Are sea otters taking a bite out of California’s Dungeness crab season?
Despite concerns, study finds crab catch is increasing, even as otter population expands
Dungeness crabs are a holiday tradition every year on tables across Northern California. But the prized crustaceans also are a prime delicacy for other local residents — sea otters that live along the Central Coast.
Scientists are studying whether to relocate sea otters north into San Fran
cisco Bay to help expand their population back to its historic range. But fishermen have been wary, concerned that the otters could reduce the number of Dungeness crabs, a $51 million industry, and one of California’s largest commercial fisheries.
Now a new study suggests the two beloved ocean luminaries may be able to co- exist. In a paper published Thursday, researchers from Duke University, the Monterey Bay Aquarium and the U. S. Geological Survey found that as the number of sea otters has grown off central California in recent decades, the catch of Dungeness crabs by fishermen in Half Moon Bay, Monterey and Morro Bay actually also has gone up, not down.
The study could increase the chances that otters will be reintroduced into San Francisco Bay nearly 200 years after they were last seen there, or to other places north of the Golden Gate Bridge.
“We looked at some of these issues, and we couldn’t find an impact,” sa id co - author A nd re Boustany, a marine biologist at Duke and principal investigator for fisheries at the Monterey Bay Aquarium. “We take the fishermen’s concerns seriously. We realize how important the crab fishery is to a lot of people. We want there to be a healthy fishery as well.”
The study, published in the journal Biological Conservation, looked at landing receipts — forms that commercial fisherman fill out and provide to state regulators. The scientists found that as the sea otter population more than doubled from 1980 to 2018, so did the size of the Dungeness crab catch per fishing trip. That growth rate was as good or better than the rate in areas north of San Francisco where sea otters do not currently live.
The scientists don’t know for sure why more otters didn’t mean fewer crabs. One theory is that the otters aren’t actually eating that many. Sea otters eat up to 25% of their body weight every day. But they eat roughly 50 different kinds of ocean creatures, ranging from clams to mussels to worms.
The study noted that Dungeness crabs only made up about 2% of the food that otters in Elkhorn Slough and along the Monterey Peninsula were documented eating in 83,000 observations back to 2007 by scientists with binoculars and small telescopes. The otters might have eaten more. Some records show they were eating crabs, but which type of crab couldn’t be identified. There are multiple other species of crabs that otters like to eat.
Second, the researchers say, sea otters don’t dive very deep — only about 75 feet maximum. Dungeness crabs often live hundreds of feet down.
“The otters can only dive so deep,” Boustany said. “They are very coastal. There’s a large area where crabs are fine and they don’t have to worry about any otters bothering them.”
Lastly, and most provocative, is research showing that the presence of otters can help boost the populations of other ocean animals. One example: Otters eat sea urchins. And sea urchins eat kelp. So when otters keep the population of urchins down, kelp forests expand. And kelp forests provide homes to lots of small marine creatures, including crabs.
A similar ecological dance plays out in bays and sloughs, where otters eat various types of crabs, which eat sea slugs. The sea slugs eat algae that lives on eel grass. When the number of crabs drops, the number of sea slugs expands. They eat more algae, allowing the undersea grass to flourish, and providing more habitat for young fish and other species.
Boustany said more study is needed. For example, a jump in sea otter numbers in southern Alaska in recent decades led to drops in the catch of Dungeness crabs there. In Alaska, there are more than 90,000 otters. Along California’s Central coast, there are about 3,000.
“Local ecosystems have evolved in the absence of sea otters,” said Mike Conroy, executive director of the Pacific Coast Federation of Fishermen’s Associations in San Francisco. “We are concerned that ill-thought reintroductions could have dramatic impacts on species, which include Dungeness crab. We have to ensure that lessons learned from the reintroduction of sea otters to Southeast Alaska are considered and planned for.”
Historically there were about 16,000 sea otters along the entire California coast. But they were hunted relentlessly in the late 1700s and early 1800s by Russian, British and American fur traders for their pelts, which are denser and softer than mink fur. Many lived in San Francisco Bay, but by the Gold Rush, they were all but gone. They were feared extinct until the 1930s, when about 50 were discovered in remote Big Sur coves. Protected by the Endangered Species Act in 1977, they began a slow comeback. Over the last decade, however, they have not been able to expand north beyond the Pigeon Point area in San Mateo County because of attacks by great white sharks. Some researchers think it’s because federal laws have protected elephant seals, sea lions and other marine mammals that the sharks eat. As a result, the population of white sharks may also be growing.
Biologists at the U.S. Fish and Wildlife Service are in discussions with scientists from other agencies, aquariums and universities to look at relocating some otters north past the shark zone so they can expand and no longer be endangered. A draft study is considered at least several years away.
“There are so many politics involved in that question,” Boustany said. “It goes well beyond the science. I would hope this research is part of the discussion.”