When climate change helped trigger a 60-fold explosion of purple urchins off Northern California’s coast, the urchins went on a feeding frenzy, devouring sugar kelp, a source of the sweetener mannitol.

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ALBION, Calif. — Early on a gray summer Saturday, an unusual assemblage — commercial fishermen, recreational boaters, neoprene-clad divers — gathered for a mission at Albion Cove, a three-hour drive north of San Francisco.

“Our target today is the purple urchin,” said Josh Russo, a recreational fishing advocate who organized the event. “The evil purple urchin.”

Five years ago, assigning wickedness to the purple urchin, a shellfish the size of a plum with quarter-inch spikes, would have been absurd.

That was before the urchins mowed down Northern California’s kelp forests.

The underwater forests — huge, sprawling tangles of brown seaweed — are in many ways just as important to the oceans as trees are to the land. Like trees, they absorb carbon emissions and they provide critical habitat and food for a wide range of species. But when climate change helped trigger a 60-fold explosion of purple urchins off Northern California’s coast, the urchins went on a feeding frenzy and the kelp was devoured.

“It would be like one of those beautiful deciduous forests turned into a desert,” said Gretchen Hoffman, a professor of marine ecology at the University of California, Santa Barbara. “But in the matter of five years.”

The dangers extend far beyond this inlet: Kelp forests exist along the cooler coastlines of every continent but Antarctica. And they are under threat both from rising ocean temperatures and from what warmer waters bring.

Already, Maine’s forests of sugar kelp, a source of the sweetener mannitol, have experienced temperature-linked declines. And in Tasmania, kelp forests have succumbed to a purple urchin outbreak. In Albion, they are trying to avoid a similar fate.

Without kelp, livelihoods lost

The divers went to work, scraping purple urchins off the bottom of the cove, hoping it would allow the kelp, which has declined 93 percent in Northern California, to grow back.

Cynthia Catton, an environmental scientist with the California Department of Fish and Wildlife, and a small team of interns sat on a boat counting the urchins that divers hauled to the surface, to get a sense of how they were faring.

The story of the kelp’s disappearance is the story of an interwoven food system breaking down, and, in the process, threatening people’s livelihoods. Some of the first people to sound the alarm about the purple urchins, Catton said, were commercial red urchin harvesters.

One of them is Gary Trumper, who has harvested red urchins for more than 30 years. Red urchins, larger than purple urchins, are commercially viable because people eat them — or more specifically, their gonads. The delicacy is better known to sushi aficionados as uni.

But the increasing purple urchin population outcompeted the red urchins for the available kelp. Without kelp, the red urchins starved.

That cut the value of Northern California’s commercial red urchin fishery from $3.6 million in 2013 to less than $600,000 in 2016. Many harvesters have moved on. “It’s probably 10 or 15 guys left doing it in the harbor,” Trumper said, sitting in a bar near the slip in Fort Bragg where he docks his boat. “But there used to be probably 100.”

The starfish and ‘the blob’

The trouble began with the starfish. Sunflower starfish, whose appendages can span more than 3 feet, normally eat purple urchins, helping to limit their numbers.

But in 2013, the starfish mysteriously began dying. There is not scientific consensus on why, but Drew Harvell, a professor of ecology and evolutionary biology at Cornell University, said she thought a virus was at least partly to blame and that warmer waters exacerbated its effects.

Sea otters, another predator of purple urchins, were hunted to near extinction in Northern California by 19th-century fur traders. Their numbers have not rebounded.

Around the same time as the starfish began dying, a mass of warm water appeared hundreds of miles off Alaska, British Columbia, Washington and Oregon. By 2014 that warm water had moved toward land, stretching from Southeastern Alaska down to Mexico.

The marine heat wave was hotter than anything humans had recorded dating back to the late 1800s. Researchers and locals called it “The Blob.” It would last into 2016.

“Human-caused global warming made it much more likely to get as extreme as it did,” said Nathan Mantua, a physical scientist at the National Oceanic and Atmospheric Administration and an author of a study linking The Blob to climate change. Over 90 percent of the heat trapped on Earth because of the greenhouse gases emitted by humans has been absorbed by the ocean, increasing its temperature.

But kelp prefers cooler waters. The bull kelp found off Northern California release spores in the fall; over the winter, the spores grow into tiny alfalfa-like plants that Hoffman said look a bit like the character Baby Groot from “Guardians of the Galaxy.”

“We don’t know very much about them because they’re very mysterious,” Hoffman said. “But we’ve done a little work on them, and they do not like high temperatures.”

The Blob also slowed the process of upwelling, in which cooler waters and nutrients move from deeper in the ocean up to the surface.

That choked off a critical supply of nourishment for the kelp.

“It’s like having your garden run out of fertilizer,” Mantua said.

“A warmer ocean generally is not as productive,” said Sonke Mastrup, a program manager at the California Department of Fish and Wildlife. “It can’t hold as much oxygen.”

Under the combination of higher temperatures and less nutrients, the kelp began to die off.

Changing behaviors

In the absence of predators and with dwindling food supplies, the purple urchins have gone on a rampage, said Mark Carr, a population ecologist at the University of California, Santa Cruz.

“They just line up like a horde of crazy individuals and storm across the reef, literally removing all algae,” he said. (Kelp and other seaweeds are algae.)

This year, they have even eaten through hard, encrusted pink algae on the surfaces of rocks, which scientists did not expect to see, said Laura Rogers-Bennett, a marine ecologist based at the Bodega Marine Laboratory.

“The impacts underwater this year have been more devastating than what we’ve seen in the past,” she said. “The perfect storm of events has been getting worse and worse over time, with 2018 being the worst we’ve seen.”

The purple urchins will likely stick around.

“They’re like cockroaches of the ocean,” Mastrup said. “They can endure starvation conditions much longer than most of the other critters.”

Even the remaining red urchins have begun adapting. While they are normally vegetarian, at deeper depths they are turning carnivorous and eating barnacles. “Which is like, one of the most bizarre story lines,” Catton said.

Saving the kelp

In an attempt to bring back the kelp, Russo has raised more than $120,000 from grants and individual divers to organize culling events like the one at Albion Cove.

The goal, said Catton, is to create kelp oases: places where kelp can safely rebound, free from purple urchins. The worry is that if too much time passes there will not be any kelp to seed future generations. “There’s a group of people who believe they can change the trajectory here of what is going on in nature,” Mastrup said. “I am hopeful but skeptical.”