Divers crushed millions of purple urchins with hammers; What happened next surprised researchers as California’s lost Kelp forests are showing signs of recovery

Can smashing millions of sea urchins really bring back California's kelp forests? A few decades ago, parts of California’s coastline were covered by dense underwater forests. Then the kelp began disappearing, leaving behind reefs dominated by hungry purple sea urchins.

The response has been anything but ordinary.

Divers have spent thousands of hours underwater using rock hammers, welding hammers and other tools to crush millions of purple urchins. In another experiment, scientists grew tiny kelp in a laboratory and planted it on underwater lines.


Now, researchers are seeing signs that these unusual efforts may be helping bring California’s kelp forests back. The report is sourced from The AP and The Guardian.

What happened to California’s kelp forests?

The decline was dramatic. Along the southern California coast, a combination of warmer ocean temperatures, pollution, overfishing and expanding populations of kelp-eating sea urchins contributed to an 80% decline in giant Pacific kelp forests over the past two decades.

Further north, California’s iconic bull kelp forests suffered an even more severe collapse, with 96% disappearing between 2014 and 2020.

The problem developed after several major changes affected the coastal ecosystem.

A warm water mass known as “the blob” developed off Alaska after 2013 and stretched southward, remaining for four years and disrupting marine ecosystems as far south as Mexico’s Baja California peninsula.

At around the same time, sunflower sea stars were devastated by a mysterious wasting disease. More than 5 billion sea stars were killed, removing the main predator of purple urchins.

With their predator largely gone, purple urchin populations exploded.

The animals consumed young kelp and scratched across the reef, leaving little opportunity for new kelp spores to establish themselves. Some areas became what scientists describe as urchin barrens, underwater landscapes dominated almost entirely by the spiny animals.

Why are divers smashing urchins?

By 2012, the Bay Foundation began working to restore kelp forests off the Palos Verdes Peninsula south of Los Angeles.

The divers discovered reefs covered with purple urchins. In some areas, densities reached 70 to 80 urchins per square meter, far above the two urchins per square meter that the foundation's tests suggested was an optimal level.

The restoration effort therefore took an unusually direct approach.

Divers went underwater carrying rock hammers and began breaking the urchins. What started largely with local divers using sledgehammers eventually shifted to smaller welding and furniture hammers and icepicks because the larger tools were difficult to swing underwater.

Over 13 years, divers working with the Bay Foundation spent 15,575 hours underwater. They smashed 5.8 million purple urchins and cleared 80.7 acres of ocean floor.

The goal was not simply to remove animals. It was to create enough space for kelp to establish itself again.

At Caspar Cove, volunteers cleared 80% of the purple urchins from one section. This summer, healthy kelp patches and schools of fish returned there.

Josh Russo, a former abalone fisher and founder of the Watermen’s Alliance, helped start the urchin-crushing effort.

“This went from being urchin barren to just full of life again,” Russo said.

How quickly can kelp return?

One of the most striking features of the restoration is how quickly kelp can respond when conditions become favorable.

Bull kelp is an annual seaweed that begins as a microscopic spore. Under the right conditions, it can grow as much as two feet per day, eventually reaching roughly 98 feet before dying during the cooler months.

That means kelp spores are already moving through the water. If they can attach to a suitable reef without being consumed by urchins, new growth can begin.

Johnson, a diver with the Bay Foundation, recalled seeing that process happen rapidly at one restoration site.

“Within three months, the kelp came back,” Johnson says. “I’ve never seen a kelp forest that dense – and it was insane to see how quickly it returned.”

But recovery has not happened everywhere.

Scientists have found that some areas cleared of urchins remain without kelp, and they do not yet know exactly why. Other restored sites, however, have remained mostly intact.

Can scientists grow kelp in a laboratory?

Urchin removal is only one part of California’s restoration work.

At Albion Bay, commercial divers removed many of the urchins in 2021. Scientists then placed tiny kelp grown in a laboratory onto 98-foot lines underwater.

In August, researchers discovered that the kelp had reached the surface and was reproducing.

“That’s the first time we know of that happening in an open coastal environment,” said Norah Eddy of The Nature Conservancy, one of the organizations involved in the experiment. “What we want is for the kelp to start putting out babies. This is showing these methods can be done in these kinds of rugged environments.”

The possibility of kelp reproducing in an open coastal environment is important because restoration ultimately needs to become self-sustaining rather than depending indefinitely on human intervention.

As Kristen Elsmore, a senior scientist at the California Department of Fish and Wildlife, said, “This is really setting the system up to hold on to the kelp that we do have until we’re in a better place.”

What could a restored kelp forest bring back?

Kelp forests are much more than large underwater plants.

They create habitat for hundreds of marine species, store carbon and help blunt the force of storm waves. Their presence also affects water quality and helps keep sediments in place.

The restoration work has already produced signs of wider ecological benefits.

California spiny lobsters have returned to restored areas, while fish including kelp bass and sheepshead are more abundant than before restoration began.

Even red sea urchins have benefited. At sites where kelp has been restored, the gonads of red sea urchins, the part valued as uni, weigh 168% more.

The difference is especially important because California once managed kelp largely as a fishery. Now, under the state’s planned approach, kelp will be managed as an ecosystem, reflecting its importance to many other species.

“Kelp forms whole forests that are supporting so many other species and so it just has this cascading effect on the near-shore ecosystem when you lose your kelp,” Elsmore said. “You’re losing a whole forest, not just one species.”

Could the kelp forests become self-sustaining?

Scientists are cautious about declaring victory.

The ocean is still changing, and kelp remains vulnerable to warm water, powerful waves and other pressures. Purple urchins can also remain dormant for years before emerging to consume new kelp growth.

There is also disagreement about how best to deal with the urchins. Some worry that crushing them could spread their eggs and make the problem worse, although Russo said he had seen no evidence of that.

Scientists also point to the importance of restoring natural predators. Sunflower sea stars are being bred in captivity with the hope of eventually reintroducing them. At least four sunflower sea stars were spotted off the Mendocino coast in the year described in the supplied material, after none had been seen there for years.

For now, urchin removal is buying time.

“The ultimate goal is for these systems to really be self-sustaining and the restoration part to really just be giving it a gentle nudge in the right direction,” Elsmore said.

The work could eventually provide lessons for other regions where kelp forests are struggling, including Tasmania, South Korea and California’s Santa Barbara Channel.

For the divers who spend hours underwater breaking urchins, the payoff is visible beneath the surface. What was once a barren reef can, in some places, become a dense forest again, with fish moving through the kelp and new growth reaching toward the surface.

For now, California’s kelp forests are not fully restored. But after years of decline, scientists are seeing something they desperately needed: evidence that the underwater forests still have the capacity to come back.

FAQs

Why are divers crushing purple urchins?
They eat young kelp and can prevent new forests from taking hold.

Has California’s kelp started returning?
Yes. Some restored areas now have kelp, fish and other signs of renewed life.