The kelp forest that formed a leafy canopy along the Northern California coast just eight years ago has almost completely disappeared, and scientists studying kelp and the species that depend on it are concerned about the inability to bounce back.
A new study by UC Santa Cruz found that the kelp forest off the Sonoma and Mendocino coasts has declined by an average of 95% since 2013. The satellite images were analyzed until 1985 to examine how a series of factors led to the sudden kelp bush. deterioration, including an explosion in the population of purple sea urchins eating it, and two marine heat waves. The research shows that the unprecedented destruction is related to unusual ocean warming and that the kelp forest is unlikely to recover any time soon, in part because the removal of the hedgehogs is so difficult.
“They can actually survive under famine conditions,” said Meredith McPherson, a graduate student at the University of Santa Cruz’s Oceanic Sciences and co-author of the study. “The impact was that there is actually no kelp forest left.”
Bulkelp (Nereocystis luetkeana) usually thrive in the rocky coastal areas of the provinces of Sonoma and Mendocino and create a habitat for many species of fish and invertebrates, including abalone, sea urchins, jellyfish and sea snails. Its disappearance has also had an impact on local tourism and other businesses – the abalone fishery that has been closed to recreational divers since 2018, and the commercial fishing of the red-sea hedgehogs in Mendocino County has been almost completely closed.
kelp0305_GR
The two hot water events that caused the decline of the kelp forest include an El Niño and what is known as’ hot water ‘stain’ that lasted together from 2014 to 2016. Around the same time, a wasteful disease affected the sunflower starfish population hit the purple hedgehog without predator.
The hedgehogs quickly took over and ate the remaining kelp and starved two other species that were popular among divers and sushi lovers – red abalone and red hedgehogs (purple hedgehogs are not as commercially viable). What remains are called urchin barren, rocky areas completely covered with the prickly purple invertebrates over hundreds of miles of the North Coast.
“What we see in Northern California is quite unprecedented in scope,” said Tristin McHugh, director of the California Oceans Conservation Program’s kel project, which is developing pilot methods to remove purple sea urchins and kelp to repair.
Although a hot water block formed in Alaska again last year, water temperatures in the North Coast have returned to normal, McPherson said, and yet the kelp has not recovered. Unlike other kelp that originate in California, it is an annual species that dies in winter, when washed up on the beach in heaps that look like long, green-brown snakes with spherical tips. It usually returns every spring.
Scientists have been monitoring the decline of the kelp forest for years with aerial photography and tidal data, but the new study was the first to use satellite images to analyze the changes in growth closer to the ocean’s temperature and nutritional level.
“We are able to see kelp relatively easily using satellite,” McPherson said.
The growth of bull kelp depends on cold conditions in the spring that bring nutrients to the surface, and it is reduced as the water temperatures rise. Although previous El Niño events – a natural pattern that causes water temperatures in the Pacific Ocean to rise for a year or two – have also caused the kelp to decline, it has usually recovered.
What was different to 2014-’16 was the purple explosion of the hedgehogs and the addition of the hot water block, which at the height of the ocean raised the temperature by almost 7 degrees above average. Although there is evidence that the stain is linked to climate change, it is more important that marine heat waves become more and more intense as global temperatures rise as a result of climate change, McPherson said.
Climate change, pollution and other factors are also blamed for the global decline of kelp forests over the past 20 years, but in California it is at its worst on the coast north of San Francisco. Monterey Bay and other parts of Central California also lost kelp forest to hedgehogs, but the areas have a mixture of bull kelp and giant kelp and also have sea otters that prey on the hedgehogs. In Southern California, where giant kelp is the main species, the forest has continued to grow better.
The most important part of the deterioration of the Sonoma-Mendocino kelp forest is that the purple sea hedgehog shows no sign of budding plants unless the sunflower starfish or another predator returns.
“Usually, some kind of physical disorder or disease would wipe out the population,” McPherson said. “It can go on for decades and decades.”
The loss of their habitat to purple sea urchins is why the California Department of Fisheries and Wildlife in 2018 put a five-year break on red pearl fishing, bringing recreational divers from across California to the North Coast. The last dive shop in the area consequently closed in Fort Bragg about a year ago.
In the state-sponsored program, run by the nonprofit Reef Check, many red-hedgehogs are hired out of work to remove purple hedgehogs from the seabed by hand. It is a slow process. In some cases, there are as many as 20 to 30 hedgehogs per square meter of seabed, and similar efforts in Norway, Japan and New Zealand have shown that it takes up to 2 per square meter to restore the kelp forest, McHugh said.
She added that the private sector should get involved if there is to be a long-term solution. It’s starting to happen. The Norwegian company Urchinomics signed a partnership with Bodega Bay property owners last year to build a ‘farm’, where purple hippos harvested off the coast are fattened to sell to restaurants and shops.
The Nature Conservancy is also working on pilot programs to determine if traps can be used to catch hedgehogs, and is building an experimental kel farm in Humboldt Bay this spring, McHugh said.
McPherson said there are other efforts that give the situation hope, such as developing railroads so that kelp can eventually be replanted if conditions are right.
“It’s a little gloomy for the North Coast,” she said. “But there is a lot of work in the area to see how we can maintain patches of kelp for recovery in the future.”
Tara Duggan is a staff writer for the San Francisco Chronicle. Email: [email protected] Twitter: @taraduggan