Every year, humpback whales travel thousands of kilometres from the cool waters of Alaska and Canada, where they fatten up, to the warm lagoons of Hawaii to breed. This ancient migration suddenly came up against an invisible threat: the marine heatwave of 2013–2016, which disrupted the entire food chain.
According to a recent study published in the journal Marine Mammal Science, the population of Hawaiian humpback whales (Megaptera novaeangliae) declined by 37 percent between the breeding seasons of 2014–2015 and 2021–2022. The scientists analysed more than 27 thousand photographs of tail flukes, which serve as a kind of "fingerprint" for identifying individuals.
Before the heatwave, adult whale survival stood at about 99 percent per year. During the anomaly, that figure fell to 68 percent — a sharp decline, unusual for long-lived mammals. In the feeding grounds off Alaska and British Columbia, emaciated animals were dying en masse.
The cause lies in the collapse of the food supply. The heatwave, known as "the Blob", sharply reduced the numbers of krill and small fish — the whales' main prey. Without enough food, adults could not build up the necessary reserves, and the birth rate and survival of calves fell.
The population has still not recovered: by some estimates, there are now roughly five thousand fewer whales than before the heatwave. Although some individuals may have moved to other areas, researchers believe that most of the animals died of starvation.
Humpback whales serve as a kind of indicator of the ocean's health. Their sharp decline signals the cascading effects of warming that affect not only whales but the entire marine ecosystem — from plankton to seabirds and pinnipeds.
Today the North Pacific is once again facing thermal anomalies, and a strong El Niño is intensifying the heating of the waters. Under such conditions, the population becomes more vulnerable to other threats: entanglement in fishing gear, collisions with ships and pollution.
Long-term monitoring of such migratory species makes it possible to better understand how climatic shifts are reshaping ocean ecosystems and what measures can mitigate the consequences for marine life.
