Clues Point to Climate Change as a Culprit in Gray Whale Deaths
Clues Point to Climate Change as a Culprit in Gray Whale Deaths

Clues Point to Climate Change as a Culprit in Gray Whale Deaths

For the past two years, the charismatic marine mammals have washed up on Pacific shores in record numbers. Scientists investigating the strandings suspect warming waters and melting sea ice are partly to blame.

Ashley Yeager
Ashley Yeager
Nov 1, 2020


Bluish-tinged tongue lolling out, the gray whale lay in near silence on the sand of Long Beach, Washington. The rhythmic whap, whap, whap of small waves lapping its belly punctuated the silence. An oily, rotten odor surrounded the behemoth, which tipped the scales at 2,200 kilograms, the equivalent of four heavy-duty SUVs. It was April of 2019, and this whale was one of 34 that Jessie Huggins, who coordinates responses to marine mammal strandings in Washington State, helped to photograph, dissect, and discard that year. Finding a few gray whales washed up on the coast each year is common, she says, but last year 215 of them washed up along the marine mammal’s migration route between Alaska and Baja. And so far this year, there have been more than 160 strandings. 

“Something’s clearly going on,” says Sue Moore, an ecologist at the University of Washington who has studied gray whales for decades. “You’ve got a big uptick in the number of animals that we’re seeing dead on the beach.”

Startled by the rapid rise in deaths, Moore and a team of scientists from the US, Mexico, and Canada have begun collating data from decades of past research on the species, Eschrichtius robustus. They’re also collecting and analyzing samples from stranded individuals, tracking booms and busts in the gray whale population, and studying changes in the animal’s feeding habitats, determined to identify what exactly is killing the marine giants.

In addition to possibly helping to save the whales, says biologist John Calambokidis, one of the founders of the nonprofit Cascadia Research Collective where Huggins works, understanding their deaths could be an “incredibly key indicator of . . . changing conditions in our whole global environment.”

Death Map

To investigate the deaths of gray whales, researchers have been tracking where they wash ashore along their migration route from Mexico to Alaska. The whales spend summers in the Arctic, where they dine on crustaceans called amphipods that live at the seafloor. Each whale can consume more than 2,000 pounds of food a day, amassing fat to fuel their yearly migration down to warmer waters off the Baja Peninsula, where they spend winters breeding and giving birth. During their 24,000-mile roundtrip journey, the whales eat almost nothing. Only occasionally along the way will they stop at snacking stations, skimming surface sea waters for squid, krill, crab larvae, and herring roe. Those snacks can often be enough for a whale to survive its yearlong fast, but may “not be enough for a pregnant female to bring a calf to term,” Moore says, or produce enough nutritious milk to support the growth of the baby’s blubber before making the trek back north. Sure enough, yearlings and adolescents accounted for most of the 2019 strandings.

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Prime suspects

A changing climate is one of a handful of leading factors that Moore, Calambokidis, and their collaborators are positing as causes of the gray whale deaths. Ship strikes, entanglement in fishing gear, killer whale attacks, and the fact that the gray whale population may have hit its carrying capacity—meaning it’s outstripped the available food supply—are the others. As if solving a classic murder mystery, the team is working to rule out these suspects one by one.

One of the first steps in the investigation is to examine the body of each gray whale that washes up on a beach. When researchers arrive at a stranding, they conduct a thorough external exam, measuring the girth of the individual, determining its sex, and looking for markings on the whale’s body that could point to possible causes of death. There may be fishing line or netting wrapped around the animal’s mouth, body, or fins, for example, or visible bruising that may be a sign that the whale was hit by a boat. Bruises that surround open gashes or rake marks in the flesh are clues that the animal was attacked by killer whales (Orcinus orca).

We’re still really struggling to try to understand what is going on with this population.

—Sue Moore, Univer­sity of Washington

Orcas are gray whales’ number one predator, often targeting their calves. Youngsters, and sometimes adults, have washed ashore along the grays’ 12,000-mile spring migration route from Baja to Alaska with signs of such attacks. Hunting the whales in packs, orcas will ram, harass, and bite the baby to separate it from its mother. Then, the orcas let their own young make the kill. In addition to telltale injuries on a gray whale’s body, death by killer whale is also evidenced by the absence of a gray whale’s tongue and jaw, which the predators typically devour, leaving the rest of it to rot. 

