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Each July and October, Susan Bengtson Nash packs a crossbow and an air rifle and heads out to sea off the shore of Brisbane in Queensland, Australia. Every time, reliably, humpback whales arrive to meet her. Of all baleen whales, humpbacks are the showiest, breaching frequently and slapping the water with their flukes and wing-like pectoral fins. As they pass by, Bengtson Nash takes aim, pulls the trigger, and a modified dart bounces off a whale’s dorsal flank with a small sample of skin and blubber.
Each biopsy is a treasure trove. It contains information about the whale’s diet, how far it traveled to find food, how much fat it carries, how stressed it is and, if the whale is female, whether it is pregnant. And, by proxy, the information about the whale’s body condition serves as a measure of the health of the Antarctic ecosystem in a changing climate.
The northeast coast of Australia is a humpback whale highway, a corridor in an annual migration between frigid, food-rich Antarctic waters where the whales gorge themselves on krill during the southern-hemisphere summer, and the tropical seas where they mate and give birth in winter.
The big question is whether climate change and other threats will outpace the whales’ recovery.
While en route, with their blubber replenished during the short summer feeding frenzy, humpback whales fast. This makes them a remarkable species to study for Bengtson Nash and her team at Griffith University’s Environmental Futures Research Institute. Within their blubber, humpbacks carry an imprint of the environmental conditions and food abundance in their Antarctic feeding grounds. “We’re looking at whales, but it’s the krill we’re interested in because they are the keystone of the Antarctic ecosystem,” Bengtson Nash says.
The humpback migration is the longest known for any mammal on the planet. Some populations cover more than 5,000 miles during each one-way journey between their Antarctic feeding areas and tropical breeding grounds. Bengtson Nash first started collecting samples from migrating humpbacks in 2006, initially to monitor chemicals known as persistent organic pollutants, or POPs, pesticides such as DDT, and industrial chemicals that, once released into the environment, build up in the fatty tissue of all living organisms and are toxic to wildlife and people. Although these chemicals have been banned in most countries, they continue to circulate widely because they take many years to degrade. Scientists in the Arctic had been tracking these pollutants for decades but there was “a complete absence of this work in Antarctica,” Bengtson Nash says. Keen to track what happens under changing climate conditions in the Southern Ocean, she knew she’d find a signal of these fat-accumulating contaminants in whale blubber.
This work is now part of the ongoing Antarctic Monitoring and Assessment Program and has the United Nations’ endorsement as part of its Decade of Ocean Science for Sustainable Development. But it led to more.
Female humpback whales can lose half their body weight during the migration because they are either pregnant or nursing young calves, and Bengtson Nash had to develop a suite of new techniques to assess their body condition. “I needed to figure out what they were eating and if they were pregnant, and this started off a whole not-really-planned growth of my research into humpback whale eco-physiology.”
Soon, the 30,000 to 40,000 humpback whales estimated to migrate through Australian waters became canaries in the ocean coal mine, signaling change in Antarctica. The first alert came when Bengtson Nash compiled the data from the project’s first decade and discovered a spike in POPs in the blubber samples during 2011.1 It was so high she first thought something had gone wrong in the lab. But then she scrutinized climate data from the whales’ Antarctic feeding area for that southern summer—and “sure enough, we had the strongest La Niña event on record,” she says.
During La Niña events, the sea surface warms in the western Pacific. Another major driver of Antarctica’s climate, the Southern Annular Mode, also pushed a belt of westerly winds that whip around Antarctica closer to the continent during the 2011 summer. These climate conditions meant sea ice stuck closer to the Antarctic coast and krill swarms retreated with it. With less krill in their feeding grounds, the whales went hungry, losing fat and showing higher levels of pollutants.2
“I pulled together all my metrics on fecundity, on diet, on body condition, and everything pointed to the fact that in 2011, our whales were in really poor condition,” Bengtson Nash says. “We also saw that far fewer females migrated, which is an indication that not enough reached the threshold to be able to commit to a reproductive event.”
This was the formal start of the Humpback Whale Sentinel Program, a long-term project to study the Antarctic sea ice ecosystem, which has continued the timeline of measurements off the east Australian coast, but which now also includes scientists in other parts of the world tracking the migration of humpback populations that use other corridors to travel from Antarctica to tropical breeding grounds in the Atlantic and Pacific. Between them, the teams’ monitoring effort covers 80 percent of the circumpolar ocean around Antarctica.
