Sea Otters Restore Degraded California Estuary, Slowing Erosion by 90%

A sea otter eating a crab in the estuarine water of Elkhorn Slough, Monterey Bay, California. Duke University / Killiii Yuyan

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Sea otters started recolonizing their former habitat in a central California coastal estuary several decades ago, and erosion has since slowed by as much as 90 percent, according to a new study.

Elkhorn Slough is an estuary dominated by salt marshes. Its marsh edges and creek banks had suffered from erosion, but when the sea otters moved back in, vegetation began to rebound and establish dense root systems that are able to increasingly withstand waves and flooding, a press release from Duke University said.

One of the big reasons for the remarkable recovery is that sea otters love to eat the marsh crabs who were devouring the coastal ecosystem’s plants.

“It would cost millions of dollars for humans to rebuild these creekbanks and restore these marshes,” said Brian Silliman, senior author of the paper and director of Duke Wetland and Coasts Center and Duke Restore, in the press release. “The sea otters are stabilizing them for free in exchange for an all-you-can-eat crab feast.”

Estuaries on the West Coast were once a vital nursery and foraging habitat for the sea otters. They had shelter in their marsh home and plenty of crabs to eat. In order to keep warm in the cold waters of the Pacific, full-grown otters need to consume about 20 to 25 pounds each day — roughly one-quarter of their body weight.

“Crabs eat salt marsh roots, dig into salt marsh soil, and over time can cause a salt marsh to erode and collapse. This had been happening at Elkhorn Slough for decades until sea otters recolonized the estuary in the mid-1980s,” said Brent Hughes, the study’s lead author and a Sonoma State University associate professor of biology, in the press release. “After a few decades, in areas the sea otters had recolonized, salt marshes and creekbanks were becoming more stable again, despite rising sea levels, increased water flow from inland sources, and greater pollution.”

Local sea otters had thrived in estuaries before fur traders hunted them almost to extinction. Those who did survive were pushed out by development, agriculture and other human activities. Meanwhile, the marsh crab population in Elkhorn Slough grew exponentially.

“(Remodeling a coastline) is usually something only large-scale physical forces, like hurricanes or extreme tidal flow changes, can do,” Silliman said in the press release. “Our study, which draws on field experiments, modeling and before-and-after measurements, underscores the far-reaching benefits that can cascade through an ecosystem when a top predator is reintroduced. It begs the question: In how many other ecosystems worldwide could the reintroduction of a former top predator yield similar benefits?”

For almost a decade, the scientists did large-scale surveys in 13 tidal creeks, as well as small-scale field experiments in five areas around the estuary.

The researchers allowed sea otters to recolonize some of the test sites while excluding them from others. They conducted observations and measurements using aerial photography and on the ground. Their research confirmed that erosion had slowed by up to 80 to 90 percent at sites with large otter populations, with marshes even expanding in some areas. Simulations using modeling demonstrated similar results.

“The return of the sea otters didn’t reverse the losses, but it did slow them to a point that these systems could restabilize despite all the other pressures they are subject to,” Hughes said. “That suggests this could be a very effective and affordable new tool for our conservation toolkit.”

The study, “Top-predator recovery abates geomorphic decline of a coastal ecosystem,” was published in the journal Nature.

“There are important theoretical implications as well,” Silliman added. “This work overturns the well-established bottom-up paradigm that coastal geomorphology is governed by interactions between physical forces and plant structure. Our results unequivocally show that predators also play a keystone role in controlling the course of these tidal creeks.”

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