On January 15, 2019, a gray whale washed up dead on a beach in southern California. The discovery of a whale carcass was rare but not unprecedented; about one dead gray whale shows up every year in January, on average, somewhere along the Californian coast. But in February another three dead gray whales washed up, and in March, twelve. April was even more morbid, when beachgoers found 21 carcasses in varying states of decay. But that record only lasted until May, when 27 whales stranded along the west coast of the United States.

Wildlife scientists began to fear that, rather than a string of isolated incidents, what they were witnessing was the start of something more substantial. The following May, NOAA officials declared the strandings to be an unusual mortality event, or UME—the official term for when the scale of a die-off rises significantly above background mortality rates.

By year’s end, 122 dead gray whales had washed up on the coast of the United States, and more than 200 along the whole North American coast. The following year, another 170 gray whales beached on U.S. shorelines. What made matters worse was that those numbers, already several times higher than the average annual total, were undoubtedly an undercount. Who knew how many gray whales had died and sunk from view?

It was in part with that uncertainty in mind that biologists tried to gain an understanding of the UME’s severity and reach. Last January, NOAA released a report estimating that the gray whale population had declined 24% as a consequence of the UME, from nearly 27,000 gray whales down to about 20,500. “In absolute terms, a decline of that scale sounds shocking,” says Jonathan Stewart, a biologist at NOAA and lead author of the report. And it has led beachgoers, wildlife advocates, and government scientists alike to ask: Why have so many gray whales been dying, what might it all mean?

Gray whales once ranged widely across northern hemisphere, in both the Atlantic and the Pacific oceans. Among the different populations, the one in the eastern Pacific is well known for the annual migration its members make, the whales swimming thousands of miles from their winter breeding grounds in Mexico north to summer feeding grounds in Arctic waters, and then back again.

Along this whale road, some grays take a quick detour into the Salish Sea region. Their presence was first noted in the early 1990s, when scientists saw that a few gray whales had pulled off on their way north, angling east for Puget Sound. These days, about a dozen individuals or so make up the so-called Sounders group. Seventy percent of those that scientists have managed to sex are males, and as a group they spend most of their time around the southern tip of Whidbey Island, as well as in the interior waters of Possession Sound and Port Susan. There they snuffle through the subtidal muds, sucking up burrowing ghost shrimps. So large are the feeding pits their scourings leave behind that they show up in satellite images.  

Although no one can say precisely how many gray whales there used to be, scientists are more certain when gray whales started to disappear: in the 19th Century, when humans hunted them nearly to extinction. So low did gray whale numbers plunge that that the species was one of the first the International Whaling Commission (IWC) protected in 1949; in the United States, the species received federal protection in 1970 under the law that preceded the Endangered Species Act (ESA). (Gray whales were also on the first endangered species list after the ESA was enacted in 1973.) Conservation measures worked wonders. Although subsistence hunts still take place in the Russian and North American Arctic, once gray whales no longer faced the threat of commercial harvest, the population in the eastern Pacific rebounded quickly. Federal managers removed the species from the endangered species list in 1994, and by the late 1990s estimated the population had more than 25,000 whales. (Both the western Pacific and Atlantic populations are much less numerous and therefore still endangered.)

The eastern Pacific gray whale’s recovery is one of the great marine conservation success stories of the 20th Century. So plentiful were gray whales along the coast of North America that biologists even began to wonder if the population was near its carrying capacity. One reason they wondered this was that gray whales had begun to die at elevated rates. In addition to an increase in the number of stranded whales in the early 1990s, over 600 whales stranded in 1999 and 2000. During this first UME, most carcasses washed up in Mexico during the winter, but higher numbers also beached in California, Washington, and Alaska. Adding to biologists’ concern was that most of the dead whales were adults and juveniles rather than calves, which tend to die more regularly. Many of the dead whales were emaciated, too, pointing to starvation as the cause. Biologists noted, however, that what had caused them to starve was unknown. Was it disease? Chemical contamination? A lack of food? No one could say for certain what ultimately had killed all those gray whales.  

Such has so far been the case during the current gray whale UME. As before, many of the dead whales—again adults and juveniles—are emaciated. As before, it is therefore tempting to point to diminished food availability as the culprit, perhaps due to climate change. But also as before, knowing that the dead whales are skinny is not the same as knowing why they are skinny. The circumstances are complicated. Rather than having a simple, linear explanation, maybe multiple factors are at work synergistically. “I don’t think climate change is good for them, per se, but I also don’t think we’re going to see such a direct a-to-b link,” Stewart says.

Stewart points out that grays whales live on the edge, so to speak, swimming several thousand miles every year between their calving grounds in Mexico and their prime feeding grounds in the Arctic. They have behaved this way for thousands of years, even as the world shifts around them. One recent study found that gray whale calf production increased in years when there was reduced sea ice.

“Gray whales have lived through ice ages and periods of pretty intense warming, they’ve seen humongous changes to their Arctic migratory corridors and coastal feeding grounds,” Stewart says. “They have shown themselves to be pretty adaptable.” One example of that capacity for adaptation, he notes, is the Sounders group. As the years passed, more gray whales joined those early pioneers, especially following the 1999 – 2000 UME. Perhaps after the present UME, more will join them.  

“Big declines like this are distressing, and people worry a lot about them,” Stewart says. But when similar things have happened in the recent past, they haven’t been a death knell for gray whales. After the 1999 – 2000 UME, the eastern Pacific population quickly rebounded, with higher-than-average calving rates. At the same time, there are still more than 20,000 gray whales, and but for a few blips—albeit significant ones—their overall trajectory has been upwards. What might be happening is that gray whales live close to some carrying capacity that isn’t a fixed number but varies from year to year. So even as he watches with concern, Stewart isn’t ready to clang the alarm bells quite yet, even in the face of so many dead whales. “I don’t think gray whales are going anywhere anytime soon,” he says.