Scientists have been left seriously concerned after thousands of blue whales "went quiet" in the ocean.
Researchers from the Monterey Bay Aquarium Research Institute (MBARI) in California have found that blue whale songs have dropped by nearly 40 per cent over the past six years.
The scientists captured acoustic recordings off the coast of the bay to try to understand the impact human activity was having on marine life.
During their studies, researchers noted that major heat waves affected the region, allowing toxic algae to bloom, which then in turn killed off the whales' 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 MBARI, told National Geographic.
In 2013, the marine heatwave known as "The Blob" began - and by 2016, a year after the study began, it spanned across thousands of miles of unusually warm water.
The Blob raised ocean temperatures by more than 2.5C.
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|Many blue whales have raised alarm bells in the scientific community by going silent
This change devasted krill and anchovy populations - leaving the whales unable to eat.
Ryan said: "It's like trying to sing while you're starving."
Scientists have said that the desolation of krill populations, which is a key food source for many marine species, marks a deeper crisis which is unfolding in the oceans due to climate change.
"There are whole ecosystem consequences of these marine heat waves," Monterey Bay Aquarium marine biologist Kelly Benoit-Bird told National Geographic.
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|Scientists have used underwater microphones, known as hydrophones, for years to listen to the sounds blue whales make to communicate
"If they can't find food, and they can traverse the entire West Coast of North America, that is a really large-scale consequence."
Although the study focused primarily on whales off the coast of California, researchers also noted the silence is happening across the South Pacific, parts of the Southern Ocean, and Argentine waters.
Researchers have used underwater microphones, known as hydrophones, for years to listen to the sounds blue whales make to communicate.
The deep, low-frequency moans and songs used by blue whales to find mates, navigate, and stay connected with their group are picked up by the devices.
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|Fin whale songs were also heard less throughout the study
The team used a 32-mile-long cable running from the California coastline along the seafloor, ending in a two-inch metal cylinder located about 3,000 feet below the surface to collect recordings.
Ryan added: "It wasn't until I plugged in a hydrophone that I realised this world of sound can help us understand human impacts, nature, and the balance between."
Other species of whales, such as humpbacks, have a more varied diet and are adapted to harsh conditions, and therefore their songs stayed the same.
In comparison, blue and fin whales suffered greatly.
Dawn Barlow, an ecologist at Oregon State University's Marine Mammal Institute said: "Blue whales are sentinels. They reflect many ocean processes.
"Where they are, and what they're doing, tells us a lot about the health of the ecosystem."