A climate-related catastrophe off the California coast has resulted in the death of 90% of the kelp from San Francisco to Oregon as an explosion of ravenous urchins devours everything in sight. And it’s happening at the same time native fish in San Francisco Bay are dying out, two studies released Monday documented.

The studies, by government, university and scientific institute researchers, offer a disturbing look at an underwater ecosystem suffering more than anyone previously suspected — along the coast, in San Francisco Bay and in the Sacramento-San Joaquin River Delta.

The “perfect storm” of ecosystem shifts triggered by climate change has caused so much havoc that human intervention will be needed to reverse things, say scientists with the University of California at Davis and the California Department of Fish and Wildlife.

“It’s very serious,” said Laura Rogers-Bennett, a Fish and Wildlife senior environmental scientist and a research associate with UC Davis’ Bodega Marine Laboratory who was lead author of the kelp study. “This is a huge environmental disaster underwater.”

The kelp study, published Monday in the journal Scientific Reports, chronicles a dramatic decline in the ocean ecosystem that started in 2013 when millions of sea stars along the coast of California withered and died. Without their main predator, the native kelp-eating purple sea urchin population skyrocketed to 60 times its historic numbers.

That was followed, from 2014 to 2017, by a marine heat wave and an El Niño weather event of unprecedented scale. The ocean heated as much as 6 degrees during that time, and toxic algae formed along the coast, killing many species.

The warm water began killing off the lush forests of bull kelp, and then ravenous urchins finished them off, leaving a barren seascape.

Only about 10% of the historic kelp population in a 217-mile-long swath along the entire North Coast of California into Oregon still exists, Rogers-Bennett said, and the problem is spreading. About 50% of the kelp south of San Francisco, including Monterey Bay, is also gone.

That, in turn, has caused the mass starvation of red abalone and other species dependent on kelp from Baja California to Alaska. The red abalone fishery was closed in 2018 because of the die-off.

“What we’re seeing now are millions and millions of purple sea urchins, and they’re eating absolutely everything,” Rogers-Bennett said. “They can eat through all the anemones, the sponge, all the kelp, the fleshy red algae. They’re even eating through calcified alga and sand.”

Things aren’t any better inland, where a new State of the Estuary Report released Monday documented the steady decline since the 1980s of virtually every native species of fish in San Francisco Bay.

The report, the first comprehensive look at the health of the bay and delta since 2015, acknowledges how extensive tidal marsh restoration work throughout the region has helped fish and other native species, but said much more needs to be done.

“Based on long-term monitoring data, native fish communities across the bay are declining,” particularly in San Francisco and San Pablo bays, said the report, put together by the San Francisco Estuary Institute in collaboration with numerous other scientists and agencies.

Letitia Grenier, a senior scientist and director of the institute’s resilient landscapes program, said fish in Suisun Marsh and the delta are also in terrible condition, largely because of a lack of ecosystem resilience caused by development.

“It’s all the big drivers — loss of freshwater flows from rivers, the loss of wetlands, problems with water quality and the huge amount of invasive species in the bay,” Grenier said.

The fish most at risk are chinook and coho salmon and delta smelt, which have suffered from loss of habitat, lack of cold water, polluted runoff, elimination of flood plain habitat and construction of dams that have cut off spawning grounds.

The institute’s findings bolster the results of a study released in August that documented the loss of nearly 750,000 acres of historic tidal wetlands along the bay, delta and nearly 450 other bays, lagoons, river deltas and coastal creek mouths throughout the West.

In the Sacramento-San Joaquin River Delta, 96.8% of the more than 380,000 acres of vegetated wetlands that once existed is gone. Only about 15% of the historic marshland habitat around the main part of San Francisco Bay still exists, according to the study.

But while most people, including delta farmers, conservationists, water district officials and even duck hunters, are focusing on the bay and delta, the ocean ecosystem is at a tipping point.

Bull kelp is a key indicator of ecosystem health because it is one of the fastest-growing plants on Earth, capable of growing 2 feet a day, Rogers-Bennett said. It thrives in cooler water temperatures, but the vast kelp forests that once existed failed to return even after the warm water period earlier in the decade ended.

The problem, she said, is that bull kelp is an annual plant — meaning it must regrow from seeds every spring and summer — and the ravenous purple sea urchins are eating even the spores, or seeds, that the kelp need to regenerate.

“We’re seeing nothing growing back as it should,” Rogers-Bennett said. “There is no kelp at the surface, no kelp underneath the water, and the bottom is barren rock with lots of sea urchins. This is a very large-scale problem and not just in one county. We’re seeing this in Northern California, Oregon, and it’s starting to spread in Central California and Southern California.”

As alarming as that is, the problem is getting very little attention, she said, largely because it is happening underwater and, for the most part, out of sight of politicians and regulators.

“If all of the forest from San Francisco to the Oregon border died back 90%, people would notice,” Rogers-Bennett said. “That’s exactly what’s happening underwater.”

The one positive is that purple urchins have fleshy gonads known to sushi lovers as uni. There is, however, no market for them right now, because although they are plentiful, they have devoured everything in sight and are also starving, which means their savory parts are no longer edible.

UC Davis researchers are working with the Bay Area shellfish company Urchinomics on ways to remove purple urchins from the sea floor and fatten them up for market.

Other strategies outlined in the Sonoma-Mendocino Bull Kelp Recovery Plan, released in June by the Greater Farallones Association and the Department of Fish and Wildlife, would include creating underwater kelp reserves, envisioned as watery oases where out-of-work abalone and urchin divers would be paid to remove invading urchins. The idea would be to preserve seed stock to repopulate bull kelp when conditions are right.

It’s important because numerous species rely on kelp, including sea otters, which use the underwater forests as cover from great white sharks. Marine biologists say the recovery of the California sea otter population from near extinction has stalled largely because of the loss of kelp and subsequent white shark predation.

“We really need to get a handle on it,” Rogers-Bennett said. “It is a climate-driven response that happened quite quickly, so it is taking a while for news to get out about the scale of the problem. We are going to need a lot of resources, people, funding, ideas, people coming together to change this climate disaster.”

Peter Fimrite is a San Francisco Chronicle staff writer. Email: pfimrite@sfchronicle.com Twitter: @pfimrite