Endangered by extinction because of their abundant pelts, the California sea otters recovered after protective measures were put in place in 1911. Their population grew steadily for much of the last century, but now the still-threatened species is bogged down at around 3,000 otters. The problem is, at both ends of their current range along the state’s central coast, they are surrounded by a sharp (and as yet unexplained) surge in shark attacks. In the hope of reintroducing breeding populations elsewhere in the otters’ historic range, wildlife managers have investigated certain estuaries that are protected pockets of water.
According to a study published in PeerJ last month, the west coast’s largest estuary – San Francisco Bay – could potentially be an excellent home for sea otters, despite being in the middle of a large urban area.
“I was surprised,” says lead study author Jane Rudebusch, a spatial ecologist at San Francisco State University’s Estuary & Ocean Science Center. “The bay is heavily urbanized. You can tell it is a busy place just by looking at it. “Tankers deliver crude oil to refineries on the coast every day, and high-speed commuter ferries are constantly racing at speeds of up to 50 mph between San Francisco, Oakland and other waterfront cities. Sediments in parts of the bay are contaminated with methylmercury and polychlorinated biphenyls, toxic chemicals that accumulate in the clams, crabs, and other animals that eat sea otters.
Rudebusch and her colleagues used existing data to create a map of the bay listing these risks by region as well as the quality of possible habitats for sea otters – especially shallow water and salt marshes. They found that the bay still has hundreds of acres of high quality habitat in areas where there is little threat from human threats. “A large part of the north bay is a sweet spot,” says Rudebusch. Much of this area is only about three feet deep and has abundant salt marshes in protected areas, including China Camp State Park and the San Pablo Bay National Wildlife Refuge.
Rudebusch is optimistic that sea otters can coexist with the many human uses of San Francisco Bay, but emphasizes that their work is an early step in a long process. She also warns that, before they can even think about reintroducing sea otters, wildlife managers “need to lay a lot more groundwork to optimize their chances of success.”
One question is whether the northern bay provides enough food for sea otters, which consume about a quarter of their body weight every day. A companion study published in PeerJ in 2019 found that the entire bay has enough prey to feed about 6,600 sea otters. However, this study did not record food abundance in certain parts of the bay. “The next big thing is the spatial assessment of the availability of prey,” says Rudebusch. “The otters will go where the food is. Will it coincide with the sweet spot in North Bay or high risk areas? “
It is also unknown whether sea otters would stay where they are reintroduced. “It’s impossible to predict what they’re going to do,” says Tim Tinker, a sea otter expert at the University of California at Santa Cruz who wasn’t involved in the new study but contributed to the study published in 2019. He suggests an attempt at reintroductions in smaller estuaries north of San Francisco Bay. These include Tomales Bay in a state park and Drakes Estero in Point Reyes National Seashore. Such test runs would help determine whether reintroduced otters are likely to remain in estuaries, as well as the best mix of sex and age to establish new populations. “These are all things that need to be considered before the reintroduction of otters to San Francisco Bay can become a reality,” says Tinker.
Another factor to consider is the local response to sea otter restoration. For example, members of the fishing industry might be suspicious of these voracious predators. There is no longer any commercial fishing in the bay itself, but it remains an important nursery for young dungeness crabs that migrate to the ocean where they are harvested as adults. And coastal developers could oppose the return of sea otters as their presence as a protected species would add an extra layer of regulation to projects.
Many conservationists, on the other hand, would be excited about the prospect of bringing sea otters back into the bay. Letitia Grenier, an ecologist at the nonprofit San Francisco Estuary Institute who was not involved in the study, notes that the bay is already rich in wildlife, ranging from salmon to migratory birds to humpback whales. The latter recently returned to enjoy huge anchovy schools. “The bay has so much space – hundreds of thousands of acres,” she says. “We should be able to share it with sea otters.”