Alarmed by the high number of gray whales that have been washing up dead on West Coast beaches this spring, the federal government on Friday declared the troubling trend a wildlife emergency.

The declaration by the National Oceanic and Atmospheric Administration — technically, the agency dubbed the deaths an “unusual mortality event” — kicks in a provision of federal law that provides funding for scientists to figure out the cause when such die-offs of marine mammals occur, from whales and dolphins in the Pacific or Atlantic to manatees off Florida.

So far this year, at least 70 gray whales have been found dead and stranded along the coasts of California, Oregon, Washington and Alaska — the most in nearly 20 years, scientists from NOAA said Friday. In recent weeks, whales have washed up in Marin, San Francisco and San Mateo counties.

On average about 35 of the giant marine mammals wash up dead on the West Coast in a year, or around three per month. Last year, 45 were found.

But the average number found dead for the first five months of the year on the West Coast is 15, so this year is seeing five times the average rate.

“There have been juveniles but adults as well. There have been males and females. It’s been all across the board at this point,” said Justin Viezbicke, NOAA’s California Stranding Coordinator.

The last time this many gray whales were found dead off California, Oregon, Washington and Alaska was in 2000, when 83 had washed up by May 31. That year, 131 died.

Scientists say they don’t yet know why the whales are dying, although many have been found malnourished, leading to theories that warmer ocean waters from climate change might be disrupting their food supply, particularly in the Arctic.

“We are seeing lots of live gray whales in unusual locations, clearly emaciated, trying to feed,” said John Calambokidis, a research biologist with the Cascadia Research Collective in Olympia, Washington.

Among those unusual locations: San Francisco Bay, Los Angeles Harbor and Puget Sound, he said. Closer to shore, whales are more at risk from dying by being hit by freighters, oil tankers and other large commercial ships.

Another theory is that the West Coast gray whale population — which has grown to more than 27,000 animals in recent decades after whale hunting ended in the early 1970s — has reached a carrying capacity of sorts, and this year’s ocean conditions have meant there is not enough shrimp, krill and small fish in some places for them to eat.

“We should be aware that in some years the environment is capable of supporting more whales than in other years,” said David Weller, a wildlife biologist with NOAA’s Southwest Fisheries Science Center in San Diego.

Scientists will continue to collect information about the dead whales, including their age, sex, tissue samples and other data, and will study it, along with records of ocean conditions, fisheries patterns and other factors as part of the detective work aiming to find the killer.

“It can take months to years to do the research,” said Deborah Fauquier, a veterinary medical officer with NOAA’s Office of Protected Resources in Silver Spring, Md. “There may be multiple causes.”

Gray whales are one of the West Coast’s iconic wildlife species. They can grow to 50 feet long, weigh 70,000 pounds and live 70 years.

Every year, thousands swim 10,000 miles between Baja California in Mexico and Northern Alaska. They draw crowds of people to state parks, beaches and whale-watching tours.

Gray whales typically head south in December, swimming down the West Coast to sheltered lagoons in Mexico where they breed and give birth. They begin swimming north up the California coast in mid-February toward feeding grounds in Alaska’s Bering and Chukchi seas and can be seen commonly from Big Sur to San Francisco during March and April because they stay closer to shore than any other species of whale.

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Watch: Orca whale pod frolics in kelp bed near Pebble Beach Once hunted by whaling ships in the 1800s for their oil until there were only about 1,000 or 2,000 individuals left, gray whales were protected in 1972 when President Nixon signed the Marine Mammal Protection Act. The last whaling company in the United States, the Del Monte Fishing Company, operated at Point Molate in Richmond. It made dog food out of whales that its crews shot with mechanized harpoon cannons. The company closed in December 1971 as the law was about to take effect.

After whaling was banned in the U.S., numbers of gray whales increased. By 1994, after they reached a healthy population, the Clinton administration removed them from the Endangered Species Act list in what is still considered one of the nation’s major wildlife success stories.

Scientists said because of the whales’ large population numbers, they should be able to bounce back and recover from this year’s die-off.

But the increase in deaths this year is sparking concern from marine biologists. That’s in part because the whales that wash up dead on the shore may represent only 10 percent of the whales that are dying in the open ocean off the West Coast.

“Most whales, and especially emaciated whales, will tend to sink when dead,” said Calambokidis. “So the numbers that wash up represent a fraction of the total. The vast majority go unreported.”