One hydrophone sitting 891 meters down in Monterey Canyon, wired to shore by 52 kilometers of cable, recorded 2,111 days out of 2,192 between July 2015 and June 2021, and humpback song climbed from 34 percent of days to 76 while blue whale song fell away after a marine heat wave.

A single hydrophone bolted to the seafloor of Monterey Bay has produced six years of near-continuous acoustic data on baleen whale behavior, and the sound it captured shows two closely related species pulling in different directions after the same environmental shock.

A cable-linked ear on the seafloor

The instrument sits 891 meters down on the continental slope outside Monterey Bay, California, part of the Monterey Accelerated Research System (MARS), a cabled seafloor observatory run by the Monterey Bay Aquarium Research Institute (MBARI). A 52-kilometer power-and-data cable links the node to shore facilities at MBARI, giving researchers a live, uninterrupted feed from the deep ocean rather than a battery-powered recorder that has to be retrieved and swapped out. From July 2015 through June 2021, that hydrophone gathered data on 2,111 of the 2,192 days in the period, a 96 percent capture rate that made it possible to track whale song trends month by month rather than in scattered snapshots. 

Researchers did not listen for whale calls by ear. Humpback whale song was flagged with a deep-learning model trained to recognize the species’ repeating song patterns, while blue and fin whale song was identified with a signal-energy method comparing sound levels in each species’ call frequency against background noise; both approaches were validated against manually reviewed spectrograms, letting the team process six years of near-continuous audio at a scale no group of human listeners could match. 

Of the 2,111 sampled days, 2,051 — 97 percent — carried at least 75 percent temporal coverage, the threshold the authors used to include a day’s detection results in the analysis.

The findings, published in PLOS ONE in February 2025 by MBARI senior research specialist John P. Ryan and colleagues from Cascadia Research Collective, NOAA Fisheries and the University of Wisconsin, describe a six-year run that began just as the Northeast Pacific was still gripped by “the Blob,” the 2014-2016 marine heat wave that pushed sea surface temperatures across a huge swath of the eastern Pacific far above normal and collapsed the productivity of the region’s cold-water food web.

Humpback and blue whales both feed in Monterey Bay’s waters, but the study, titled “Audible changes in marine trophic ecology: Baleen whale song tracks foraging conditions in the eastern North Pacific,” found that the two species came out of the heat wave on very different trajectories, tracked entirely through how often males of each species were heard singing. Male humpback and blue whales sing structured, repeating songs primarily on breeding grounds and, to a lesser but well-documented extent, on feeding grounds, and the frequency of that singing has increasingly been used by researchers as a proxy for body condition and feeding success — animals that are underfed spend less time singing and more time searching for food.

Diverging trajectories

Humpback whale song climbed steadily and almost without interruption across the study, present on 34 percent of days in the first year and on 76 percent of days by the sixth year, a more than twofold increase that the authors describe as a continuous interannual rise unmatched by the blue whales recorded at the same site over the same years. Blue whale song moved differently. Detections were suppressed during the peak of the heat wave, then rose for three straight years alongside humpback and fin whale song as krill recovered.

Song then fell for two consecutive years as krill grew scarce again, before rising once more in the study’s sixth and final year. Unlike humpbacks, blue whales never showed a sustained, uninterrupted rise across the full six years — their song stayed more tightly tied to the krill supply they depend on.

Why the species split

Ryan and his co-authors tie the split to how each species finds food. Blue whales specialize heavily on krill, small shrimp-like crustaceans whose populations are highly sensitive to warm water and were sharply reduced during and after the heat wave. Humpback whales are far more flexible foragers, able to shift between krill and small schooling fish such as anchovies when one prey type becomes scarce, and anchovy populations in central California rebounded strongly in the years following the heat wave. That flexibility, the researchers argue, let humpbacks recover their foraging success — and their singing — while blue whales, tied to a food source that took longer to recover, lagged behind.

“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 said in comments accompanying the study’s release, a distinction that matters for conservation planning given that the eastern North Pacific blue whale population is already listed as endangered and estimated at under 2,000 whales.

What continuous listening makes possible

The result is also a demonstration of what long-duration cabled ocean observatories can do that ship-based surveys or short deployments cannot. Visual surveys of whales require calm seas, daylight and enough water clarity to spot a surfacing animal, and they capture only a moment in time. A hydrophone wired to shore records around the clock regardless of weather, sea state or time of day, turning six years into a single, gap-resistant dataset. MBARI has made portions of the MARS acoustic archive publicly available, and 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, alongside its role in peer-reviewed research.

Because the recovery of prey species can be uneven and slow, the authors caution against reading a single strong season as evidence that either population has fully bounced back. The value of the MARS record, they note, lies in its continuity: an instrument that rarely goes silent turns whale song into a long-term biological indicator of ocean conditions, one that can flag a struggling food web well before population counts or fishery landings do. With marine heat waves projected to become more frequent as ocean temperatures rise, that kind of early acoustic warning system is likely to become more valuable, not less.