For the east Australian whales, known as breeding population E1, 2016 turned out to be a bumper year, but it was followed by another lean year in 2017. This was the Antarctic summer when a tenth of the Larsen C ice shelf cleaved off as a massive iceberg; a hole almost as big as South Carolina, known as a polynya, opened up in the winter sea ice off east Antarctica; and summer sea ice coverage was the lowest on record, 27 percent below the mean annual minimum. “That became the second poor year for the whales,” says Bengtson Nash.
This highlights a finely tuned connection between the whales’ winter condition and the summer abundance of krill in Antarctica, which, in turn, depends on the extent of sea ice. The freeze and thaw of Antarctic sea ice, which forms like an apron around the entire continent and doubles its size each southern winter, represents Earth’s largest seasonal change. For krill, it provides a vital nursery ground.
The early larval stages of these small shrimp-like animals live on the underside of sea ice, grazing on algae to fuel their development into ocean-swarming adults. Fluctuations in sea ice cover can change krill stocks. During years with less sea ice, the under-ice nursery shrinks and produces less krill. Even a marginal drop in krill abundance produces ripple effects along the food web, including humpback whales. Biologists argue that the whales’ extreme migration behavior evolved in tandem with Antarctic krill. The whales can only operate at the metabolic extremes of their annual journey, moving between feast and famine during each cycle, if they find enough food off Antarctica. Because of their reliance on krill, they are among the first to show measurable changes in response to a changing climate.
“We’re getting a true ecosystem response to climate change,” Bengtson Nash says. “So far, we’ve been very good at monitoring from satellites. We can see sea ice extent and we can monitor temperature in both air and sea. We can get a good handle on those physical aspects of change. But what’s much harder to get is the ecosystem response to change. That’s what we’re capturing with this program.”
Off the coast of Brazil, another group of humpback whales, breeding population A, gathers at the Abrolhos Bank, a 120-mile-wide extension of the continental shelf. Milton Marcondes has been tracking these whales since 1997, and is one of several international researchers who recently began contributing biopsies to the sentinel program.
Since the first survey in 2002, whale numbers here have ballooned from about 3,400 to 25,000, and Marcondes estimates the group is growing at an annual rate of 12 percent. “The population shows a remarkable capacity to recover from whaling,” he says.3
But the whales now face multiple new threats through climate change, ship traffic, entanglement in fishing gear, plastic pollution, and oil and gas development off the coast of Brazil. And as the population grows, so does the number of strandings. “With the recovery of the humpback whale population, strandings are now more frequent, but we had some years with unusual mortality events,” Marcondes says.
The whales now face multiple threats through climate change, pollution, and oil and gas development.
Those years—2010 and 2017—match the lean years for humpbacks off the Australian coast. Both are linked to decreased krill in Antarctic feeding areas. Further, 2021 broke the record for strandings, with 230 humpbacks—almost twice as many as in 2017—beached off the coast of Brazil. “Some of these whales that died in 2021 were feeding in Brazil—an unusual behavior since it’s a breeding ground—and some were very thin,” Marcondes says.
The biopsy samples haven’t been analyzed yet, but Bengtson Nash is concerned that 2021 will prove another bad season for the whales. “Our first clue that something was off was when we were operating in Hervey Bay, which is a famous whale-watching place, and they were waiting for the whales. They did turn up, but they arrived late and disappeared early.”
Without plentiful years to recover after times of sub-optimal feeding, she expects to see cumulative effects. In the Southern Hemisphere, the chemical pollution in itself is not at harmful levels—whales in the Northern Hemisphere are exposed to many times as much—but when female whales lose fat during the migration, the ratio of pollutants to body weight increases. “It’s a given that because of the size difference between the mom and the fetus, the fetus is getting a much higher dose,” Bengtson Nash says. “It’s very unusual for humpback whales to strand, but we do know that it’s predominantly calves and juveniles. And if the mother hasn’t been in good enough condition, they might experience premature weaning or not enough provisioning in utero. And that means when they embark on their first independent migration, they’re already in deficit, and they’re really vulnerable to not making it.”
