Over the past four years, hundreds of gray whales have washed up dead on the beaches of Mexico, the U.S. and Canada, most of them skinny and malnourished – and scientists have been trying to figure out what’s causing the majestic animals to die en masse for so many years.

Now, researchers from Oregon State University have an answer.

The die-offs, according to a study published this week in the journal Science, follow changing conditions in the Arctic Ocean – namely, declines in sea ice cover that lead to a dearth of the crustaceans that gray whales love to eat.

And when the ice periodically returns, it restricts the animals’ access to feeding grounds, so the whales have even less to eat, the researchers found.

While Arctic conditions vary naturally year to year, climate change has magnified the changes in recent years, said the study’s lead author, Joshua Stewart, an ecologist and assistant professor with Oregon State’s Marine Mammal Institute.

“When the whales have both little food available and not much access to their feeding areas in the same year, that’s when we get these big impacts and die-offs,” he said. “And the reason this mortality event is lingering and is more severe than before is likely to be climate-related.”

UNUSUAL MORTALITY

Whales were once populous on Earth but humans hunted them to near extinction through commercial whaling. Today, despite conservation efforts, only a few whale species are on their way to recovery and most populations remain small – which makes them difficult to study and challenging to draw conclusions about their biology, ecology and population dynamics, Stewart said.

Gray whales are a notable exception, often held up as an icon of human conservation measures. While also hunted to very low numbers, they made a stunning recovery after gaining protection from the International Whaling Commission in 1947. They were removed from the Endangered Species Act in the mid-90s.

Part of why the gray whales recovered so spectacularly from near-extinction is because, unlike many other large whales, they spend most of their time migrating close to shore, Stewart said. Their migration path takes them 12,000 miles along the Pacific Coast, from Baja California, Mexico, to the Arctic – one of the longest mammalian migrations – a journey they make twice each year. Being close to shore, they’re less exposed to modern human-made threats that plague other whale populations, including getting tangled up in fishing gear or being hit by large ships.

But they’re vulnerable to human-caused climate change.

Shortly after gray whales were considered recovered, they suddenly started dying in large numbers, leading federal agencies in 1999 to declare an “Unusual Mortality Event.” Within a few years, the population recovered and scientists never found the root cause of the die-off.

In 2016, the population was estimated to be nearly 27,000 gray whales – one of the highest estimates since federal officials started counting in 1967.

Then, starting in 2019, gray whales again began dying en masse and scientists launched investigations into the deaths.

Over the past five years, 688 stranded gray whales have been recorded in Mexico, the U.S. and Canada, according to NOAA Fisheries, also known as the National Marine Fisheries Service. Scientists say that’s just a fraction of the total number of dead whales because most sink to the seafloor.

Scientists estimate the gray whale population has since declined by almost half, to about 14,500 whales as of last winter. The issue vaulted into local consciousness earlier this year when several whales – both gray and sperm whales – washed ashore in Oregon. The National Oceanic and Atmospheric Administration tracks whale beachings, including in Oregon and Southwest Washington, and the agency has counted 233 of them along those coasts since 1989.

That die-off continues, Stewart said, though it’s finally showing signs of waning.

It has lasted about twice as long as the previous die-off and has led to twice as large of a decline in the gray whale population, he said, a fact that’s likely attributed to climate change.

UNCERTAIN FUTURE

According to the new study, climate change in the Arctic has caused a chain reaction that’s hurting whales.

Warming temperatures have led to melting ice cover. That has meant less algae, which grows on the underside of sea ice and which feeds the benthic amphipods that whales eat. Less algae means fewer amphipods that whales need to survive.

And while less sea ice also potentially allows whales to access their feeding areas for longer, that’s only useful up to a point, Stewart said.

“It doesn’t matter how many days they have access to feeding areas if their food is in short supply,” he said.

Conversely, when the ice returns, the whales can’t access most of their feeding grounds – and what they can access has few amphipods to speak of.

The study’s findings, Stewart said, are significant not just for gray whales but also for other whale species that are starting to recover, including the humpback whale – which might see similar patterns of die-offs as its environments are regularly reshaped by warming temperatures.

Whales, in turn, could be a bellwether for other species.

“The study shows us that climate impacts are going to be affecting species of all shapes and sizes and distributions, even the biggest ones that can move long distances and live for a long time,” Stewart said. “They don’t get to escape these impacts just because they’re big and mobile.”

The future of the gray whale remains uncertain, Stewart said, largely depending on conditions in the Arctic, which are quickly deteriorating, with Arctic sea ice declining, ocean acidification making Arctic waters unlivable for many creatures and erosion degrading coastal habitats.

Still, he’s cautiously optimistic, partly because gray whales have survived through hundreds of thousands of years of dramatic environmental change, including ice ages and periods of global warming, he said.

“They’re resilient. They have the ability to adapt to changes in their environment and have been able to scrape by and recover, so I hope that they’ll be able to do that moving forward as well,” he said.

Even so, climate-caused changes mean the Arctic likely won’t be able to support as many gray whales as it has in the recent past, Stewart said.

And that’s “tragic,” he said, not just because whales are valuable for tourism, beneficial for coastal economies and play a significant role in capturing carbon from the atmosphere. (A whale can sequester 33 tons of carbon dioxide  on average, according to a report by the International Monetary Fund.)

“These are magnificent, huge, long-lived animals that are just extraordinary to have around,” he said. “It’s amazing to share the planet with them.”