Marine Heatwaves Force Humpback Whales to Power Down, Threatening Pregnancy and Reproduction

The record sits in a whale's mouth. Baleen, the keratin plates humpbacks use to filter food from seawater, grows continuously, and as it grows it locks in a chemical timeline of what the animal was eating and how its body was spending energy. Researchers at the University of Hawai‘i at Mānoa have now read that timeline from stranded female humpbacks in Hawai‘i, and it tells them what happened inside the whales during one of the Pacific's worst marine heatwaves.

The heatwave was the 2014–2016 event known as the Blob. Reports from NOAA, academic researchers and regional monitoring programs had already documented troubling signs: fewer humpback sightings in both Hawai‘i and Alaska, fewer calves, and widespread evidence of nutritional stress. What was missing was the physiology behind those counts. The new study, published in iScience, aims to supply it.

The team analyzed nearly eight years of biochemical records preserved in the baleen. They used compound-specific isotope analysis of amino acids, paired with bioenergetic models, which are computer models that estimate how animals use and allocate energy. Together these let them reconstruct how the whales routed nutrients and drew on energy reserves before, during and after the heatwave.

The finding is that the whales essentially powered down. They slowed their metabolism to stretch limited food supplies. That lack of nutrition made it harder to maintain an early pregnancy, even in whales that had conceived. The result offers a physiological explanation for the drop in calf numbers seen during the Blob years.

One whale added a longer view. Its decades-long life was recorded in its baleen, which preserved a five-year record of how shifting ocean conditions affected its body late in life, showing the cumulative toll of environmental change.

“We’ve known from previous reports that humpback whales declined in number and body condition during the Blob,” said lead author Ching-Tsun Chang, a former graduate student in oceanography at UH Mānoa and now an assistant professor at National Sun Yat-sen University in Taiwan. “What we show here is the physiological story behind those patterns.” Co-author Kristi West, director of the UH Health and Stranding Lab, called baleen “an extraordinary archive” that lets scientists reconstruct an animal's nutritional and reproductive history in fine detail.

The work matters because humpbacks are capital breeders: they rely on energy stored during feeding seasons to fuel migration, pregnancy and lactation. When feeding is disrupted, as in a marine heatwave, such species have limited ability to make up the shortfall.

That gives the findings a forward edge. The Pacific is entering a developing 2026–2027 El Niño, expected to be one of the strongest on record, with unusually warm conditions across the North Pacific. Seasonal forecasts from international climate centers project widespread surface warming that could reduce prey in key humpback foraging regions. The event is distinct from the Blob, but the study's authors say it underscores the need to understand how whales respond physiologically to extreme stress.

“As marine heatwaves become more frequent, capital-breeding species like humpback whales may face increasing challenges in a rapidly warming ocean,” said co-author Brian Popp, professor of Earth Sciences at UH Mānoa. What to watch now is whether the coming El Niño brings the prey losses forecasters project, and whether humpback sightings and calf counts follow.