Analyzing the autopsy data of 50 mostly young whales that stranded on the Pacific beaches of the United States and Canada in 2019, Calambokidis and his colleagues found evidence that 19 of the animals were killed by orcas; an additional eight had rake marks indicating they’d been attacked. Recent acoustic recordings detected more orca vocalizations near the Bering Strait, where many grays go for the summer to feed, than recordings from previous decades did. These acoustic detections could mean there are more orcas in the gray whales’ feeding habitat, Moore explains. Or it could mean there are the same number of orcas but they have become more vocally active during the periods that the gray whales are in the Arctic.  

Recent aerial surveys did not show an uptick in orca numbers, however, and “there have always been killer whales up in that region,” explains Moore, who says there’s no strong evidence that orca numbers have gone up significantly. As a result, she and others reason, it’s unlikely that orcas are solely to blame for the recent spate of gray whale deaths.

More likely, according to Calambokidis, is that the whale population had hit its carrying capacity and there wasn’t enough food to go around. Whale counts and population modeling suggest the species has recently seen a huge surge in numbers, from an estimated 19,000 in 2007 to 27,000 in 2017. “That was pretty eye-popping,” Moore says. “That’s a big population of gray whales.” And now, the whales washing up on the beach seem to be exceptionally skinny, even taking into account that the animals have typically fasted for three to four months in the winter. Of the 53 whales necropsied along the west coast of North America in 2019, 38 were considered emaciated.

SLENDER PROFILE: When examining a stranded whale, researchers consider the depth of the dip behind the whales head. If the whale has a significant dip (top), it is considered skinny and in poor health, but if there is no dip (bottom), the whale is in good condition.

The findings are reminiscent of the last uptick in whale strandings, which started in 1999. That year, at least 273 gray whales washed up on the Pacific beaches of North America; the next year, there were 361 recorded strandings. Many of those dead whales were also emaciated, seemingly suffering from nutritional stress, or possibly starvation. As with the recent spike in deaths, researchers suspected the whale population had hit its carrying capacity, having recently rebounded from overhunting earlier in the century, when the animals’ numbers had dropped to as few as 1,000 from perhaps as large, at one point, as 100,000. With international protections that went into effect in the 1940s, along with the added security provided by the 1972 Marine Mammal Protection Act, the population rebounded so well that gray whales were removed from the endangered species list in 1994. Then came the exceptional number of dead whales washing ashore.

The similar pattern now unfolding, with an uptick in deaths following a population boom, fingers carrying capacity as a prime suspect. But, Calambokidis notes, it’s too soon to close the case on the recent gray whale deaths. That’s because the carrying capacity argument doesn’t address some key unknowns about the food supply, such as whether the whales’ diet is changing due to “the dramatic changes that are occurring in the Arctic environment,” he says.

Moore agrees that warming temperatures and melting sea ice in the Arctic have to be considered. “The reason I have a little problem with [the argument that] all these whales are starving because they’ve hit this hard carrying capacity,” she says, “is that gray whales are very adept at eating a lot of different things.” They particularly like to chow down on amphipods during the summer months in the Arctic, and also eat krill and other small crustaceans as well as herring roe while migrating along the North American coast.

Arctic warming as a possible killer

For more clues to what’s killing gray whales, researchers are slicing deeper into the animals that have washed ashore. If a beached whale has been dead for a day or less, scientists can cut into the animal’s skin and blubber to measure the thickness of the fatty layer and take samples to analyze its lipid levels. “How thick the blubber is and, more importantly . . . the level of oil and lipids in the blubber and some internal body cavities tell you about the health of the animal,” Calambokidis says. 

While the blubber thickness has been seemingly similar across the years, whales that have washed ashore recently have less oily blubber and more watery blubber compared with whales autopsied prior to 2019, he says. “We’ve had cases where the blubber layer is still thick, but we’ll cut, and we can see it’s dry and fibrousy.” He says it’s “like all the oil and material has been . . . sucked out of it.”

Recently stranded whales have less oily blubber and more watery blubber compared with whales autopsied prior to 2019.

In nine emaciated whales studied last year, the researchers also noticed that the animals’ blubber was discolored, appearing as a washed-out pink rather than the more rosy color of healthy blubber. An analysis of the discolored blubber under the microscope revealed a disproportionate increase in collagen fibers relative to fat cells, along with structural changes to some of the fat cells themselves. The results suggested that the whales weren’t getting the nutrition needed to maintain healthy blubber, leading some researchers to suspect that the marine mammals’ diet could be changing, becoming less nutritious. “Polar seas are warming twice as fast as in other places, and we know that that is changing aspects of the ecosystem,” says Moore. 

