The Sea Otters Holding a California Salt Marsh Together

A sea otter eats about a quarter of its own body weight every day. It has no blubber and keeps warm on fur alone, and fur that dense costs a lot of fuel. In one California estuary, that appetite turns out to be holding the shoreline together.

Elkhorn Slough winds inland from Monterey Bay through a maze of tidal creeks lined with salt marsh. For decades those marsh banks have been slumping into the water, pushed along by nutrient pollution, rising sea level and the scour of the tides. Then the otters moved in, and the collapse slowed down.

The Crab Problem

The marsh has a native digger. Lined shore crabs burrow into the creek banks and eat the roots of the marsh plants, and those roots are what hold the mud in place. A bank riddled with burrows and stripped of roots loses its strength. The next strong tide takes a slice of it, and the marsh edge retreats a little further every year.

Nothing about that is unusual. Crabs have always lived in the marsh. What had been missing was something that ate them in large numbers.

The Otters Come Back

Sea otters very nearly did not survive to fill that role. Fur hunters in the 1700s and 1800s killed them across the North Pacific, and by 1938 California's entire population came down to about 50 animals found off the coast of Big Sur. The southern sea otter has been rebuilding from that handful ever since, and it numbers around 3,000 today.

Otters started recolonizing Elkhorn Slough in the mid-1980s and spread into its salt marsh creeks over the past decade. Many of them started life as stranded pups. Raised in captivity by adult females that could not be released, the pups learned to forage before going back to the wild, and 37 of them were released into the slough between 2002 and 2016. Most stayed put: 90 percent of the females and 80 percent of the males. Those otters and their offspring account for more than half of the slough's otter population growth over 15 years.

Proving It With Cages

A study published in 2024 put the pieces together. The researchers matched erosion records going back to the 1930s, built from aerial photographs and ground measurements, against sea otter counts from the 1980s onward. As the otters increased, erosion slowed, even though the pressures on the marsh kept rising.

A pattern like that can be a coincidence, so the team tested it directly. At five creeks they fenced off sections of marsh bank with cages that kept otters out and left neighboring sections open. Where otters could reach, there were fewer crabs and fewer burrows, the banks were stronger, and less of them washed away. A wider survey of 13 creeks, comparing them before and after otters arrived, found the same thing across the estuary. In creeks with high otter densities, bank erosion slowed by an average of 69 percent.

The Same Job in the Kelp

Elkhorn Slough is the newest place the sea otter has turned out to be a keystone species, and it is not the first. On the open coast, otters eat sea urchins, and urchins graze on giant kelp. With otters around, urchin numbers stay in check and the kelp forests stand. Take the otters away and the urchins multiply and graze the kelp down.

The otter's whole way of life is built around that constant eating. It dives for crabs, clams and urchins, tucks what it finds into loose pockets of skin under each forearm, and brings the haul to the surface. There it floats on its back and cracks the hard shells open, sometimes by pounding them against a rock balanced on its chest. All of it fuels a coat of up to a million hairs per square inch, the densest fur of any animal and the only thing standing between the otter and cold Pacific water.

Where They Live Now

Sea otters are the largest members of the weasel family. Around 168,000 of them live off Alaska and Russia, from Southeast Alaska through the Aleutian Islands to the Kuril Islands. The California population is small and confined to the central coast, which is why it remains listed as Threatened under the Endangered Species Act, while the species as a whole is Endangered on the IUCN Red List.

That makes Elkhorn Slough more than a curiosity. Protecting a threatened predator is usually argued for the predator's sake. Here the sea otter spent the last century coming back from 50 animals, and part of what it is now holding in place is the marsh itself.

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