On California’s north coast, marine biologists, divers and fishery managers are preparing for battle against an army of sea urchins – but it isn’t clear that they stand a fighting chance.
Lush forests of bull kelp grew here just several years ago, providing refuge for many fish and food for red abalone, a coveted sea snail. Today, purple sea urchins – and not much else – cover the rocky bottom over a seemingly endless area of seafloor. Ravenous grazers, the urchins proliferated about four years ago after a mysterious epidemic wiped out their main predator, the sunflower sea star. At about the same time, a temporary spike in sea surface temperatures – the result of the warm-water mass scientists called “the blob” – caused large groves of kelp to perish along the west coast. The urchins mowed down what remained, leaving a seafloor almost completely devoid of vegetation.
In normal balanced kelp ecosystems, purple urchins play a role in keeping algae biomass in check. But when it falls out of balance and they take over, as is happening on the north coast, biologists call these landscapes “urchin barrens.”
This transformation caused a cascade of devastating repercussions. Without kelp to eat, the red abalone population crashed, prompting the state to put an indefinite ban on recreational harvesting. A small fishery for red urchins – commercially valuable, unlike the smaller purples – also collapsed as the urchins’ prized golden gonads shriveled with the stress of starvation. Kelp forests also play an important role in marine ecosystems, soaking up carbon and offering valuable habitat to fish, invertebrates and marine mammals such as sea otters, which normally prey on urchins and keep their numbers down. The disappearance of kelp along other parts of California’s shores has restricted sea otters’ range and been linked to an increase in shark attacks, as sharks tend to avoid areas of kelp.
Using kayaks and boats, a giant vacuum cleaner and – in some cases – hammers, people are now working to cull the teeming masses of urchins and restore the kelp. “There are miles and miles that have been turned into urchin barrens in Sonoma and Mendocino,” said Dan Abbot, a volunteer diver with Reef Check, a program that monitors the health of seafloor ecosystems.
Similar kelp collapses in other parts of the world, however, show that these efforts face an uphill battle. The relationship between kelp forests and urchin barrens is a unique duality, with each environment type being considered an “alternative stable state” to the other. Once the ecosystem flips from a kelp forest to an urchin barren, it’s not easy to reverse. For example, urchin barrens in the Aleutian Islands and on a stretch of the coast in southwestern Hokkaido, Japan, have persisted for decades.
According to Craig Johnson, a biologist with the University of Tasmania in Australia who is studying his region’s urchins and kelp forests, urchin barrens may last decades or centuries in the absence of dramatic environmental disruptions, such as changing ocean temperatures or alterations in urchin-killing diseases and predators. He said human restoration activity can feasibly turn urchin barrens into kelp forests, but only at small scales. “At large spatial scales, we need other solutions,” he said in an email.
When these “flips” are linked to rising ocean temperatures and climate change, they may be especially difficult to turn around. Since warm seawater tends to contain fewer nutrients, kelp that thrive in colder conditions can die en masse when temperatures increase. This has happened on the east and west coasts of mainland Australia, Tasmania and northern Europe, as well as in California. High densities of urchins in such situations can effectively prevent kelp from recovering.
In California, the odds – and sheer numbers of urchins – seem stacked heavily against restoration efforts. Nobody has counted all the animals, but at densities of anywhere from 10–100 per square meter, the region’s purple urchins probably number several billion along the coast of Sonoma and Mendocino counties, ground zero for the phenomenon. Now many divers and some scientists have also reported that farther to the south, in Monterey County, purple urchin numbers are growing at an alarming rate while kelp beds shrink.
On that stretch of coast, a healthy population of otters, which prey on urchins, would normally play a key natural role in keeping kelp ecosystems in balance. So scientists have been surprised. “We had expected that the presence of sea otters in Monterey would keep this from happening, but that’s not what we’re seeing,” said Cynthia Catton, a California Department of Fish and Wildlife biologist working on the region’s urchin-culling programs.
