PARIS: The number of humpback whales in the North Pacific has plunged by 20 percent in less than a decade, and marine heat waves may be the main culprit, according to a study on Wednesday that portends a troubled future for the majestic marine mammals.
Thanks to conservation efforts and the end of commercial whaling in 1976, the humpback whale population in the region continued to increase until 2012.
But whale numbers have plummeted over the past decade, researchers reported in the journal Royal Society Open Science.
A team of 75 scientists compiled the largest photographic identification dataset ever created for a large marine mammal to track humpback whale populations in the North Pacific from 2002 to 2021.
Using images of the whale’s unique tails, the team was able to record about 200,000 sightings of more than 33,000 individuals.
Until 2012, the humpback whale population had been steadily increasing and it was widely assumed that it would eventually reach its natural “carrying capacity” – the number of whales the ocean can support.
Instead, they experienced a sharp decline in population.
From 2012 to 2021, the number of humpback whales declined by 20 percent, from approximately 33,000 individuals to just over 26,600.
For the subset of whales that wintered in Hawaii, the decline was even more pronounced: 34 percent.
This proved to be a very significant difference.
From 2014 to 2016, the Northeast Pacific was ravaged by the strongest and longest marine heat wave ever recorded, with mild anomalies sometimes exceeding three to six degrees Celsius, altering the marine ecosystem and the availability of humpback whales.
“My jaw was on the floor,” study author Ted Cheeseman, a whale biologist and PhD student at Southern Cross University in New South Wales, told AFP. “It’s a much bigger signal than we expected.”
“Our estimate is that about 7,000 whales mostly starved to death,” he said.
Even in healthy populations, it’s normal for numbers to fluctuate, but such a sudden decline in long-lived species points to a major disruption in the oceans.
In this case, the researchers speculate that the extreme sea heat actually lowered the carrying capacity for humpback whales.
“Instead of the whales reaching the ceiling, the ceiling came crashing down on the whales,” Cheeseman said.
The fact that humpback whales have not been able to change their already flexible diet is a telling indicator of the overall health of the ocean.
“It’s not just the loss of food for the whales,” Cheeseman added, noting the decline in puffin, sea lion and seal populations. “A warm ocean produces less food.”
Some commercial fisheries also felt the impact.
According to the Intergovernmental Panel on Climate Change, marine heat waves – already more frequent and intense – are projected to increase globally over the course of this century.
For hundreds of years, whalers from all over the planet have hunted humpback whales for their oil, meat and baleen, their feeding filter system.
By 1986, the IUCN listed this species as globally threatened.
Humpback whales continue to face threats today, primarily from boat strikes and entanglement in fishing nets.
But international restrictions on commercial whaling have allowed the global humpback whale population to return to more than 80,000 adults.
But today, nature conservation goes hand in hand with climate measures.
“It’s a great achievement that these whales are no longer in imminent danger of extinction as they were 50 years ago,” Cheeseman said.
“And yet there is a new reality of changing oceans that we have to live with.
