In 1972, a humpback whale nicknamed Festus was first spotted off the mountainous coast of Southeast Alaska. He returned every summer for 44 years, entertaining whale watchers, locals and biologists as he fed in the cold, nutrient-rich waters of the North Pacific before returning to Hawaii to breed in the winter.
But in June 2016, Festus was found floating dead in Glacier Bay National Park. The main cause of death was starvation, which scientists say was likely caused by the most extreme marine heat wave ever recorded. New research published Wednesday by Royal Society Open Science shows that the North Pacific humpback whale population declined by 20% between 2013 and 2021 after warmer waters upended the ecosystem.
“The [2014-2016] A marine heat wave has really reduced ocean productivity in a way that has seriously undermined humpback whale populations,” said Ted Cheeseman, a biologist at Southern Cross University in Lismore, Australia, who led the study.
Humpback whales, which can weigh as much as 40 tons and grow to 55 feet (17 meters) long, are known for their melodious underwater songs and showy breaching displays. But the animals are almost extinct due to centuries of hunting. By 1976, the number of humpback whales in the North Pacific had probably dwindled to 1,200 to 1,600 individuals.
After the International Whaling Commission banned commercial whaling in 1982, humpback whales made a remarkable recovery. The new study estimates a peak of nearly 33,500 humpback whales in the North Pacific in 2012, and an average population growth of 6% between 2002 and 2013. This 40-year upward population trend was so impressive that humpback whales were removed from the US. Endangered Species Act 2016.
That same year, however, an extreme marine heat wave continued to warm waters in the northeast Pacific Ocean. Maximum sea temperatures measured between 2014 and 2016 were 3-6 degrees Celsius above average. This left fewer nutrients for phytoplankton, the plants at the base of the marine food web. The effects spread throughout the ecosystem, leaving less food for everything from sardines to seabirds and sea lions.
The new research shows that about 7,000 humpback whales disappeared from the North Pacific between 2013 and 2021, a decline likely due to a lack of food. “It was definitely an unusual mortality event,” Cheeseman said. “Humpback whales are flexible and willing to switch from krill to herring or from anchovies to salmon. But if the productivity of the entire ecosystem decreases, it will be very painful.”
Persistent heat waves can cause whales and other marine animals to starve, as was the case with Festus. It can also lead to “skinny whales,” Cheeseman says. “Instead of looking nicely curved, the whales are awkwardly angular.” Skinny whales are more susceptible to disease, and skinny females are less likely to reproduce.
Research on Antarctic humpback whales has shown that warmer ocean conditions mean less food for whales, resulting in lower pregnancy rates. Ari Friedlaender, an ecologist in the Department of Ocean Sciences at the University of California, Santa Cruz, who led the Antarctic study and was not involved in the North Pacific research, believes the 2014-2016 marine heat wave is likely “ has had an impact on population pregnancy rates” and also “led to the demise of a certain number of animals” in the North Pacific.
Similar findings have come from long-term surveys of humpback whales in the Au’au Channel between Maui and Lanai. The number of mother calves in this Hawaiian channel fell by almost 77% between 2013 and 2018, indicating a rapid decline in humpback whale reproductive rates.
“If you lose the quality of the habitat, your carrying capacity decreases. It just can’t hold that many animals,” said Rachel Cartwright, a humpback whale researcher at the Keiki Kohola Project in Maui and co-author of the new study. “What we saw during the heat wave gave us a very good insight into how [humpbacks] will respond to future nutritional stress. There is no sign of us going back to the top.”
Festus, like all humpback whales, was easy to identify because its truck-sized tail fin showed unique black and white markings, much like a human fingerprint. To estimate the abundance of its species in the North Pacific over the past two decades, Cheeseman and colleagues used the largest individual photo identification database ever compiled for a whale species. The database is called Happywhale and consists of hundreds of thousands of images of humpback tailworms contributed by 46 research organizations and more than 4,000 citizen scientists from several countries.
Cheeseman founded Happywhale in 2015 to “create a living database” that provides abundant, accessible information to make it easier to answer important questions about the health of the ocean and its animals. He calls the online database “Facebook for Whales” in part because it uses similar image recognition algorithms. Powered by photos voluntarily uploaded by community contributors and hundreds of scientists around the world, Happywhale has a 97-99% accuracy rate for identifying humpback whales, and is also used to track more than a dozen other marine species .
Martin van Aswegen, a doctoral candidate at the University of Hawaii at Manoa, uses drones to study Hawaiian-born humpback whales. Over the past six years, Van Aswegen has calculated the length, width and body mass of more than 7,500 whales, tracking them from breeding grounds in Hawaii to their feeding grounds in southeastern Alaska. He uses the Happywhale database to identify the whales he measures.
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The lack of food sources during the marine heat wave “ultimately led to reproductive failure in 2018,” says Van Aswegen. Only three humpback whales made the trip from Hawaii to Alaska, and by the end of the feeding season, all three were gone.
During a shorter marine heatwave that ravaged the northeastern Pacific Ocean in 2021, Van Aswegen found that the 24 females with calves lost average weight during the feeding season, while these mothers would normally gain about 16 kg per day. “We have never seen lactating females actually lose weight at the feeding grounds,” says Van Aswegen.
Long-term monitoring efforts such as the drone-based humpback whale measurements and the collaborative collection of tailfin images through Happywhale “are absolutely critical because they allow us to look at the impacts of large-scale oceanographic events,” said Lars Bejder, the director of the Marine Mammal Research Program at the University of Hawaii at Manoa and co-author of the recent study. “These animals are true sentinels of the ocean. Healthy oceans make for healthy whales and vice versa.”
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