SAN FRANCISCO – At least 70 dead or dying gray whales have stranded on the West Coast this year. The number is so high the National Oceanic and Atmospheric Administration declared it an “unusual mortality event” Friday and launched a scientific investigation into why so many of the giant mammals are dying as they swim toward their summer feeding grounds in Alaska.

Death rates this high have been seen only once before. In 2000, the year with the highest number of strandings on record, 131 gray whales were found on U.S. shores: 61 in California, two in Oregon, 23 in Washington state and 45 in Alaska.

In an average year, there are about 35 strandings in an entire season. Last year, there were 45 strandings overall.

Thousands of whales are dying:Scientists have run out of public beaches for the carcasses to rot

If the strandings continue at their current pace, this could be the deadliest year for gray whales on record. The whales are halfway through their annual journey from the warm waters of Mexico to their Arctic feeding grounds, so more will probably die, the researchers said.

“For Washington state, we are on track to surpass pretty dramatically the deaths in 2000. We were at 23 in 2000, and we’re already at 26. In 2000, at this point in the year, we only had 14 deaths,” said John Calambokidis, a research biologist with the nonprofit Cascadia Research in Olympia, Washington, who studies whale populations on the West Coast.

The cause of death appears to be starvation, though that's under investigation.

Many of the stranded whales in 2019 have been malnourished, said David Weller, a research wildlife biologist with NOAA’s Southwest Fisheries Science Center in La Jolla, California.

Gray whales make one of the Earth's most epic migrations each year. Filter feeders, they spend late spring through fall in the Arctic, eating and building up a thick layer of blubber. Their food of choice is small sea animals called amphipods. The whales scoop up sediment from the seafloor and filter out the tiny shrimp-like creatures.

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 their journey and their time in the south, they eat almost nothing, living off the fat they stored up. Because so many of the dead whales are found emaciated, one theory is that the whales didn't get enough food last spring and summer.

That could be because of very anomalous warm temperatures in the Northern Bering Sea last year, said Sue Moore, a biological oceanographer at the University of Washington in Seattle.

The ice in the area is melting earlier, which produces less of the type of algae that the amphipods eat. That, in turn, could affect how many of them there are for the whales to eat, she said.

Gray whales are very flexible in the diet they can eat, so they are willing to forage for other food if their preferred amphipods aren’t available in sufficient numbers.

However “there are a lot of complicated factors in how the ecosystem is changing,” Moore said.

Though the indication is that the warming waters resulted in less food for the animals, the researchers are investigating all options. That could include something in the environment or disease or something humans are doing, Moore said.

"We're still in the beginning stages of the investigation," she said. "It could take months to a year to finally identify the cause."

Could be as many as 700 dead

The number of dead whales is almost certainly much higher than the 70 that have washed ashore. Not all whales that wash ashore are found, and most whales, especially emaciated whales, tend to sink when dead.

Those 70 could represent as few as 10% of the actual gray whales that have died, Calambokidis said.

The population of gray whales overall has been quite healthy in recent years. In 2016, the count was about 27,000 individuals, Weller said. As many as 1,000 calves are born each year. This year, the number born is lower.

When pregnant females don’t have sufficient access to the Arctic feeding areas, "there seems to be a lag in calf production in that year or the year after,” Weller said.

This season, more of the giant mammals, which can reach 50 feet and 40 tons, are being injured by ships. The researchers said that might be because they venture into bays and harbors in search of food as they make their way north.

"All of that brings the animals into greater contact with shipping traffic and fishing activities," Calambokidis said.

The public can play an important role in helping the effort to find out what's harming the whales by immediately contacting officials if they see a dead or distressed whale.

“Don't approach any animals in distress, as it can hurt the animals or you. And it’s also illegal,” said Deborah Fauquier, a veterinary medical officer with NOAA’s Office of Protected Resources.

For those who want to aid the research effort, NOAA established an Unusual Mortality Event contingency fund where money is collected to support the scientists' work.