Study: Sea otters eating invasive crabs

By Tim Didion and Drew Tuma

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    MONTEREY, California (KGO) — When we first met the hungry otters of Elkhorn Slough last year, they were chowing down on a species of burrowing crabs blamed for destabilizing the shoreline. Thinning their numbers and rescuing sections of the estuary, located at Moss Landing in Monterey Bay. Now, researchers believe the otters could be a potential solution for controlling an invasive species that’s invaded ecosystems up and down the West Coast: green crabs.

“Well, so back in the early 2000s, I was just starting out as a graduate student at UC Santa Cruz, and the green crab population was growing explosively at Elkhorn Slough Reserve,” says estuary ecologist Rikke Jeppesen, Ph.D.

Jeppesen began trying to figure out why the green crabs, native to Europe, were thriving. But somewhere in the middle of her research project, a strange thing happened: the trend began reversing, at least in Elkhorn Slough. So, what had changed in this tiny sliver of the California Coast? She believes it was the return of native sea otters, in greater numbers.

“Given that the of population size is about 100 and 120 currently in Elkhorn Slough, they can probably eat somewhere between 50,000 and 120,000 green crabs the slough per year, and I mean, 100,000 green crabs consumed by otters, that’s a lot,” Jeppesen explains.

So she and her colleagues began mapping the slough. Pinpointing the location of sea otter populations, versus green crabs. Research coordinator Kerstin Wasson, Ph.D., says the modeling seems to confirm the trend, more otters, fewer crabs.

“So we used circumstantial evidence about spatial patterns. Like within Elkhorn Slough, the areas that have a lot of otters don’t have a lot of green crabs,” says Wasson.

And the green crabs are nothing if not invasive. Spreading rapidly since being accidentally introduced into San Francisco Bay in the late 1980s. And coastal communities from California to Washington state are spending millions in time and money to control the crabs, which compete with native sea creatures.

Could the otter be a help?

“I think there’s a lot of promise that sea otter in introduction to these areas, would decrease the European green crab populations as well as leading to healthier seagrass beds and salt marshes, and all the other benefits that come from having a top predator and restoring food webs in the system,” Wasson believes.

Several groups are pushing state and federal officials to consider the idea of reintroducing sea otter populations to areas that were decimated by fur hunters more than a century ago. The idea is expensive and likely to face opposition from the fishing industry. But in a world of changing climate and changing priorities, researchers say it’s important to study all our options.

“However, with changing climate and changing conditions, natural extreme events, it’s very hard to predict the future. So although we feel like the system right now is more balanced than maybe it was 20 years ago, I don’t think we should be too safe, says Jeppesen.

And the monitoring will continue at Elkhorn Slough which is a national research reserve. Groups including the Elkhorn Slough Foundation team with major research centers to track changes to its environment.

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