Sea otters are making great strides toward recovery in California, and new research shows that their growing numbers are directly correlated with the decline of one of the most invasive species in the marine world: the dreaded green crab. 

“I’ve studied green crabs in estuaries on three coasts and two continents for decades, and this is one of the first pieces of good news we’ve gotten,” estuarine ecologist and lead author Rikke Jeppesen said in a statement.

The study, which was published in the scientific journal Biological Invasions, was conducted at Elkhorn Slough National Estuarine Research Reserve — a coastal estuary also known as a “cradle of biodiversity” in Monterey Bay. 

The research team estimated that sea otters local to the region eat 120,000 green crabs a year. 

It’s an essential job, given that green crabs are aggressive predators that habitually outcompete native species and decimate fields of seagrass. 

“We set traps and are delighted that we no longer catch large green crabs,” Jeppesen said. “This is one more great reason to support recovery of top predators in coastal habitats.”

To stay warm and well-fed in freezing waters, sea otters eat nearly a quarter of their body weight in food every day — and their diet is not just limited to crabs. Sea otters also snack on sea snails, clams, mussels, fish, and sea urchins. 

The last item on the menu is especially important, as sea urchins are densely overpopulated and capable of destroying entire kelp forests — carbon-capturing canopies that are vital to ocean health. 

So in addition to keeping the green crab population down, sea otters also play a crucial role as a top predator by cracking down on kelp-munching sea urchins. 

And it doesn’t stop there. 

In the stretch between Monterey and San Francisco, recent studies show that sea otters feed on destructive shellfish like the striped shore crab. 

For decades, the crustaceous species has been slowly increasing the risk of coastal flooding by decimating shorelines with their feeding habits. 

“[Otters] don’t completely reverse erosion but slow it down to natural levels,” Marine ecologist Brent Hughes told VOA News. 

In the grand scheme of things, sea otters have had quite the comeback story. 

In the early 20th century, sea otters were almost driven extinct by poaching and harvesting. 

After government protections were put in place in 1911, they slowly began to make strides towards recovery — but they are still listed as an endangered species under the International Union for Conservation of Nature. 

Today, modern threats include oil pollution, orca predation, and habitat loss. 

But sea otters have also increasingly become subject to research in the marine science community in recent decades — often with the intention of sourcing further evidence for their conservation. 

And the correlation between green crabs and sea otters adds even more credibility to those conservation efforts. 

In their paper, the Elkhorn Slough research team argued that when sea otters thrive, the ocean thrives too. 

Jeppesen and his colleagues wrote: “Our investigation highlights that investment in recovery of top predators [like sea otters] can increase beneficial food web interactions and resilience of the entire ecosystem.”

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Header image via Lilian Carwell/USFWS