San Francisco Chronicle - (Sunday)
North Coast kelp forest almost gone
Heat helps sea urchins take over Sonoma, Mendocino
The kelp forest that only eight years ago formed a leafy ocean canopy along the Northern California coast has almost completely disappeared, and scientists who study kelp and the species that depend on it are worried about its inability to bounce back.
A new study from UC Santa Cruz found that the kelp forest on the Sonoma and Mendocino coast has declined by an average of 95% since 2013. It analyzed satellite imagery going back to 1985 to investigate how a series of factors led to the kelp forest’s abrupt decline, including an explosion in the population of purple sea urchin, which eats it, and two marine heat waves. The research shows the unprecedented destruction was related to unusual ocean warming and that the kelp forest likely won’t recover any time soon, partly because removing the urchins is so difficult.
“They can actually survive under starvation conditions,” said Meredith McPherson, a graduate student at UC Santa Cruz’s Ocean Sciences department and coauthor of the study. “The impact has been that basically there is no kelp forest at all left, really.”
Bull kelp (Nereocystis luet
keana) usually thrives in the rocky coastal zones of Sonoma
and Mendocino counties and creates a habitat for many types of fish and invertebrates, including abalone, sea urchin, jellyfish and sea snails. Its disappearance also has had impacts on local tourism and other businesses — the abalone fishery closed to recreational divers in 2018, and Mendocino County’s commercial red sea urchin fishery is almost completely shut down.
The two warm water events that helped cause the kelp forest’s decline include an El Niño and what was known as warm water “blob” that together lasted from 2014 to 2016. Around the same time, a wasting disease struck the sunflower sea star population, leaving the purple urchin without a predator.
Those urchins quickly took over, eating the remaining kelp and starving off two other species popular with divers and sushi lovers — red abalone and red urchin (purple urchin is not as commercially viable). What is left are called urchin barrens, rocky areas completely covered with the spiky purple invertebrates over hundreds of kilometers of the North Coast.
“What we’re seeing in Northern California is quite unprecedented in terms of its scale,” said Tristin McHugh, kelp project director at the Nature Conservancy’s California Oceans Program, which is developing pilot methods to remove purple sea urchins and restore kelp.
Though there was fear another warm water blob was forming last year off Alaska, water temperatures in the North Coast have returned to normal, McPherson said, and yet the bull kelp hasn’t recovered. Unlike other kelp native to California, it’s an annual species that dies off in winter, when it washes onto shore into piles that resemble long, greenbrown hoses with bulbous tips.
It normally returns each spring.
Scientists had already been monitoring the kelp forest’s decline for years with aerial photography and tidal data, but the new study was the first to use satellite imagery to more closely analyze changes in growth along with ocean temperature and nutrient levels.
“We’re able to see kelp relatively easily from space using satellite,” McPherson said.
Bull kelp growth depends on cold upwellings in spring that bring nutrients to the surface, and those are reduced when water temperatures rise. Though previous El Niño events — a natural pattern that causes Pacific water temperatures to go up for a year or two — have also caused the kelp to decline, it has usually recovered.
What was different about 201416 was the purple urchin explosion and the addition of the warm water blob, which at its peak raised ocean temperatures by almost 7 degrees above average. While there’s some evidence that the blob was linked to climate change, what is more important is that marine heat waves are becoming more frequent and intense as global temperatures rise because 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’s at its worst on the coast north of San Francisco. Monterey Bay and other parts of Central California have also lost kelp forest to urchin barrens, but those areas have a mix of bull kelp and giant kelp and also have sea otters that help prey on the urchins. In Southern California, where giant kelp is the main species, the forest has persisted better.
The most concerning part about the decline of the SonomaMendocino kelp forest is that unless the sunflower sea star or another predator returns, the purple sea urchin shows no sign of budging.
“Usually some kind of physical disturbance or disease would wipe out the population,” McPherson said. “It could go on for decades and decades.”
The loss of their habitat to purple sea urchin is the reason the California Department of Fish and Wildlife in 2018 put a fiveyear pause on recreational fishing of red abalone, which usually brings recreational divers from all over California to the North Coast. As a result, the last dive shop in the area closed in Fort Bragg about a year ago.
Many outofwork, professional red urchin divers have been hired in a statesponsored program managed by the nonprofit Reef Check to remove purple urchin from the sea floor manually. It’s a slow process. In some cases, there are as many as 20 to 30 urchin per square meter of sea floor, and similar efforts in Norway, Japan and New Zealand have shown it’s necessary to get down to 2 per square meter for the kelp forest to recover, McHugh said.
She added that the private sector has to get involved if there’s going to be a longterm solution. That’s starting to happen. The Norwegian company Urchinomics last year signed a partnership with Bodega Bay property owners to build a sea urchin “ranch,” where purple sea urchins harvested from the coast will be fattened up to sell to restaurants and stores.
The Nature Conservancy is also working on pilot programs to see if traps could be used to capture urchins and is in talks to build an experimental kelp farm in Humboldt Bay this spring, McHugh said.
McPherson said there are other efforts that give hope to the situation, such as developing spore banks so kelp can eventually be replanted when conditions are right.
“It is a bit gloomy for the North Coast,” she said. “But there’s a lot of work in the area to see how we can maintain patches of kelp for restoration in the future.”