Sounding the alarm.

The declining sound of blue whales’ songs off California’s coast has alarmed scientists, as they fear it signals broader ecological issues.

Researchers monitored the songs of three whale species—blue, fin, and humpback—across the North Pacific Ocean within the California Current Ecosystem (CCE) over a span of six years.

The study employed an underwater hydrophone to examine the frequency of these whales’ structured sound sequences, known as songs, to determine their presence in this feeding area and explore their potential in behavioral ecology studies.

The study, published in Public Library of Science in February, found a decrease in song detection for both blue and fin whales across two of three interannual changes after 2017.

Recordings began in 2015, at the peak of a multi-year marine heatwave.

The unusual warming began in 2013 when a large, dense pool of hot water, nicknamed The Blob, traveled from the Bering Sea and Gulf of Alaska down to the waters off the West Coast of the US.

At the same time, the CCE food web toxicity was at high levels because of an extreme harmful algal bloom (HAB) that caused the “most widespread poisoning of marine mammals ever documented, including in whales,” the study read.

Waters were recorded to be 4.5 degrees hotter than average, and covered about 2,000 miles of the Pacific Ocean by 2016.

The heatwave created a thriving environment for the bloom, killing off the krill population, and in turn quieting the waters with fewer blue whale songs.

“When you really break it down, it’s like trying to sing while you’re starving,” John Ryan, a biological oceanographer at the Monterey Bay Aquarium Research Institute who coauthored the report, told National Geographic. “They were spending all their time just trying to find food.”

Ryan described it as the “most widespread poisoning of marine mammals ever documented,” creating hard times for the whales.

Blue whale songs decreased by 40 percent during the study period.

The study suggests blue whales had to forage in a larger area in 2019 because of a low abundance of krill in the tracked area.

Blue whales – the world’s largest mammal – maintain a strict krill-based diet, but their humpback counterparts forage on a wider range of krill and fish.

The Blob did not make an impact on the population of foraging fish — anchovies and sardines — and the study found an increase in auditory detection of humpback whale songs.

Interannual change from the smaller whale species increased from 34 percent to 76 percent of days over the six years.

Disturbed by the sound of silence, scientists are trying to understand the long-term damage caused by “The Blob” and similar anomalies believed to have tripled in occurrence since the 1940s, according to the outlet.

“There are whole ecosystem consequences of these marine heat waves,” study co-author and Monterey Bay Aquarium marine biologist Kelly Benoit-Bird told the outlet. “If they can’t find food, and they can traverse the entire West Coast of North America, that is a really large-scale consequence.”

The longer foraging period results in less reproductive effort by the marine species, creating a smaller population of the blue whale.

“Where they are, and what they’re doing can tell you a lot about the health of the ecosystem,” regon State University ecologist Dawn Barlow told National Geographic. “The Blob really highlighted how long-term these consequences can be.”

“Science shows that climate change is impacting the oceans,” she said, “Listening and learning from these places is essential to our future. Now more than ever, it’s important to listen.”