Back From the Brink: Big River Kelp’s Dramatic Comeback

After a decade of relentlessly bad news about kelp in California, it’s at first hard to parse what Tristin Anoush McHugh is saying. “We’re seeing so much kelp,” she reports. “Massive, massive amounts.” 

For the past two years, McHugh, the kelp project director at the Nature Conservancy, has led a California Sea Grant- and California Ocean Protection Council (OPC)-funded consortium of academic researchers, nonprofit organizations, laboratories, contractors and state agencies to restore recently degraded kelp habitat at the mouth of the Big River in Mendocino Bay. 

The seaweed forest here has been a particular heartbreaker in the ongoing saga of kelp in California. In the mid-2010s, kelp died off en masse along the state's coast, triggered by an extended marine heatwave and a surge of kelp-eating native purple urchins. But the forest at Big River seemed to dodge the catastrophe, growing back year after year, even as 96% of kelp elsewhere in the region vanished. “Big River was our ‘not all is lost’ site,” says McHugh. 

Then, in 2022, divers noticed early signs of deforestation at Big River. One year later, more than 90% of the kelp there had vanished, adding to the vast losses elsewhere along the North coast. Without kelp and the habitats it provides, fisheries from abalone to salmon have crashed in parts of the state, disrupting industries as well as traditional Indigenous harvesting practices and food sovereignty. Dive tourism has collapsed too, leaving local economies reeling.

And now the Big River kelp is back, Lazarus-style. Results from fall 2025 show that the local kelp canopy increased by over 900% since starting restoration efforts and compared to the historical lows of 2023. “We're near 2022 levels again,” says McHugh. 

A Multipronged Approach

She doesn’t sound jubilant. Having studied kelp for over a decade, McHugh knows that much could go wrong again. “Another marine heat wave recently hit the coastline,” she says, naming just one threat. 

Still, McHugh is “cautiously optimistic.” Not only that Big River's kelp has survived the worst, but also that the approach used here could serve as a blueprint for conservationists elsewhere facing sudden kelp loss.

She attributes Big River's success to a multipronged approach: combining proven restoration methods and timing them strategically to match the kelp's life cycle.

The first step was removing the kelp's main threat. Between 2024 and 2025, commercial urchin divers hand-collected over 82,000 lbs of purple sea urchins from five acres of ocean floor around Big River. They collected another almost 36,000 lbs at a companion site farther down the coast. 

“A Straight Up Front of Purple Urchins”

Purple sea urchins feed voraciously on kelp, but their populations used to be kept in check on the West Coast by an equally ravenous predator: Sunflower sea stars (Pycnopodia helianthoides) hunt urchins on the seafloor. These massive predators can measure up to 39 inches and often display more than 20 arms. Until 2013, these sea stars were plentiful along California’s coast.

Then a mysterious disease attacked them, killing the sea stars in droves. The disease caused lesions and twisted arms, ultimately disintegrating the sea stars' entire bodies. 

Only in 2025, researchers finally identified the culprit behind it: a bacterial pathogen called Vibrio pectenicida. The bacterium killed close to an estimated six billion sunflower sea stars, nearly the entire California population. 

With their biggest predator gone, urchin populations shot up by 10,000% between 2014 and 2022. Eventually, they reached the reefs of Big River. In 2021, “we were like, there's a little bit of urchins over here,” remembers McHugh. “In 2022, we realized, oh, there's a lot more. Then we dove the site again in 2023, and it was a straight up front of purple urchins.”

On average, just under one urchin perched on every square foot of seafloor in the area, ravenously chomping down on any kelp it could find. Fishermen have traditionally avoided harvesting these urchins because their 'uni' — the gonads prized as a delicacy — are small compared with other urchin species.

So after the squad of commercial sea urchin had combed the seafloor and picked up urchins by hand, finding uses for them all became key to the project’s success. Some were sold to food markets, while others became compost, soil amendments or were tested in experimental biomarble products.  

Rebuilding the Underwater Forest

To pacify the remaining urchins, McHugh’s team distributed lab grown bull kelp on the ocean bottom, like hay onto a pasture. Purple sea urchins actually prefer dead drift material to living kelp and only eat fresh plants when desperate, according to McHugh.

She says the trick worked. “We went from seeing urchins running rampant, eating whatever they could,to urchins moving back into their crevices and just hanging out.” 

It still left the problem that large parts of the seafloor had already been stripped bare of kelp. “At the center of the site, it was like scorched earth,” says McHugh. 

To bring life back to these barren patches, the team needed to replant the region's dominant species: bull kelp (Nereocystis luetkeana), a seaweed that shoots up from spores, or seeds, to 60 feet stems in mere months. 

The project team worked hard to learn how to cultivate bull kelp, setting up tanks with UV lighting at Moss Landing Marine Laboratories to grow multiple generations of kelp from spores for outplanting. Then they wove kelp sprouts into twine and tied the twine to PVC tubes, which they anchored to the seafloor with cinderblocks. The tubes and natural buoyancy of the kelp kept the twine floating, ensuring the new outplants stayed safely out of reach of any urchins still wandering the bottom. 

Within two months, bull kelp had shot up from these temporary floating nurseries, called ARKEVs (Arrays to Recover Kelp Ecosystem Vegetation). The kelp matured, releasing trillions of spores, re-colonizing the barren seafloor around it.

Reducing the urchin population in tandem with replanting kelp “really accelerated the pace and scale of the recovery,” says McHugh. California Sea Grant and the Ocean Protection Council funded the $1.6 million project, which brought together 10 partner organizations including the Nature Conservancy, Monterey Bay Aquarium, University of California, Davis and the California Department of Fish and Wildlife. The United Nations Decade of Ocean Science for Sustainable Development recently endorsed the webstory the project created. 

A Blueprint for Recovery

The kelp restoration project celebrated its success last October at the second North Coast KelpFest! in Mendocino and Fort Bragg,  which attracted over 2,400 visitors. McHugh hopes to keep building on this interest. Kelp forests fulfill similar ecological roles as coral reefs, creating habitats for marine organisms from tiny crustaceans to fish and sea otters. Yet McHugh says they face a visibility problem — literally. “Kelp forests grow in cold water. They're hard to get to. The visibility is just not as good,” she says. “More Californians know about the Great Barrier Reef than they do about kelp forests.”

For now, the kelp forest at Big River mouth has recovered enough that “we have some resilience baked into the system,” McHugh says. Last year, the project team even spotted a handful of sunflower sea stars in the restoration areas.  “That was a major hope spot,” says McHugh, although the sea star wasting disease remains a threat. 

The restoration work will continue. While the original funding is about to end, the team used their success to secure a new federal grant through the Pacific Coast Ocean Recovery Initiative. This will allow them to keep harvesting urchins and replanting kelp through at least 2027, extending the restoration site from five to 25 acres.

“We need to keep our eyes on the water,” says McHugh. “We know how quickly things can turn, and we should be ready.”