Did a marine heatwave cause 7,000 humpback whales to starve to death? | Whales

In 1972, a humpback whale nicknamed Festus was first spotted off the mountainous coast of south-east Alaska. He returned each summer for 44 years, entertaining whale watchers, local people and biologists as he fed in the cold, nutrient-rich waters of the North Pacific before returning to Hawaii to breed during the winters.

But in June 2016, Festus was found floating dead in Glacier Bay national park. The primary cause of death was starvation, which scientists believe was likely caused by the most extreme marine heatwave on record. New research, published on Wednesday by Royal Society Open Science shows the humpback population in the North Pacific declined by 20% between 2013 and 2021 after warmer water upended the ecosystem.

“The [2014-2016] marine heatwave really diminished the productivity of the ocean in a way that critically undermined humpback whale populations,” says Ted Cheeseman, a biologist at Southern Cross University in Lismore, Australia who led the study.

Humpbacks, which can weigh up to 40 tonnes and reach 17 metres in length, are well known for their melodic underwater songs and showy displays when breaching. But the animals nearly went extinct due to centuries of hunting. By 1976, humpbacks in the North Pacific had likely dwindled to 1,200 to 1,600 individuals.

After the 1982 International Whaling Commission banned commercial whaling, humpbacks made a remarkable recovery. The new study estimates a peak of nearly 33,500 humpbacks in the North Pacific in 2012, and an average population growth rate of 6% between 2002 and 2013. This 40-year upward trend in the population was so impressive that humpbacks were removed from the US Endangered Species Act in 2016.

That same year, however, an extreme marine heatwave was still warming waters in the north-east Pacific. Maximum sea temperatures recorded from 2014 to 2016 were 3-6C above average. This left fewer nutrients for phytoplankton, the plants at the base of the marine food web. The impacts cascaded across the ecosystem, leaving less food for everything from sardines to seabirds to sea lions.

The new study shows that about 7,000 humpbacks disappeared from the North Pacific between 2013 and 2021, a decline that was likely due to a lack of food. “It was definitely an unusual mortality event,” says Cheeseman. “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.”

Sustained heatwaves can cause whales and other marine animals to starve, as was the case with Festus. It can also lead to “skinny whales”, says Cheeseman. “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 humpbacks in Antarctica has shown that warmer ocean conditions mean less food for whales, which results in lower pregnancy rates. Ari Friedlaender, an ecologist in the Ocean Sciences Department at the University of California Santa Cruz who led the Antarctic research and was not affiliated with the North Pacific study, believes that the 2014-16 marine heatwave probably “impacted the pregnancy rates in the population” 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 humpbacks in the Au’au channel between Maui and Lanai. Mother-calf encounter rates in this Hawaiian channel fell by nearly 77% between 2013 and 2018, suggesting a rapid drop in humpbacks’ reproductive rate.

“If you lose the quality of the habitat, then your carrying capacity goes down. It just can’t support as many animals,” says Rachel Cartwright, a humpback whale researcher with the Keiki Kohola Project in Maui who was a co-author on the new study. “What we saw during the heatwave gave us some really good insight into how [humpbacks] are going to respond to future nutritional stress. There’s no sign of us going back to the peak.”

Festus, like all humpbacks, was easy to identify because his truck-sized tail fluke sported unique black-and-white markings, much like a human’s thumbprint. To estimate his species’ abundance over the past two decades in the North Pacific, Cheeseman and colleagues used the largest individual photo-identification database ever compiled for a whale species. Called Happywhale, the database is made up of hundreds of thousands of images of humpback tail flukes contributed by 46 research organisations and more than 4,000 citizen scientists hailing from several countries.

Cheeseman founded Happywhale in 2015 to “create a living database” providing 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. Fuelled by photos that are voluntarily uploaded by community contributors and hundreds of scientists worldwide, Happywhale has a 97-99% accuracy rate for identifying humpbacks, and is also used to track more than a dozen other marine species.

Martin van Aswegen, a PhD candidate at the University of Hawaii at Manoa, has been using drones to study Hawaii-born humpback whales. Over the past six years, Van Aswegen has calculated the length, width and body mass of more than 7,500 whales, following them from the breeding grounds in Hawaii to their feeding grounds in south-east Alaska. He uses the Happywhale database to identify the whales he measures.

The lack of food resources during the marine heatwave “ultimately resulted in reproductive failure in 2018”, says Van Aswegen. Only three humpback calves made it from Hawaii to Alaska, and by the end of the feeding season all three were missing.

During a shorter marine heatwave that blasted the north-east Pacific in 2021, Van Aswegen found that, on average, the 24 females with calves lost weight over the feeding season, when typically, these mothers would gain about 16kg a day. “We’ve never seen lactating females actually lose weight on the feeding grounds,” says Van Aswegen.

Long-term monitoring efforts such as the drone-based humpback measurements and the collaborative collection of tail fluke images through Happywhale “are absolutely critical because they allow us to look at the effects of large-scale oceanographic events”, says Lars Bejder, the director of the Marine Mammal Research Program at the University of Hawaii at Manoa and a co-author on the recent study. “These animals are truly sentinels of the ocean. Healthy oceans make healthy whales and vice versa.”

Find more age of extinction coverage here, and follow biodiversity reporters Phoebe Weston and Patrick Greenfield on X for all the latest news and features