by In 1972, a humpback whale named Festus was first discovered off the mountainous coast of southeast Alaska. It returned every summer for 44 years, entertaining whale watchers, locals and biologists as it fed in the cold, nutrient-rich waters of the North Pacific, before returning to Hawaii to breeding during the winter.
But in June 2016, Festus was found dead floating in Glacier Bay national park. The main cause of death was starvation, which scientists believe was most likely caused by the largest marine heat wave on record. New research, published by the Royal Society Open Science on Wednesday, shows that the daffodil population in the North Pacific declined by 20% between 2013 and 2021 after warmer water dumped on the ecosystem.
“The IS [2014-2016] A marine heat wave actually reduced ocean productivity in a way that undermined the humpback whale population,” 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 meters in length, are famous for their melodious underwater songs and showy displays when breaching. But the animals almost became extinct due to hundreds of years of hunting. By 1976, there were probably 1,200 to 1,600 individuals in decline in North Pacific humpbacks.
After the International Whaling Commission banned commercial whaling, the humpbacks recovered significantly. 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 dramatic that humpbacks were extirpated from the US. The Endangered Species Act of 2016.
That same year, however, an extreme marine heat wave was still warming waters in the northeast Pacific Ocean. The maximum sea temperatures recorded from 2014 to 2016 were 3-6C above average. This left less nutrients for the phytoplankton, the plants at the bottom of the marine food web. The impacts rippled 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 goes down in productivity, it makes them lose a lot of time.”
Sustained heat waves can cause whales and other marine animals to starve, as was the case with Festus. It could also lead to “skinny whales”, says Fer Cáis. “Instead of looking nicely curved, the whales are awkwardly angled.” Thin whales are more susceptible to disease, and thin females are less likely to reproduce.
Research on humpback whales in Antarctica has shown that warmer ocean conditions mean less food for whales, leading to lower pregnancy rates. Ari Friedlaender, an ecologist in the Department of Ocean Sciences 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 heat wave “probably affected pregnancy rates in the population ” and also “as a result of the extinction of certain numbers of animals” in the North Pacific.
Similar results were obtained from long-term humpback surveys in the Au’au channel between Maui and Lanai. Mother-calf contact rates in this Hawaiian channel declined by nearly 77% between 2013 and 2018, suggesting a rapid decline in humpback reproduction.
“If you lose habitat quality, your carrying capacity is reduced. It can’t sustain as many animals,” says Rachel Cartwright, a humpback whale researcher with the Keiki Kohola Project in Maui who co-authored the new study. “What we saw during the heatwaves gave us a very good idea of how [humpbacks] going to respond to future nutritional stress. There is no sign that we are going back to the peak.”
Festus, like all humpbacks, was easy to identify because it had unique black-and-white markings on its truck tail, like a human thumb. To estimate the abundance of their species over the past two decades in the North Pacific, Cheeseman and his colleagues used the largest single photo-identification database ever compiled for a whale species. Called Happywhale, the database consists of hundreds of thousands of images of spiny tail whales contributed by 46 research organizations and more than 4,000 citizen scientists from different 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. With photos voluntarily uploaded by community participants and hundreds of scientists around the world, 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, is using drones to study humpback whales born in Hawaii. Over the past six years, Van Aswegen has been calculating the length, width and body mass of more than 7,500 whales, following them from their breeding grounds in Hawaii to their feeding grounds in southeast Alaska. It uses the Happywhale database to identify the whales it measures.
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The lack of food resources during the marine heat waves 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 heat wave that hit the northeast Pacific Ocean in 2021, Van Aswegen found that, on average, the 24 females with calves lost weight during the feeding season, when these mothers would gain about 16kg per day usually. “We have never seen females that are lactating weight on the nutritional grounds,” says Van Aswegen.
Long-term monitoring efforts such as the drone-based dorsal measurements and the collaborative collection of tail whale images through Happywhale are “absolutely critical because they allow us to observe 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 co-author of the recent study. “These animals are true inhabitants of the sea. Healthy oceans make healthy whales and vice versa.”
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