Along the northern Pacific coast of Colombia, humpback whales mingle with hammerhead and whale sharks in the Gulf of Tribugá, a near-pristine stretch of coastline fringed by the jungle of the Utría National Nature Park. Natalia Botero Acosta, who works for the non-profit Macuáticos Colombia Foundation, has been tracking this group at both ends of their migration. Genetic data show the whales feed almost exclusively on krill along the western coast of the Antarctic peninsula, and photographs of flukes and fins have helped her to identify 850 individuals that come into the gulf to breed. Her particular interest is in the whales that act as escorts for females with newborn calves. “That association has always been fascinating to me,” she says. “Why is the escort there, how long do they stay together?” Together with Ari Friedlaender at the University of California, Santa Cruz, Botero Acosta has been exploring how stress hormones change during the whales’ migration. Friedlander contributed to a recent study that quantified the amount of krill that baleen whales consumed before the period of commercial whaling.4
Humpbacks are a medium-sized species among baleen whales—a group that also includes blue, fin, sei, and minke whales. The 20th-century period of industrial-scale whaling, with its explosive harpoons and factory ships, brought all of the baleen whales close to the brink of extinction.
The study estimated that before that time, whales ate an unfathomable 430 million metric tons of krill every year, or twice as much as all the krill in the ocean today. The whales’ disappearance should have left a huge krill surplus. Instead, the opposite happened: The krill population collapsed by more than 80 percent because the missing whales no longer returned nutrients to the ocean in the form of iron-rich excrement that acted as manure, stimulating the growth of algae, krill food. In the north Atlantic, iron blows in with dust from the Sahara, but in the ocean around Antarctica, it is rare. The larger whales played a bigger role than humpbacks in fertilizing the ocean with iron, but the recovery of all baleen whales could restore this ecosystem function.
The big question is whether climate change and other threats will outpace the whales’ recovery. Bengtson Nash says a climate-driven crash in krill populations would have serious repercussions for the Antarctic ecosystem as a whole, but humpback whales may well be adaptable enough to change their migratory lifestyle. “I come from Denmark, and when the herring stocks collapsed there, we saw that the harbor porpoise stopped migrating within a year. We have to remember this is a behavior, and it’s facilitated because krill is such a wonderful food source that has enabled [humpback whales] to live like this.”
Already, humpback whale populations in the northeast Pacific are adapting to life without krill by switching to anchovy. Swarms of krill—perfect bundles of fat—are a great energy source for the whales, but “if they can’t reliably locate those swarms any longer, we have to prepare ourselves that there will be a fallout, Bengtson Nash says. “But with time, they will find other food sources and we will see a change in their behavior.”
Such flexibility may buffer them against fluctuations in the ocean. “The reason why humpback whales are a good sentinel is that we will see change because they are an adaptable species,” Bengtson Nash says. “We know from the Northern Hemisphere that they are perfectly capable of feeding on other prey. We know that they’re perfectly willing to forgo migration.”
Globally, humpback whales have recovered to about 70 percent of their pre-whaling population size and they have shown remarkable capacity to adapt to change. In the Southern Hemisphere, there are seven distinct populations, all embarking on long migrations in search of food in Antarctic waters. The number of humpbacks migrating along Australia’s eastern and western coastlines has been climbing, despite regular lean years, and the country has removed them from its list of threatened species in March, 2022.5
In contrast, the Oceania population, which moves north along New Caledonia, remains listed as endangered on the International Union for Conservation of Nature red list and shows no recovery trend. And some whales in the Brazilian population are now veering off their traditional routes, making detours and mixing with other groups as they chase after swarming krill, carrying a deeper message of change around Antarctica. 
Lead art: solarseven / Shutterstock
References
1. Groß, J., et al. Interannual variability in the lipid and fatty acid profiles of east Australia-migrating humpback whales (Megaptera novaeangliae) across a 10-year timeline. Nature Scientific Reports 10, 18274 (2020).
2. Bengtson Nash, S.M., et al. Signals from the south; humpback whales carry messages of Antarctic sea-ice ecosystem variability. Global Change Biology 24, 1500-1520 (2018).
3. Marcondes, M.C.C., et al. The Southern Ocean Exchange: porous boundaries between humpback whale breeding populations in southern polar waters. Nature Scientific Reports 11, 23618 (2021).
4. Savoca, M.S., et al. Baleen whale prey consumption based on high-resolution foraging measurements. Nature 599, 85–90 (2021).
5. Bestley, S., et al. New insights into prime Southern Ocean forage grounds for thriving Western Australian humpback whales. Nature Scientific Reports 9, 13988 (2019).
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