The clear, coastal waters of Monterey Bay, California, are renowned as a diver’s paradise and home to rich kelp forests teeming with life. Though biodiverse, Elkhorn Slough, an estuary at the mouth of the Bay, provides a striking contrast.
The waters of Elkhorn Slough are shallow and often murky, making it a challenging place to dive. “I don’t do it for fun, I can tell you that,” says Brent Hughes, a coastal ecologist at Sonoma State University. Yet, for more than two decades, Hughes has immersed himself in this habitat, blindly counting blades of seagrass as he gropes along the estuary’s silty bottom.
Like trees in a forest, seagrass is the backbone of Elkhorn Slough’s ecosystem. Its leaves form a flowing underwater meadow that provides shelter for juvenile fish and scaffolding for algae. Grazing invertebrates, such as sea slugs, feed on the algae and are themselves food for crabs. The entire ecosystem sustains more than 300 bird species, hundreds of harbour seals and sea otters. Beyond their role in supporting wildlife, estuaries such as Elkhorn Slough buffer flooding and sequester carbon at rates rivalling those of tropical rainforests.
But 20 years ago, Elkhorn Slough was in crisis. “It was not a pretty sight,” recalls Hughes. “There was very little seagrass and algal mats were everywhere.” The culprit was no mystery: agricultural runoff.
California’s Salinas Valley, dubbed the ‘salad bowl of the world’ for its intensive production of lettuce, broccoli and strawberries, started sending vast quantities of fertiliser-laden runoff into the slough in the 1950s. “Elkhorn Slough has some of the highest nutrient concentrations ever recorded on planet Earth,” says Hughes. This nutrient overload fuelled explosive algal growth that smothered seagrass beds and triggered a cascade of ecological decline.
Crabs, which thrived in the altered habitat, feasted on the small grazing invertebrates that normally keep algae in check. Their burrowing also destabilised the marsh banks, accelerating erosion. Elkhorn Slough began to disintegrate. Between 1956 and 2003, more than half the estuary eroded away. Land managers were desperate for solutions but nothing seemed to work.
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The remarkable recovery of Elkhorn Slough
Then, something remarkable happened. In the mid-1980s, the nearly extinct seagrass began to return. Within three decades, it had increased sixfold. Elkhorn Slough was recovering but no one knew why. The seagrass recovery defied conventional wisdom. Pollution levels remained high and no natural climatic event, such as El Niño, could explain the resurgence. It wasn’t until Hughes got his hands on an unlikely dataset that the mystery was finally unravelled. No one could have foreseen that the solution was sea otters.
The ability of north America’s smallest marine mammal to transform an ecosystem might seem surprising but it is well-documented in kelp forests. In the kelp’s case, the primary threat is sea urchins. These spiny, bottom-dwelling invertebrates troll the ocean floor on hundreds of tiny tube-feet, vacuuming up kelp. With their dexterous paws, sea otters are adept urchin-hunters. Floating on their backs and using stone tools as leverage, each sea otter can crack open 1,000 sea urchins in a single day.
By preventing urchins from over-consuming kelp, otters allow the entire ecosystem to flourish, creating what scientists call a trophic cascade.
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Sea otters are uniquely adapted to their coastal environment. Unlike other marine mammals, they lack a layer of insulating blubber, instead relying on a prodigious metabolism to stay warm in the frigid Pacific, consuming up to 25 per cent of their bodyweight per day. Their dense, plush coats – the thickest fur in the animal kingdom, with more than a million hairs per square inch – help them to retain heat but also made them a prime target for 18th-century fur trappers.
By 1910, unregulated hunting had reduced sea otter populations from an estimated 250,000 individuals to fewer than 2,000. Though hunting bans and conservation efforts saved them from extinction, California’s current population of 3,000 otters occupies just a fraction of their historic range. Most live in offshore kelp forests but, in the 1980s, a small group of males ventured into Elkhorn Slough.
At first, scientists regarded this as a curiosity. Estuaries such as Elkhorn Slough seemed an unusual habitat for sea otters. But from the otters’ perspective, the calm waters, abundant food and absence of predators, including orcas and great white sharks, made the slough an appealing refuge.
Reintroducing otters
In the early 2000s, scientists from the Monterey Bay Aquarium began reintroducing orphaned otters into Elkhorn Slough as part of a bold experiment. One stormy Thanksgiving night, biologist Teri Nicholson and her team received an urgent call: a stranded sea otter pup had been spotted along the shore. Despite scouring the coastline for hours, they couldn’t locate the pup’s mother.
That same night, a female otter at the aquarium had given birth to a stillborn pup. Mother otters sometimes adopt orphans in the wild, so Nicholson decided to introduce the crying pup to the female. At first, the female seemed hesitant, sniffing the pup before retreating. “She seemed to indicate, no, you’re not my pup,” says Nicholson. But as the pup’s cries continued, the female approached again. This time, she picked up the pup and cradled it. “She never put him down after that,” says Nicholson.