Gray whales dine on small, shrimp-like crustaceans called amphipods that they siphon off the bottom of the Bering Sea and the Chukchi Sea in the Arctic. Warming seas mean less sea ice, which in some ways could be seen as good for the whales because it opens up wider swaths of Arctic seafloor for a longer period to graze. But the growth of bottom-dwelling amphipods relies on the ice, and particularly on the algae that grow underneath it. When the sea ice melts at the beginning of summer, the algae sink to the seafloor, feeding amphipod populations. With warming, Moore explains, that sea ice pattern has been very different, with “the extreme surprise of winter sea ice really being absent in 2018 and 2019.” That means algae didn’t collect on the ice or sink to the seafloor, and that the amphipods didn’t get the carbon they needed to thrive. Without those algae, the crustaceans may fail to bulk up on lipids, the very type of molecule that’s depleted from the dead gray whales’ blubber.

How Climate Change Puts Whales At Risk

Over the past two years, hundreds of gray whales have washed ashore along North America’s west coast. Researchers are learning that the answer to the question of why these whales are dying is complex, but likely involves warming ocean waters and melting Arctic sea ice brought on by climate change. So far, the evidence is circumstantial, and scientists are still seeking more-definitive data about what’s killing the marine giants.


Malnourished whales are probably not as good at fending off orca attacks, toxins, and maladies that make them more vulnerable to death.

Without sea ice to collect algae, which would normally fall to the seafloor when the ice melts seasonally, shrimp-like crustaceans called amphipods
that use the algae as a source of carbon become less nutritious food for gray whales, affecting mammals’ ability to pack on blubber for the winter.

Having less ice opens more-northern territories for whales to feed. Reduced ice also leads to an increase in commercial shipping along the Northern Sea Route, raising chances for ships to collide with whales.

The whales may ingest toxins, such as the neurotoxin domoic acid, from the sediment as they filter feed, which could be damaging to animals that are unhealthy as a result of warming waters.

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The link between less-healthy amphipods and lower lipid levels in gray whales is circumstantial at this point, but whales might be emaciated because they are eating
amphipods that have less lipids, making them less calorie-dense than the algae-fattened amphipods that the whales are used to feasting on. The whales may also be eating other critters with lower calorie counts than the algae-fed amphipods. When Calambokidis, Huggins, and others examined the guts of whales that washed ashore in Washington State, they found remnants of wood chips, bark, eel grass, kelp, and certain crustaceans that gray whales don’t normally eat, suggesting the animals were so hungry they may have been foraging for suboptimal food. “Another line of research I’d like to see . . . is what is the relative caloric value of those . . . amphipods, which we know store a lot of lipid, versus something like a krill or a mysid, which is another type of invertebrate but not as lipid rich,” Moore says. Krill and mysid are other favorite foods of gray whales that might give them sustenance, but quite possibly not the fat they need to survive yearly fasts during their migrations.

“When you really start getting into the ecology, and the nutritional value of different types of prey that we know these animals eat, the story gets a little bit more complex than, ‘There’s a lot of whales [and] they’re all starving, because they’ve eaten their way through their food supply,’” Moore says. 

BEACHED: An adult female gray whale measuring more than 12 meters long washed up at California’s Point Reyes National Seashore in April 2019.

Calambokidis agrees that identifying a single cause of death for gray whales is unlikely. Any climate-driven changes in their diet might not kill the grays directly, but could make them more vulnerable to killer whale attacks, toxins, and ship strikes, for example. Indeed, compared to the 1999–2000 die-offs, there have been more recorded ship strikes and entanglements among stranded whales in the last two years. The whales are now feeding in different areas, including in shipping lanes near Seattle, San Francisco, and Los Angeles, and also in areas where there’s more fishing and crabbing gear, explains Frances Gulland, a marine mammal veterinarian at the University of California, Davis. 

The pattern of deaths is also different this time around, she notes. “It’s been very spotty, and there are dead whales, but it’s not the continuous elevated mortality that we saw in 2000.” Because of the lack of continuity in the deaths, Gulland agrees with the others that what’s killing gray whales is multifaceted, with climate change acting as an accomplice. “I think the mistake is to be looking for just one thing,” she says. 

 “We’re still really struggling to try to understand what is going on with this population,” Moore says. And determining what’s killing them is essential, not only for the whales, but also for surveying the health of the ocean. Gray whales “are great oceanographers,” she says. “They reflect back to us what’s going on in the water. They are our ocean sentinels.”