7,000 Humpback Whales May Have Starved to Death During the ‘Blob’ Heatwave

Between 2012 and 2021, around 7,000 humpback whales died in the North Pacific Ocean, likely from starvation caused by a marine heatwave.

Researchers came to this sobering conclusion after reviewing observations from a massive citizen science database. They traced the species’ population trends over the last two decades in a new paper published Wednesday in the journal Royal Society Open Science.

Once hunted to the brink of extinction, humpback whales have made a major comeback—aided in large part by a moratorium on commercial whaling that the International Whaling Commission implemented in 1986. (The moratorium remains in place today.)

The team thought that, buoyed by the whaling ban, humpback whales’ numbers would have increased in the last two decades, with their growth eventually leveling off. But they were surprised to find the recent decline, instead.

During the first half of the study period, from 2002 to 2012, the number of humpback whales living in the North Pacific Ocean doubled from 16,875 to 33,488. But between 2012 and 2021, the population dropped by 20 percent, leaving 26,662 humpbacks alive.

The numbers came from crowdsourced records on a platform called Happywhale, which encourages users around the world to submit photos they’ve taken of whale tails, also known as flukes. Using artificial intelligence, the platform can then identify individual whales based on the unique markings on their tails. To date, users have submitted more than 782,000 photos from 285,000 encounters, which has led to the identification of more than 109,000 individual whales worldwide.

This massive dataset is a treasure trove for researchers, including the team behind the new paper. But the sharp decline it revealed left them with a major question: Why?

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Scientists suspect the unprecedented marine heatwave of 2013 to 2016—the largest ever recorded around the world—caused the whales to starve to death. During this period, North Pacific Ocean temperatures were 4 to 10 degrees Fahrenheit higher than normal, depending on the location and date.

This heatwave, known as “the blob,” caused ripple effects up and down the food chain—including the proliferation of less-nutritious zooplankton, which led to smaller fish. Those fish, in turn, provided fewer calories for humpback whales, seabirds and other marine creatures that depend on them for energy. A million Pacific seabirds also died because of the marine heatwave, past research has found.

Under these conditions, whales become thinner, more vulnerable to disease and less likely to reproduce. In addition to causing a whale die-off, the heatwave likely impacted pregnancy rates among the North Pacific humpbacks, Ari Friedlaender, an ecologist at the University of California Santa Cruz who was not involved with the study, tells the Guardian’s Brianna Randall.

Other factors may also have contributed to the humpback whales’ demise, including collisions with ships and entanglement in fishing gear. But those two causes alone cannot account for the deaths of so many whales in such a short period of time, according to the paper. As such, the marine heatwave was likely the main culprit.

“It was definitely an unusual mortality event,” says study lead author Ted Cheeseman, a conservation biologist at Australia’s Southern Cross University and the co-founder of Happywhale, to the Guardian. “Humpback whales are flexible and willing to switch from krill to herring or anchovies to salmon fry. But when the whole ecosystem decreases in productivity, it hurts them big time.”

Researchers are concerned about the long-term effects of the loss of 7,000 individuals on the humpback whale population. But, more broadly, they fear that marine heatwaves will become more common because of human-caused global warming, leading to additional die-offs and other problems in the future.

For instance, an estimated ten billion snow crabs died because of marine heatwaves in the eastern Bering Sea off the coast of Alaska in 2018 and 2019. That mass die-off forced the state to cancel its snow crab season in both 2022 and 2023, which affected local fishers and communities that rely on the crustaceans for income.

“This is unlikely to be a one-time event, and if we do not rapidly curb the causes of climate change globally, more marine heatwaves decreasing ocean productivity worldwide will be our future,” Cheeseman tells New Scientist’s Michael Le Page.

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Sarah Kuta is a writer and editor based in Longmont, Colorado. She covers history, science, travel, food and beverage, sustainability, economics and other topics.