The pair formed a strong bond but the captive female’s poor health made releasing them together impossible. So Nicholson released the pup into the slough alone, unsure if he would survive. Can a pup raised in a tank adapt to life in the wild, she wondered.
The pup stayed in the slough for three days. When his radio collar began pinging several miles off the coast, Nicholson feared the worst. She dispatched a rescue team but, to her amazement, the pup had integrated with a raft of wild otters in the northern reach of Monterey Bay. Despite his artificial upbringing, he was behaving like a wild otter. Years later, he returned to Elkhorn Slough as an adult, sired dozens of pups and lived for another decade.
Today, Elkhorn Slough boasts the highest concentration of sea otters in California, with 60 per cent of the population released as surrogate-reared pups by the Monterey Bay Aquarium. Recent fossil evidence suggests that estuaries were once a major part of the sea otters’ range, and their return to Elkhorn Slough hinted at a hidden role in the ecosystem.
Sea otters and seagrass
When Hughes began studying seagrass in Elkhorn Slough, he paid little attention to top predators such as sea otters, instead focusing on the bottom-up effects of nutrients and plant productivity on the health of the estuary. The first clue that sea otters might be affecting seagrass was lucky. While Hughes and his team kept tabs on seagrass, a tour boat captain named Yohn Gideon was keeping tabs on otters. Every day for 15 years, starting in 2000, Gideon asked his passengers to tally the number of sea otters they saw during the tour. When Hughes overlaid this data with his records of seagrass abundance, the connection was unmistakable. “They fit like a glove,” says Hughes. “Sea otters go up, seagrass goes up. Sea otters go down, seagrass goes down.” But just what were sea otters doing, and what did it mean for Elkhorn Slough?
On misty spring mornings in 2013, then-PhD student Kathryn Beheshti would scan the still waters of Elkhorn Slough with a pair of binoculars. “Sometimes you would find 10, 15, even 20 otters rafting together,” she says. “You could see harbour seals hauled out, sometimes a sealion or leopard shark swimming by. It was the most biodiverse place I have ever been.” She pauses, and adds, “And you’d also see lots and lots and lots of crabs.”
Like sea urchins, hyper-abundant crabs can have profound effects on their habitat. “Their burrows turn the banks into Swiss cheese,” says Beheshti. “Metres of marsh can be lost in a storm, and that’s marsh that’s lost forever.” Unlike kelp-eating urchins, crabs don’t eat much seagrass. They prey on grazing invertebrates such as sea slugs, which normally keep algae in check. Without grazers, algae smothers seagrass, blocking sunlight and stalling photosynthesis. The result is a slow collapse of the ecosystem.
Could sea otters indirectly protect seagrass by eating crabs, Beheshti and Hughes wondered. To test this hypothesis, they placed cages around plots of seagrass to exclude foraging otters. Inside the cages, crab populations flourished, seagrass declined and erosion increased. In open plots where otters could forage freely, seagrass thrived. “Otters just move in and keep house,” says Hughes. Their research described a never-before-documented trophic cascade and helped to solidify the sea otter’s keystone role in estuaries. By eating crabs, sea otters help to stabilise the very sediment the estuary is built upon.
Future projects
The discovery of sea otters’ role in estuaries has implications far beyond Elkhorn Slough. Seagrass meadows and estuaries across the Pacific Coast face similar threats from pollution and erosion. Reintroducing sea otters could help stabilise these habitats, particularly in areas such as San Francisco Bay, which, according to recent research, could support up to 6,000 otters.
The US Fish and Wildlife Service is exploring plans to reintroduce sea otters across their historic range, from north of San Francisco to Oregon. “Where would you put them? That’s the million-dollar question,” says Hughes. “All signs point to estuaries.”
Sea otters alone can’t solve all of Elkhorn Slough’s problems. Agricultural runoff continues to pollute the estuary, and the ecosystem remains vulnerable. Still, the recovery of Elkhorn Slough offers a rare story of hope in conservation. It shows that even in the most degraded habitats, recovery is still possible. “Most ecologists focus on doom and gloom,” says Hughes. “It’s refreshing to find these bright spots.”
For those who have ventured into the murky waters of Elkhorn Slough, the beauty beneath the surface is undeniable. On rare clear days, divers can glimpse seagrass meadows stretching taller than a person. “It’s incredible,” says Beheshti. “Few people scuba dive in California seagrass meadows but it’s like discovering another world.”
Among the seagrass, sea otters dive and twirl, foraging on clams, urchins, abalone and, of course, crabs. As sea otters continue to reclaim their place in California’s estuaries, they remind us of nature’s resilience and the critical role of predators.
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Top image: close up of a sea otter in Elkhorn Slough. Credit: Getty
Additional photography by Luciane Coletti