By STACY LIBERATORE, U.S. SCIENCE & TECHNOLOGY EDITOR
Many blue whales have mysteriously gone silent in the ocean, raising alarm bells among the scientific community.
Acoustic recordings were captured off the coast of Monterey Bay, California to understand the impact of human activity on marine life.
Researchers at the Monterey Bay Aquarium Research Institute found that blue whale songs had dropped by almost 40 percent over the past six years.
During this time, the team identified that major heat waves had engulfed the region, allowing toxic algae to bloom and kill off the mammals’ food supply.
‘It caused the most widespread poisoning of marine mammals ever documented. These were hard times for whales,’ John Ryan, a biological oceanographer at the Monterey Bay Aquarium Research Institute, told National Geographic.
The marine heatwave known as The Blob began in 2013 and covered thousands of miles with unusually warm water by 2016, a year after the study began.
The Blob raised ocean temperatures by more than 4.5°F across, devastating krill and anchovy populations. ‘It’s like trying to sing while you’re starving,’ said Ryan.
With krill, a key food source for many marine species, being wiped out, scientists warn that this marks a deeper crisis unfolding in the oceans as climate change accelerates.
Blue whales are falling silent off California’s coast, and scientists fear it’s a warning sign the ocean is in deep trouble
‘There are whole ecosystem consequences of these marine heat waves,’ Monterey Bay Aquarium marine biologist Kelly Benoit-Bird told National Geographic.
‘If they can’t find food, and they can traverse the entire West Coast of North America, that is a really large-scale consequence.’
While the team focused on whales off the coast of California, they noted the silence is happening across the South Pacific, parts of the Southern Ocean, and Argentine waters.
Since blue whales depend heavily on sound to communicate, researchers have used underwater microphones, called hydrophones, for years to listen in.
These devices pick up the deep, low-frequency moans and songs that blue whales use to find mates, navigate, and stay connected with their group.
The new study collected recordings from a 32-mile-long cable that runs from the California coastline along the seafloor, ending in a two-inch metal cylinder located about 3,000 feet below the surface.
‘It wasn’t until I plugged in a hydrophone that I realized this world of sound can help us understand human impacts, nature, and the balance between,’ Ryan said.
Humpbacks have a more varied diet and are adapted to harsh conditions, so their songs stayed the same.
Humpbacks, with their diverse diet and resilience to tough conditions, kept singing as usual. But blue and fin whales, relying almost solely on krill, were heard singing far less than in past years
In contrast, blue and fin whales feed almost entirely on krill, and their songs were heard less often than in previous years.
The team noted that this was all due to the Blob.
In normal years, krill arrived in such numbers that fishing nets turned pink from their sheer volume, but during the heatwave, they all but disappeared.
‘When we have these really hot years and marine heatwaves, it’s more than just temperature,’ explains oceanographer Kelly Benoit-Bird, a Monterey Bay Aquarium marine biologist and co-author of the paper.
‘The whole system changes, and we don’t get the krill. So the animals that rely only on krill are kind of out of luck.’
Not only did krill numbers drop, but their behavior likely shifted too.
With the usual upwelling disrupted by heat, krill scattered, making it even harder for whales to feed.
‘We don’t hear them singing,’ said Ryan. ‘They’re spending all their energy searching.
‘There’s just not enough time left over, and that tells us those years are incredibly stressful.’
In the waters between New Zealand’s islands, scientists tracking blue whales from 2016 to 2018 also stumbled onto an eerie silence during the years of ‘The Blob,’ similar to what was recorded off California’s coast.
‘We were interested in understanding blue whale ecology,’ said Dawn Barlow, an ecologist at Oregon State University’s Marine Mammal Institute and lead author of the study.
‘And without trying, we ended up studying the effects of marine heatwaves—which, in this day and age, is hard to avoid when working in the ocean.’
Barlow and her team used underwater recorders in the South Taranaki Bight to monitor two types of blue whale calls, including low-frequency D calls linked to feeding and rhythmic songs associated with mating.
During unusually warm years, they heard fewer D calls in the spring and summer, suggesting whales were foraging less.
That fall, song intensity dropped as well, pointing to reduced efforts to reproduce.
‘When there are fewer feeding opportunities, they put less effort into reproduction,’ Barlow explained, noting that the silence is a warning.
‘Blue whales are sentinels,’ Barlow said. ‘They reflect many ocean processes. Where they are, and what they’re doing, tells us a lot about the health of the ecosystem.’
