the predatory sea stars to wasting disease is an example of what can occur if populations are kept unchecked.

In estuary environments such as Elkhorn Slough in Monterey Bay, the otters have shown to improve state’s precious eelgrass beds by eating crustaceans, Harris said.

“Eelgrass habitat is a critical nursery for small finfish and invertebrates,” Harris said. “The work that Hughes has done in Elkhorn Slough suggests that otters can play this ecosystem service role by helping to reestablish a healthy environment through their foraging pressure. Otters could potentially provide that same function in San Francisco Bay.”

Crustaceans tend to prey on smaller invertebrates that eat the algae that compete with eelgrass beds for light and nutrients, Hughes said.

Eelgrass is one of the rarest habitats in California, with only about 11,000 to 15,000 acres estimated to remain, according to the National Oceanic and Atmospheric Administration. Tomales Bay, Humboldt Bay, San Francisco Bay, San Diego Bay and Mission Bay support more than 80% of the known eelgrass in the state, according to the agency.

While Sawyer said he supports the recovery of threatened species such as the otter, the complexities and challenges that San Francisco Bay poses should be very thoroughly vetted and studied before any formal proposal moves ahead.

“This has a long way to go in terms of any viability,” Sawyer said.

For Hughes, the study is meant to provide decision-makers updated information and how they plan to use it is up to them.

“Estuaries aren’t really in the recovery plan for sea otters. We’re highlighting that as kind of a big mistake,” Hughes said. “… We’re just highlighting that there is a lot of benefits and managers should consider estuaries in the future should they want to expand the range.”