Commercial sea urchin diver Jon Holcomb is helping coordinate one of several efforts with other divers and the state to help bring back the kelp. Holcomb has been using a large suction tube called an airlift that draws the purple urchins from the rocky bottom, upward to his boat. Onshore, his urchins are collected by a waste disposal company and turned into compost. Since January, he said he has collected about 120,000 urchins from a small cove beside the town of Caspar, in Mendocino County, and he hopes to deliver another 1.5 million by July. His goal is to remove purple urchins from small areas where some bull kelp still remains.
“We want to create a sort of seed bank in these places, so that when the environmental conditions become favorable again, maybe the kelp can start growing back,” said Sonke Mastrup, the California Department of Fish and Wildlife’s invertebrate fisheries program manager.
Recreational divers – most of them abalone fanatics – are contributing to the effort, with plans to clear Ocean Cove, in Sonoma County, starting in May, according to diver Josh Russo, president of the Watermen’s Alliance. Though some sport divers have been smashing urchins, Russo says the divers he is working with will be removing the animals from the water entirely. (Smashing urchins violates a state law that forbids the wanton waste of marine resources.)
State officials, while they won’t allow smashing, are backing Russo’s plan. According to Mastrup, fishery managers are strongly considering increasing the daily purple urchin bag limit for sport divers from 35 urchins to 20 gallons – which equals several hundred animals.
While they face serious challenges of scale, California’s urchin-fighting army may take hope in past interventions in other regions that have had some success in converting urchin barrens into kelp forest.
In Southern California, a coordinated effort to cull overpopulated urchins using hammers – for which state officials had granted a permit – has worked, though also at a very small scale. According to Julie Du Brow, communications director with the Bay Foundation, divers with hammers have reverted 18 hectares (44 acres) of a 60-hectare (152-acre) sprawl of urchin barren off the Palos Verdes Peninsula to lush kelp forest. Over four years, they’ve reduced urchin densities from as high as 100 per square meter to just two.
And after a long absence in the northern Gulf of Maine, kelp forests are expanding – in what seems to be the indirect result of intense commercial fishing for another species, the green sea urchin.
Doug Rasher, a marine biologist studying these ecosystem changes at the Bigelow Laboratory for Ocean Sciences in Maine, said an abundance of predatory fish – especially large cod – once kept urchin numbers in check in earlier times, allowing kelp forests to thrive. When cod became overfished, urchin numbers grew, gnawing down the region’s kelp forests. But in the 1980s, a sea urchin fishery developed and drove down urchin densities, as fishers had discovered the value of their reproductive organs, an Asian delicacy. The kelp has rebounded dramatically, he said.
“It feels as though the reefs have started to come alive again,” Rasher said.
In Maine, the kelp comeback happened by happenstance. In the Australian island state of Tasmania, where Johnson works, local scientists are directing a methodical effort to cull urchin numbers and protect what is left of diminished kelp and seaweed beds. The efforts have been only marginally successful. There, warming waters triggered a lasting die-off of giant kelp starting about 20 years ago and also allowed the subtropical long-spine urchin to spread southward. After establishing itself along the southeast Australian coast, where an absence of natural predators has allowed the urchin to dominate the ecosystem, it spread to Tasmania, and scientists are concerned that its presence will make the kelp collapse a permanent change.
To keep urchin barrens from growing in Tasmania, fishery managers are trying a strategic ecological intervention: increasing local populations of spiny lobsters, an important urchin predator that intense fishing pressure had reduced to low numbers. The effort has produced meager results, said Johnson, prompting a return of lost seaweed beds – but only in a few small and isolated places where small barrens were just beginning to appear. He is also currently working with a team of scientists on developing an underwater robot that would patrol the seabed, smashing urchins. However, he said knocking urchin numbers down sufficiently to allow seaweed to regrow in larger areas is essentially impossible.
It is this gloomy long-term fate that divers in California hope to avert. Holcomb is both optimistic and realistic about the future.
“If we eliminate the urchins that threaten the kelp, the kelp should regrow,” he said. But recovery of the entire ecosystem, he knows, will not come easily. The urchin barrens currently cover tens of thousands of acres of seafloor. “The conditions we’re seeing now – they will last for decades, for sure,” he said.