For the first time, scientists temporarily captured baleen whales and tested their hearing, a controversial step that brings researchers closer to understanding how some of the largest and most intelligent animals on Earth perceive their world.
In 2023, researchers captured two juvenile minke whales off the coast of Norway, applied gold-plated electrodes to their skin with suction cups and then examined their brain waves as different frequencies of sound played. Each young whale was at least 12 feet long and weighed about 1 ton.
The results of the study, published Thursday in the journal Science, surprised whale researchers because they suggest the creatures can hear at much higher frequencies than expected. The mammals’ ability far exceeds that of typical humans, the research suggests.
“It was a bit shocking,” said Dorian Houser, director of conservation biology for the National Marine Mammal Foundation and the study’s lead author. “They have this ultrasonic hearing, which we think is probably related to avoiding the killer whale as a primary predator since they use echolocation signals to hunt their prey at the same frequencies.”
The surprising findings come amid growing concern about the effects of ocean noise from sources like naval sonar, oil and gas exploration and ship traffic, which can alter marine mammals’ behavior, cause hearing loss and even kill them. The new information could shift what noisy activities are allowed in the ocean under the Marine Mammal Protection Act or lead to restrictions on some equipment.
“There might be higher frequency sounds that are produced from ocean exploration activities that were never considered a potential problem for baleen whales that need a second look,” said Susan Parks, a professor of biology at Syracuse University in New York, who was not involved in the study.
This study also closes a chapter of deep disagreement within the whale research community. Scientists had never captured baleen whales and tested their hearing before because the animals are so large it could be dangerous to both the whales and researchers.
Some scientists and activists fiercely opposed the four-year research effort — called the Minke Whale Hearing Project — over concerns that whales would become stressed and potentially die during their brief stints in captivity. They argued it wasn’t possible to safely capture the whales or ensure they wouldn’t develop long-term stress responses.
The Whale and Dolphin Conservation, a global nonprofit advocacy group that focuses on protection of these animals, in 2021 organized and sent an open letter from scientists and veterinarians to the Norwegian government, urging the project be “stopped as it may lead to considerable trauma for the whales targeted, without contributing to useful science.”
What the largest whales on Earth could hear has long remained a mystery to scientists. Because baleen whales — of which there are 16 species, including Humpback, Blue and Fin whales — are too large to study in captivity, biologists have had to infer their hearing ability from studies of whale behavior, how other animals interact with them and from models of their anatomy.
Scientists know, from recordings and other observations, that baleen whales rely on hearing for social communication and that it’s important to their survival. Humpback whales sing complex songs associated with courtship and mating, and make “clicks” when foraging, for example.
“To get an actual direct measure of hearing, that was something that’s been kind of a Holy Grail,” Houser said.
Minke whales were a good target for audio testing because they’re the smallest baleen whale and a species whose hearing scientists don’t understand very well.
“For minke whales, I don’t think there are a lot of details on how they use sound day-to-day,” Parks said, adding that there are other baleen species “where we’re just matching species to sound.”
To perform the tests, the Minke Whale Hearing Project researchers devised a complex trap near Norway’s Lofoten Islands to capture whales, relying on the species’ predictable migration path along the coastline.
The researchers set up more than a mile of nets to guide the whales into a channel between two islands, then blocked off their exits with barrier nets. The researchers next corralled the whales into a preexisting fish farm enclosure, where they used a “hammock”-like net to pull the creatures up closer to the surface.
In June 2023, the researchers caught two young whales and, with on-site veterinarians, performed 30 minutes of testing on one whale and 90 minutes on the other before releasing them. The researchers attached electrodes for brain stem monitoring, took blood samples and attached satellite tracking tags to follow up on the animals’ behavior and welfare.
Houser said the effort required about 20 tons of netting and 12-15 people working around the clock to corral, monitor and test the whales. Although the whales’ blood sugar rose and their heart rates changed, he said they tolerated testing well and continued their typical behavior afterward.
“Those satellite tags do show that they return to being minke whales again,” he said, adding that the animals’ welfare was the researchers’ top priority.
A minke whale that was never caught nor tested died when it became entangled in a net and drowned after a storm forced the research crew off the water and dislodged an anchor securing its nets.
Regina Asmutis-Silvia, executive director of the Whale and Dolphin Conservation in North America, said the organization remained opposed to the project.
“Releasing the whales alive does not mean that these individuals will not suffer long term stress responses to sounds or activities they associate with this experiment and their lives, and health, may be compromised as a result,” Asmutis-Silvia wrote in an email, adding that it was “tragic and ironic to harm individual whales to substantiate just how harmful human sounds are on these species.”
Asmutis-Silvia said similar science could have been completed through noninvasive means, like studies of behavioral responses or stress hormones, and that the science produced by the project had significant limitations.
“This is a sample size of two juvenile male minke whales from a particular region so it’s not clear how this translates more broadly to other age classes in that area, never mind other species,” she said.
Brandon Southall, a scientist and consultant who has helped develop acoustic exposure criteria for marine animals for federal regulators, said he thought the research would contribute to future limits set on ocean sound to keep marine mammals healthy.
“They did everything according to best practices and carefully and professionally. The fact that it did yield the first results and it guides conservation outputs, I think on balance it was useful and informative,” Southall said.
Southall said the new research did not measure the lowest ranges of the whales’ hearing ability, which is more challenging because of how neural signals operate in animals’ bodies.
He said low-frequency sounds, like ship propellers or seafloor piledriving for wind farms, travel great distances in the ocean and are more likely to cause marine mammals harm.
“We should think, before someone does another study like this, what more could we get out of it,” Southall said.
Houser agreed that it’s more difficult to study low-frequency sounds. But he said researchers had not reached the lower limit of what signals they could detect. He said the Minke Whale Hearing Project is on hold, but he’d like to pursue the topic further if he could secure funding.
