Researchers utilizing a cabled deep-sea hydrophone outside Monterey Bay, California, have tracked a six-year divergence in whale song trends following the 2014-2016 marine heat wave known as “the Blob,” according to a study published in PLOS ONE in February 2025. While humpback whale song more than doubled over the study period, blue whale song remained tightly suppressed and tied to fluctuating krill supplies.
Cabled Seafloor Observatory Tracks Deep-Ocean Soundscapes
The instrument sits 891 meters down on the continental slope, operating as part of the Monterey Accelerated Research System (MARS) run by the Monterey Bay Aquarium Research Institute (MBARI). A 52-kilometer power-and-data cable links the node directly to shore facilities. This setup provides a live, uninterrupted feed rather than relying on battery-powered units that require physical retrieval, according to study records. From July 2015 through June 2021, the hydrophone captured data on 2,111 of the 2,192 days in the period, achieving a 96 percent capture rate that allowed researchers to monitor acoustic trends month by month.
Rather than relying on human listeners, the team processed the near-continuous audio using specialized analytical methods. Ryan and co-authors from Cascadia Research Collective, NOAA Fisheries, and the University of Wisconsin utilized a deep-learning model to flag humpback whale song patterns. They identified blue and fin whale calls using a signal-energy method that compared sound levels in each species’ call frequency against background noise. Both approaches were validated against manually reviewed spectrograms.
Did you know? MBARI has made portions of the MARS acoustic archive publicly available. The same feed has been used to let members of the public listen to live whale song from Monterey Bay through a smart speaker skill.
Humpback and Blue Whale Songs Diverge After Marine Heat Wave
The study, titled “Audible changes in marine trophic ecology: Baleen whale song tracks foraging conditions in the eastern North Pacific,” details how two local baleen species reacted differently after “the Blob” collapsed the region’s cold-water food web. Male humpbacks and blue whales sing structured, repeating songs primarily on breeding grounds and, to a lesser extent, on feeding grounds. Researchers use the frequency of this singing as a proxy for body condition and feeding success, noting that underfed animals spend less time singing and more time searching for prey.
Humpback whale song climbed steadily across the six years, appearing on 34 percent of days in the first year and 76 percent of days by the final year. Blue whale song followed a different path. Detections dropped during the peak of the heat wave, rose for three consecutive years as krill recovered, fell for two years as krill grew scarce again, and then climbed once more in the sixth year. Unlike humpbacks, blue whales never displayed a sustained, uninterrupted rise.
Foraging Flexibility Explains Population Recovery Differences
John P. Ryan and his co-authors attribute the species split to feeding strategies. Blue whales specialize almost entirely on krill, which are highly sensitive to warm water and suffered severe declines during the heat wave. Humpback whales exhibit much greater foraging flexibility, shifting between krill and small schooling fish like anchovies when one prey type disappears. Central California anchovy populations rebounded strongly after the heat wave, allowing humpbacks to restore their foraging success and resume singing.
“Compared to humpback whales, blue whales in the eastern North Pacific may be more vulnerable due to not only a smaller population size but also a less flexible foraging strategy,” Ryan stated in comments accompanying the study’s release. This vulnerability is particularly significant because the eastern North Pacific blue whale population is already listed as endangered, with estimates placing numbers below 2,000 animals.
Pro Tip: Continuous acoustic monitoring allows scientists to detect stress in marine ecosystems long before traditional population counts or fishery landings register a decline.
Frequently Asked Questions
How do researchers record whale song continuously?
Researchers use cabled seafloor observatories like the MARS hydrophone, which is connected to shore by a 52-kilometer power-and-data cable. This setup bypasses the battery limitations of traditional underwater recorders.
Why do male baleen whales sing less when food is scarce?
Singing requires significant energy. When food sources like krill or anchovies are depleted, male humpback and blue whales spend less time vocalizing and more time actively searching for prey.
Are eastern North Pacific blue whales endangered?
Yes. According to researchers, the population is estimated at under 2,000 individuals, making them particularly vulnerable to environmental shifts and warm-water marine heat waves.
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