Fish populations off California’s coast have plummeted more than 70 percent in the last four decades, and scientists’ best guess is that climate change is to blame.

The precipitous decline has occurred ecosystemwide in the California current, a stretch of the Pacific that runs from the Pacific Northwest to Baja California, according to a recently published study. The decline can’t be pegged on a lone cause, such as overfishing or chemical pollutants.

There is one correlation: ever increasing ocean temperatures.

“The decline has been going on for four decades, and it hasn’t ever reversed, so it seems to us that it might be related to global climate warming,” said Tony Koslow, the lead author on the study and a researcher at Scripps Institution of Oceanography in La Jolla.

Natural variability might see fish populations fluctuate over a decade. But the decline this time around has been steady for half a century, leaving few options other than climate change.

Even as scientists examine evidence of decadeslong trends of declining fish populations, short-term threats have wracked marine ecosystems off California’s coast.

Last week, California officials warned people against eating Dungeness crabs because of a natural acid toxin that has contaminated them. An algae that produces the acid has grown exponentially because of warm ocean temperatures. The algae has made its way up the food chain and into the crabs.

Populations of certain commercially fished species also have suffered in recent years. The sardine fishery, for instance, was closed this year because estimated fish levels were below a certain threshold.

And for the past two years, increasing numbers of emaciated sea lions have been stranded on California’s shores. With warmer water, certain fish populations are lower and sea lions are seeing their food sources dwindle. Instead of sardines and anchovies, whose numbers are down, the sea lions are eating low-quality squid and rock fish.

“The food quality that’s available to the mothers, and they come back and then feed their pups, is now lower-quality food,” said Sam McClatchie, a National Oceanic and Atmospheric Administration scientist.

These changes largely have been attributed to a patch of warm ocean water lingering off California, called “the blob.”

But the new study, published last month in the journal Marine Ecology Progress Series, addressed trends that take decades to unfold, not months. And it revealed a decline that has more do with global climate change than any one particularly warm patch of ocean water.

The study combined two sets of data that hadn’t been analyzed before.

One was taken from cooling water intakes at coastal power plants that also suck in and kill some fish.

The other data came from one of the most extensive collections of fish larvae and ocean conditions in the country, compiled by the state’s California Cooperative Oceanic Fisheries Investigations program. For more than 60 years, scientists have measured ocean conditions off the Central and Southern California coast.

Four times a year, boats travel 500 kilometers offshore, taking measurements of temperature, nitrate levels, dissolved oxygen and other variables, from just below the surface to 210 meters deep.

The boats also collect water samples. Those are sent to the lab, where researchers count fish larvae under a microscope and sort them by species.

Counting fish larvae is a good stand-in for counting adult fish populations: if there are lots of larvae, scientists are confident there were a lot of adult fish reproducing to create those larvae. And sampling larvae in the top 200 meters captures most larvae-producing species, since that’s where they reproduce.

That fish numbers are declining isn’t surprising. What the new study shows is how evenly numbers are dropping for fish species across the spectrum, for the whole marine ecosystem, and not only for a handful of commercially fished species.

In the recent study, larvae populations in the open ocean had declined 72 percent since 1970, while fish levels at power plant intakes dropped 78 percent.

Such consistent declines call into question many of the usual suspects for fish decline, such as overfishing, chemical pollutants and power plant water intakes, according to the researchers.

“We’re seeing this consistent pattern across species,” said Eric Miller, an author on the paper and scientist at MBC Applied Environmental Sciences, an environmental consulting firm. “It’s beyond fishing, it’s beyond pollution, it’s beyond power plant ocean intakes. It’s something in the environment itself.

“You can’t look locally to try to understand all the problems and concerns we have in the ocean. It’s something that’s going on in a much bigger, regional scale,” he added.

Only ocean temperatures correlated with fish declines. The scientists found that cold-water fish are declining the fastest, Miller said. And while warm-water fish are replacing them, they’re doing so at a slower rate than the cold water fish are disappearing.

While chemical pollution isn’t benign, it couldn’t have done such widespread damage to the ecosystem, Miller added.

Chemicals like DDT have killed many marine and other creatures. For instance, the pesticide became famous and then was subsequently banned in the U.S. because it thinned bald eagles’ egg shells, causing that bird’s population to crash in the 1960s. But the current declines in fish population can’t be explained by one chemical – nor by the power plant water intakes, which would have a very local effect on the fish population.

Population declines have been observed far from the coast, where chemicals enter the water and fish get sucked into cooling systems.

“You can’t look at any single thing and say, ‘That’s the problem.’ There’s something much more pervasive,” he said.

While heavy fishing directly reduces numbers of certain fish species in the open ocean such as sardines and anchovies it probably isn’t causing the ecosystemwide declines.

Fish that live at middle depths have suffered – and they don’t wind up in fishermen’s nets either on purpose or accidentally.

Relying on fish counts from commercial fishermen’s nets can mask population declines for fish that form large schools, according to another study Miller published several years ago.

Such fishery data calculates population based on a formula that compares the number of fish caught to how much effort is expended catching those fish.

Even as fish populations decline, fishermen are skilled enough to find them easily and still catch large numbers of them. The large catches suggest the population is robust, even as fish numbers dwindle.

For that reason, Miller and his colleagues relied on fishery-independent data in the new study.

They included open-ocean data collected by the state’s California Cooperative Oceanic Fisheries Investigations program, which was created in the early 1950s to figure out why the sardine stocks collapsed.

The collapse of the sardine fishery, a major business in the 1930s, was documented in John Steinbeck’s 1945 novel “Cannery Row.”

Today, the oceanic data collection program is considered the gold standard for data collection and has been used for about 600 scientific studies examining fisheries.

It was instrumental in uncovering how El Niño increases the heat content of the ocean, lowers nutrient production and wrecks certain fisheries, such as squid.

Those decades of data reveal the Pacific Ocean isn’t the same place it was half a century ago, said John McGowan, a study author and researcher at Scripps.

“The California current is changing,” he said.

Contact the writer: aorlowski@ocregister.com. Twitter: @aaronorlowski