In 1972, a humpback whale nicknamed Festus was first spotted 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 breed during the winter months.
But in June 2016, Festus was found dead while swimming in Glacier Bay national park. The main cause of death was hunger; Scientists believe this was likely caused by the most extreme marine heatwave on record. New research published Wednesday by Royal Society Open Science shows that the humpback population in the North Pacific fell by 20% between 2013 and 2021 after warm waters disrupted the ecosystem.
“ [2014-2016] “The marine heatwave actually reduced the productivity of the ocean in a way that severely weakened humpback whale populations,” says Ted Cheeseman, a biologist at Southern Cross University in Lismore, Australia.
Humpback whales, which can weigh up to 40 tons and reach 17 meters in length, are known for their melodic underwater songs and flamboyant displays when breaching. However, due to centuries of hunting, the animals were almost extinct. By 1976 the number of humpback whales in the North Pacific had probably fallen to 1,200 to 1,600 individuals.
Humpback whales made a remarkable recovery after the International Whaling Commission banned commercial whaling in 1982. The new study estimates that the North Pacific will peak at about 33,500 humpback whales in 2012, with an average population growth rate of 6% between 2002 and 2013. This 40-year upward trend in population was so impressive that humpback whales were removed from the United States. Endangered Species Act of 2016.
But that same year, an extreme marine heat wave was still warming the waters in the Northeast Pacific. Maximum sea temperatures recorded from 2014 to 2016 were 3-6C above average. This left less food for phytoplankton, the plants at the base of the marine food web. The effects cascaded throughout the ecosystem, leaving less food for everything from sardines to seabirds to sea lions.
The new study shows nearly 7,000 humpback whales were lost in the North Pacific between 2013 and 2021; this decline was probably due to lack of food. “It was definitely an unusual death,” Cheeseman says. “Humpback whales are flexible and willing to switch from krill to herring or anchovies to salmon fry. But when the productivity of the entire ecosystem decreases, it takes a huge toll on them.”
Persistent heat waves can cause whales and other marine animals to starve to death, as happened in Festus. This can also lead to “weak whales,” Cheeseman says. “Instead of looking beautifully curved, the whales look oddly angular.” Skinny whales are more susceptible to diseases, and skinny females are less likely to reproduce.
Studies of humpback whales in Antarctica have shown that warmer ocean conditions mean less food for whales, leading to lower pregnancy rates. Ecologist Ari Friedlaender of the Department of Ocean Sciences at the University of California Santa Cruz, who led the Antarctic study and was not affiliated with the North Pacific study, believes the 2014-16 marine heat wave likely “impacted pregnancy rates in the population.” and also “caused the death of a certain number of animals” in the North Pacific.
Similar findings were obtained from long-term studies of humpback whales in the Au’au channel between Maui and Lanai. Mother-calf encounter rates in this Hawaiian tract dropped approximately 77% between 2013 and 2018; This suggests a rapid decline in the reproductive rate of humpback whales.
“If you lose the quality of the habitat, your carrying capacity decreases. “It can’t support that many animals,” says Rachel Cartwright, a humpback whale researcher at the Keiki Kohola Project on Maui and a co-author of the new study. “What we saw during the heatwave gave us a really good idea of how this happens. [humpbacks] They will respond to future nutritional stress. “There is no sign that we will return to the top.”
Like all humpback whales, Festus was easy to identify because the truck-sized tail fluke had unique black-and-white markings that looked just like a human’s fingerprint. Cheeseman and colleagues used the largest database of individual photo identifications ever compiled for a whale species to estimate the species’ abundance in the North Pacific over the past two decades. The database, called Happywhale, consists of hundreds of thousands of images of the humpback tail fluke, contributed by 46 research institutions and more than 4,000 citizen scientists from various countries.
Cheeseman founded Happywhale in 2015 with the goal of “creating a living database” that provides a wealth of accessible information to make it easier to answer important questions about the health of the oceans and their animals. He calls his online database “Facebook for whales” in part because it uses similar image recognition algorithms. Fueled 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 calculated the length, width and body mass of more than 7,500 whales and tracked them from their breeding grounds in Hawaii to their feeding grounds in southeastern Alaska. It uses the Happywhale database to identify the whales it measures.
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Van Aswegen says a lack of food resources during a marine heatwave “ultimately led to reproductive failure in 2018.” Only three humpback calves made it from Hawaii to Alaska, and all three were lost by the end of the feeding season.
Van Aswegen found that during a shorter marine heat wave that hit the northeastern Pacific in 2021, 24 females with calves lost weight, on average, during the feeding season, when these mothers gained about 16 pounds per day. “We’ve never seen lactating females actually lose weight in the nutritional realm,” Van Aswegen says.
Director Lars Bejder says long-term monitoring studies, such as drone-based hump measurements and the collaborative collection of tail fluke images through Happywhale, are “absolutely critical because they allow us to look at the impacts of large-scale oceanographic events.” from the Marine Mammal Research Program at the University of Hawaii at Manoa. and co-authored the latest study. “These animals are truly the guardians of the ocean. Healthy oceans produce healthy whales and vice versa.”
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