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Abalone hunters and other recreational divers forced to stand by idly for years as tiny purple urchins overran the ocean floor off the North Coast are scheduled to converge en masse over Memorial Day weekend to try their hand at resetting nature.

At least 100 participants are expected to gather at Ocean Cove on the Sonoma Coast for a two-day blitz aimed at clearing as many of the dollar-sized urchins from the cove as possible. They hope it will give some of the region’s ravaged bull kelp and the beleaguered red abalone that feed on it a fighting chance at recovery.

The event is one of several in the works by the Watermen’s Alliance, a coalition of spearfishing clubs claiming more than 1,000 members throughout California. The campaign, involving public and private partners, aims to try to restore the kelp forest that once dominated offshore waters in Sonoma and Mendocino counties.

Sport-fishing groups also have raised tens of thousands of dollars to put commercial red urchin divers to work at several coastal locations, in hopes they can coax the bull kelp into re-establishing a fraction of its former territory.

The scale of the problem is vast.

“We’re seeing swarms of purple urchins absolutely eating anything alive,” said Jon Holcomb, a commercial urchin diver from Fort Bragg who has worked closely with state the Fish and Wildlife Department and the Watermen’s Alliance on the issue. “Urchins, abalone, anything. It’s dramatic.”

Solutions are in short supply, and the best method of attack remains in dispute amid regulatory restrictions and concern that too much human interference in the ocean could have adverse environmental effects.

“The science is at best uncertain on this approach,” Valerie Termini, executive director of the Fish and Game Commission, told her members during a meeting last month. “It’s sort of, ‘We’re trying something.’ ”

For recreational divers, the opportunity to cull the voracious purple critters is an antidote to years of frustration experienced while watching them proliferate, decimating miles of coastal kelp beds that once harbored California’s last productive abalone fishery, as well as provided habitat for other marine wildlife.

Many would just as soon smash the urchins to bits where they lie, though officials and scientists warn that would violate state wildlife code and risk accidental spawning through the release of reproductive material that would only boost the population.

State officials and scientist have urged a more cautious, restrained approach, one that critics say amounts to “dragging their feet.” Divers watching the abalone shrink in their shells and die of starvation have struggled to restrain themselves, meanwhile. This year, they have been forced to quit hunting for abalone altogether, the popular season canceled by state officials due to the imperiled status of the fishery.

Healthy bull kelp forests are seen as a key factor in the recovery, and this is the time of year those forests should be expanding their reach in near-shore waters where abalone normally thrive.

“The kelp needs to sprout every two years, at a minimum,” said Josh Russo, Watermen’s Alliance president. “If the seeds don’t land and create a viable plant, after two years, it dies. So now is really kind of a crunch time. If it’s not now, it’s never.”

The state Fish and Game Commission cleared a hurdle last month to support recreational divers in their campaign, unanimously approving an emergency regulation that raises the daily bag limit for sport-caught urchins from 35 individual invertebrates to 20 gallons, with no limit on possession.

Though not the unlimited catch for which some divers had petitioned, the increased limit makes it feasible for sport hunters to try to make a dent in the purple urchin population, estimated recently at more than 60 times its historic density.

“As divers, we just want to do the right thing, and at this point, it might be too late,” said Monte Rio diver and guide Matt Mattison. “I’d like to think we still have enough natural bull kelp seed out there.”

Consternation over the collapse of the kelp forest has been growing for years amid profound ecological shifts some scientists have come to call a perfect storm. The biggest changes included a toxic algal bloom off the Sonoma Coast that in 2011 took a toll on red abalone and other invertebrate species; the eruption in 2013 of sea star wasting disease, which wiped out important urchin predators; and a resulting population explosion among small purple urchins.

The bull kelp that defined the North Coast’s near-shore habitat already was under stress due to the “Warm Blob” — a vast and persistent area of unusually warm water that spread down the Pacific Coast from Alaska in 2014 — when ravenous purple urchins, their growth unchecked, mowed through the kelp forest. By 2016, the kelp canopy had been reduced by more than 90 percent, compared with 2008 surveys.

The result is what’s known as an “urchin barren,” a rocky floor dominated by purple spiky creatures and little else.

Even the large red urchins that Holcomb for decades targeted for their “uni,” a delicacy used in sushi, have largely disappeared, out-competed by purple urchins too small to harvest commercially. Holcomb’s small specialty fishery is no longer viable, so he has been working instead to perfect a vacuum-like “airlift” device that sucks up the purple urchins as a pilot project under authorization from state Fish and Wildlife officials.

It’s painstaking work that requires hours under water, as he and a partner scrape urchins from the rock and direct them to the vacuum tube.

But he’s been able to work on a limited, grant basis since New Year’s, most recently thanks to the Watermen’s Alliance, which is funding his efforts to clear Caspar Cove of purple urchins.

Including about $45,000 in grants from local fish and wildlife commissions in Mendocino and Sonoma counties, the alliance and its partner, the Sonoma County Abalone Network, have pooled more than $100,000 in funding for urchin removal on the two coasts.

One big obstacle to expanding the effort: Little demand exists for the purple urchins as a food source. Most are smaller than 2 inches in diameter and starving themselves.

State law, meanwhile, prohibits “wanton waste” of game fish, so any harvested urchins have to be diverted to some productive use, even if it’s not food, state officials said.

The calcium-based shells can be used as soil amendment, for example, said Cynthia Catton, an environmental scientist with Fish and Wildlife, stationed at the UC Davis Bodega Marine Lab.

Holcomb has experimented with making light strings using bleached urchin shells.

Still, while the sport divers may have no particular use for the purple urchins they plan to collect, they must be harvested lawfully and in a manner that causes as little disturbance in the ocean as is possible. Those curbs have led some critics to urge a more aggressive approach to urchin removal, some citing smaller-scale examples in Southern California where crushing large volumes of the urchins has helped.

But Catton, who has led state efforts to monitor and understand recent changes in the North Coast’s kelp forest, has held her ground, saying lab experiments demonstrate that crushing the urchins can release gametes and fuel population growth.

“These are large-scale disturbances, and if we’re smashing them, not only are we disturbing the area, but we’re also inputting potentially a large amount of reproductive material and food,” she said. “Those are things that we won’t see the consequences of for a few years.”

You can reach Staff Writer Mary Callahan at 707-521-5249 or mary.callahan@pressdemocrat.com. On Twitter @MaryCallahanB.

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