Are sea otters taking a bite off crab season?
Despite concerns from fishing groups, a new study finds crab catch increasing, even as sea otter population expands
Fishermen have been concerned that otters could reduce the number of Dungeness crabs.
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 Francisco 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,” said co- author Andre 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 showed 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.
“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.” — Andre Boustany, a marine biologist at Duke and principal investigator for fisheries at the Monterey Bay Aquarium