After being hunted to near extinction, sea otters are back on the Central Coast. A researcher from UC Santa Barbara says the otters play an important role in stabilizing the marsh of Elkhorn Slough in Monterey County.
As one of the state’s largest coastal salt marshes, the protected area is home to numerous birds, plants, fish, and marine mammals. It’s also a hub for marine research.
Ron Eby is a long-time volunteer at the Elkhorn Slough Reserve. He coordinates volunteer scientists to observe and count the sea otters in support of ongoing studies.
“We come out twice a month and we do it on consecutive weeks because with the tidal cycles, that way we get to see otters in their various conditions,” he said.
Southern sea otters were common on the California coast until hunted to near extinction in the 19th century. A small number survived near Big Sur and now, with protections in place, Eby said the population in the state has rebounded to about 3,000, with 120 in Elkhorn Slough.
He said otters returned to the area around the 1990s. He’d see them venture into the salt marsh then head back out to sea. But now, they live there full time.
“A lot of the males go offshore, whereas all of the females, every female otter here in Elkhorn Slough, has remained here in Elkhorn Slough. It's just amazing,” Eby said.
UC Santa Barbara researcher Kathryn Beheshti said sea otters typically live in the ocean near kelp forests, so settling in the salt marsh was a bit of a surprise.
“The recovery of that sea otter population has expanded over the last 100 years and now sea otters are really common within the Monterey Bay area and have been moving into Elkhorn Slough over the past few decades, so that’s a relatively new phenomenon. We didn't know that they actually used the salt marsh habitat,” said Beheshti.
She said the salt marsh seems to offer a level of comfort not found in the open sea — like fewer threats and a chance to haul out, which means to get out of the water and warm up.
“In the part of Elkhorn Slough that is actually restricted, you’ll see mothers with their pups all resting on the marsh plain and in the tidal channels,” Beheshti said.
And there’s plenty of food, too. Otters eat constantly due to their high metabolism, and they really like crab. In Elkhorn Slough, the crab population has grown so large over the years that it’s damaging the structural integrity of the marsh.
“You can see at the edges of those tidal creeks, there’s just tons and tons of burrows, it looks like Swiss cheese, and that’s from these shore crabs,” Beheshti said.
She said the crabs burrow into the creek banks and forage on the roots of plants. This causes the banks to erode and collapse, leaving the area more prone to the effects of sea level rise and flooding.
One day, while at the marsh with fellow scientists, she said they wondered if the otters’ appetite for crab might change the landscape.
“And so, we’re watching them eat all these crabs and looking at the banks that are riddled with the same burrows made by these crabs, and making these connections of, 'huh, I wonder if these sea otters are doing anything to stabilize the creeks by removing and eating these massive amounts of these burrowing shore crabs,'” Beheshti said.
So, they monitored the sea otters and the crabs in a long-term study that combined historical data with field work.
“We actually manipulated plots in the salt marsh where we allowed otters in versus plots where we excluded them with these little fenced enclosures. Those enclosures allowed crabs to move in, but kept the sea otters out,” she explained.
Beheshti said the areas with sea otters had a stabilizing effect on the creek beds — they devoured enough crabs to slow erosion by about 69 percent.
She said the study offers new insights into the important role of top predators.
“Because of our impact on the natural environment, a lot of these systems have seen the loss of these top predators, and so potentially the reintroduction or the recovery of these top predators could increase the stability and resilience of these systems to future change,” she said.
The research on otters in Elkhorn Slough is published in the journal Nature.
