SAN FRANCISCO — At least 53 dead or dying gray whales have washed up on West Coast beaches this spring, a death rate that’s only been seen once before. The great mammals are starving to death and scientists have theories as to why but so far no full explanation.

The number of deaths is likely much higher because it’s estimated that only 10% of dead whales actually end up on shore, said John Calambokidis, a research biologist with the non-profit Cascadia Research in Olympia, Washington, who studies whale populations on the West Coast.

That could mean as many as 530 whales have died, a large number for a population that is estimated to be just over 20,000 and that only began to rebound in recent decades after being hunted almost to extinction in the late 1800s.

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The strandings have occurred up and down the West Coast, on major public beaches and in sheltered coves. What they have in common is the heart-wrenching image of these giants of the sea dying as they try to reach their feeding grounds, but not making it.

Whales that wash ashore offer a window in the health of marine ecosystems, said Kyle Van Houtan, chief scientist at the Monterey Bay Aquarium in Monterey, California.

“They’re great indicators for what’s happening in the ocean and the animals are telling us what’s going on right now,” he said.

What they're saying is that something's wrong.

A months-long fast

Gray whales make one of the Earth's most epic migrations each year. Filter feeders, they spend late spring through fall off the Alaska coast, eating and building up a thick layer of blubber. They forage for their food of choice, small sea animals called amphipods, scooping up sediment from the sea floor and filtering out the tiny shrimp-like creatures.

Then, in October, they begin their 6,000-mile annual journey south toward sheltered warm water lagoons in Mexico’s Baja California Peninsula and the Gulf of California, where the females give birth. During the three to four months they are traveling or in the waters off Mexico, they eat almost nothing, living off the fat they’ve stored up, said Calambokidis.

But they may not have found enough food last year to make it through their fasting months. In recent years, whales observed making the journey north were "emaciated and thin,” he said.

“Recent autopsies are suggesting that malnourishment is likely to blame,” said Elliott Hazen, a research ecologist with NOAA's Southwest Fisheries Science Center in La Jolla, California.

Younger whales seem to be especially at risk, with dead juvenile whales turning up on beaches emaciated, said Justin Viezbicke, the California stranding network coordinator for the National Oceanic and Atmospheric Administration. He is based in Long Beach, California.

“In Southern California, we’re seeing strandings in juveniles between one and two years of age, when they’re about 30 feet long,” he said.

Births also appear to be down overall, with scientists seeing fewer calves overall this year. “That fits with the emaciated animals we’re seeing,” said Calambokidis.

Gray whales can live to be 70 years and can grow to be up to 50 feet long and 40 tons. They were known as “devil fish” by whalers because they aggressively fight to protect themselves and their calves when attacked.

From San Diego to Washington state

The last large die-off occurred in 1999 - 2000, when 62 dead whales washed up on West Coast beaches, said Viezbicke, who is in charge of whale strandings for NOAA along the West Coast.

This year’s event could be large or larger. “We’re closing in on that number and it’s still early in the migration," he said.

The whales are floating to shore from beaches as far south as San Diego and as far north as the Washington state border. A 41-foot female washed ashore in Ocean Beach in San Francisco the first week in May. Two others stranded just outside of Los Angeles, including one that came ashore in Malibu on March 24. Several have appeared in Oregon and two washed up in a park in the Washington state city of Everett earlier this month.

The 53 dead reflect strandings in California, Oregon and Washington. Whales dying in Mexico aren’t part of NOAA's numbers. Whales that die along British Columbia in Canada and along the Alaskan coast also aren't included. Two dead whales have already been sighted near Anchorage in recent weeks, one on Tuesday.

A stranded whale is both a sign of a problem in the ocean and also a difficulty for those on land. Thirty to forty tons of rotting whale makes a stench that can be smelled more than a mile away.

"It's the most wretched smell you've ever smelled in your life," said Amanda Tuttle, an Anchorage, Alaska, resident who's seen several stranded whales. Last winter, she saw one that washed up on the shore in Anchorage.

"I was instantly dry-heaving," she said.

Disposing of the bodies is a major undertaking. In remote areas they can be left to decompose, but in urban areas, "because of concerns around the smell and shark attractant, local authorities prefer to have them removed," Viezbicke said.

They can be buried where they are, which requires heavy earth moving equipment, or removed to landfill for disposal or towed out to sea. The last two can be very costly.

"And with towing, there's a risk that the carcass will come back ashore," he said.

Changing climate could be the culprit

Scientists don’t yet know why so many of the gray whales are starving, though there are several theories. A leading one is that the whales’ food supply in Alaska last year was diminished by weather patterns, either because of normal variations or related to longer-term climate change.

“We have been facing record low levels of sea ice and earlier melting, which does not bode well for the organisms that the gray whales feed upon,” said Hazen.

For the die-off that happened from 1999-2000, there was a strong El Niño event in 1997-1998. That cyclical weather pattern raises water temperatures in the areas of the Pacific including where the whales feed. Warmer waters tend to be less nutrient dense, which could affect how large the amphipod population is. There was a large El Niño event in 2015-2016 and 2018 showed positive El Niño conditions, both of which could have affected food levels.

Some of the whales are dying by becoming entangled in large fishing nets or being hit by boats and propellers. Though that’s probably because they’re looking for food and going to places they wouldn’t normally swim through in search of it, Viezbicke said.

It's especially an issue with young whales that don’t have the decades of knowledge about what’s safe and what’s not that their parents have.

“You’re a younger whale, you’re not doing as well as you’d like to be, you’re looking in new places to feed,” Viezbicke said.

Even with the recent spate of deaths, overall gray whales are doing “fantastic” compared to where they were before they were finally protected by international treaties in 1946, said Viezbicke. It’s believed that at one point the population may have been as low as 2,000, so today’s 20,000 to 23,000 represent an enormous win for conservation efforts.

That raises another possibility, that the gray whales have simply reached the biological limits of their usual environment and the younger and weaker animals are not surviving as they jockey for access to food.

“This could be part of the natural ebb and flow of the population. Only time will tell, we’re making educated guesses now,” Viezbicke said.

But Hazen is concerned that what's happening now is not part of normal weather variations but instead a new normal — a normal that is affecting the ocean’s ability to support large mammals like whales and sea lions.

“Warm and cold periods have always been part of the natural variability of the ocean, but the longer duration, the greater strength, and even the greater frequency of some of these low productivity events seem to have been worsened by human contributions to the changing atmosphere,” he said.