By Todd Woody | Bloomberg
A hungry whale is a quiet whale. A new first-of-its kind study found the marine mammals vocalized less after a marine heat wave decimated their prey, making whale songs a barometer of the effects of climate change on ocean ecosystems.
The waters of the Monterey Bay National Marine Sanctuary off central California are rich feeding grounds for blue, fin and humpback whales that cruise the coast during their annual migrations. Whales sense their surroundings largely through sound and create complex vocalizations, or songs, when they’re searching for mates and food.
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Scientists at the Monterey Bay Aquarium Research Institute (MBARI) and their colleagues analyzed whale songs recorded by underwater microphones in the marine sanctuary between 2015 and 2021, a period that included a years-long marine heat wave that researchers have linked to climate change. It devastated populations of krill, anchovies and sardines that the cetaceans eat.
“The whales all exhibited the lowest levels of song when conditions were the worst,” but vocalized more when the heat relented, said John Ryan, an biological oceanographer at MBARI and the lead author of the peer-reviewed paper published last month in the journal PLOS One. “Because they experience these very strong changes in their ecosystem in ways that we can perceive, they are good ecosystem sentinels.”
Such data could also provide early warning when whales leave foraging grounds due to changing ocean conditions. “If they’re seeking refuge in a new area that has more prey available, then we need to figure that out so we can protect those refuges,” said Vanessa ZoBell, a postdoctoral scholar at Scripps Institution of Oceanography’s Whale Acoustics Lab at the University of California at San Diego who was not involved in the study.
The ocean absorbs nearly a third of the world’s carbon dioxide emissions and 90% of the excess heat generated by that pollution. Whales in turn play a role in the marine carbon cycle, sequestering CO2 in their gargantuan bodies while pooping in such great quantities as to stimulate the growth of other carbon-consuming organisms.
Identifying individual songs from specific species amid the cacophony of ocean sounds and then correlating those vocalizations with the abundance or absence of whale chow proved a complex undertaking. The researchers relied on machine learning developed by the National Oceanic and Atmospheric Administration (NOAA) and Google artificial intelligence researchers to parse humpback whale songs from background noise.
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Ryan and his colleagues analyzed data from a long-running NOAA survey of krill, anchovies and other whale favorites to match the prevalence of prey to the songs’ volume. Researchers also ventured out in small boats and used crossbows fitted with darts to collect whale skin samples to confirm changes in their diets in years when prey was plentiful or scarce.
“We can use sounds like whale song as a metric for understanding biodiversity and ocean health, and if all those sounds go away, it kind of lets us know that a change might be happening in a certain area,” said ZoBell.
Blue whales — the planet’s largest animal — subsist on tiny crustaceans called krill. As populations of this food source recovered from the marine heat wave, detections of blue whale singing soared. When krill numbers fell again, so did the whales’ songs. Unlike blue whales, humpback whales can also eat anchovies and sardines, and their choruses returned when there was a population explosion of those fish.
Ryan said that given how important singing is to whales’ reproduction, social interactions and foraging, their silence signals trouble. A whale has no energy for song “if it’s harder to gather the food resources it needs to sustain a body that can weigh more than 300,000 pounds.”
“Beyond being beautiful compositions that are wonderful to listen to, these songs are a window into their lives and their resilience through a heat wave as well as their vulnerability,” he added.
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