Climate change is driving increasingly intense marine heatwaves, raising concerns about their effects on marine wildlife. A new study led by researchers at the University of Hawai‘i (UH) at Mānoa has revealed how extreme ocean warming disrupts the metabolism, nutrition and reproductive success of humpback whales, shedding light on the biological mechanisms behind population declines observed during previous warming events.
Earlier reports from NOAA, academic researchers and regional monitoring programmes documented troubling changes during the 2014–2016 Pacific marine heatwave, known as the Blob. These included fewer humpback whale sightings in Hawai‘i and Alaska, declines in calf numbers and widespread signs of nutritional stress. However, the physiological mechanisms behind these population-level changes remained unclear. The new study, published in iScience, helps explain how extreme ocean temperatures affect whales internally.
“These findings give us a window into how whales actually cope with extreme environmental stress,” said lead author Ching-Tsun Chang, a former graduate student in the Department of Oceanography at the UH Mānoa School of Ocean and Earth Science and Technology and now an assistant professor at National Sun Yat-sen University, Taiwan. “We’ve known from previous reports that humpback whales declined in number and body condition during the Blob. What we show here is the physiological story behind those patterns: how whales adjust their metabolism, how nutrient use shifts, and how reproduction may be compromised.”
How Humpback Whales Respond to Extreme Ocean Warming
To investigate the physiological effects of marine heatwaves, researchers analysed nearly eight years of biochemical records preserved in the baleen of stranded female humpback whales from Hawai‘i. Baleen, the filtering structure whales use to feed, grows continuously and preserves a timeline of biochemical signals that can reveal changes in an animal’s nutritional and reproductive history.
“Baleen is an extraordinary archive,” said co-author Kristi West, Director of the UH Health and Stranding Lab and associate researcher in Human Nutrition, Food and Animal Sciences at the UH Mānoa College of Tropical Agriculture and Human Resilience. “It lets us reconstruct an animal’s nutritional and reproductive history with fine detail. By pairing these biochemical records with bioenergetic models, we can finally see how climate extremes ripple through a whale’s body.”
The team combined compound-specific isotope analysis of amino acids with bioenergetic models, which estimate how animals use and allocate energy. This approach allowed researchers to reconstruct how whales used nutrients and mobilised energy reserves before, during and after the marine heatwave.
The results revealed the physiological mechanisms behind the declines documented during previous warming events. During the heatwave, whales essentially “powered down”, slowing their metabolism to stretch limited food supplies. This reduced nutritional availability made it more difficult for females to maintain early pregnancies, even after becoming pregnant.
Researchers also examined one whale’s decades-long history, which revealed the cumulative effects of environmental change. Its baleen preserved a five-year record showing how changing ocean conditions affected its body late in life.
Why Marine Heatwaves Threaten Whale Reproduction
Humpback whales are capital breeders, meaning they rely on energy reserves accumulated during feeding seasons to support migration, pregnancy and lactation. This reproductive strategy makes them particularly vulnerable when marine heatwaves disrupt feeding conditions and reduce food availability.
When whales cannot replenish their energy reserves, they have limited opportunities to compensate for nutritional shortages. The findings suggest that reduced nutrient availability during marine heatwaves can affect not only whales’ metabolic processes but also their ability to sustain pregnancy and successfully reproduce.
These physiological effects help explain why previous monitoring programmes observed declines in calf numbers and signs of poor body condition during the 2014–2016 Blob. The study connects those population-level observations with changes in how whales regulate their metabolism, use nutrients and allocate energy to reproduction.
What the Developing 2026–2027 El Niño Could Mean for Humpback Whales
The findings arrive as the Pacific enters a developing 2026–2027 El Niño, which is expected to be one of the strongest on record, bringing unusually warm conditions across the North Pacific.
Seasonal forecasts from international climate centres project widespread surface ocean warming that could reduce prey availability in key humpback whale feeding regions. Although this developing event is distinct from the 2014–2016 Blob, the anticipated warming highlights the importance of understanding how whales respond physiologically to extreme environmental stress.
“As marine heatwaves become more frequent, capital-breeding species like humpback whales may face increasing challenges in a rapidly warming ocean,” said Brian Popp, co-author and professor of Earth Sciences at the UH Mānoa School of Ocean and Earth Science and Technology. “Understanding these physiological responses is essential for anticipating how populations will fare in the decades ahead.”
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