Can the song of the large mysticetes tell us about the abundance of their prey, and therefore about the state of the food web that carries them? A team at the Monterey Bay Aquarium Research Institute tested this hypothesis by listening to the same point in the Pacific Ocean for six consecutive years. The results, published on 26 February 2025 in PLOS One, show that two species monitored in the same place and at the same time moved in opposite directions, according to their diet.
The listening set-up
The recordings come from a cabled hydrophone installed in the Monterey Bay National Marine Sanctuary, at 36.713° north and 122.186° west, at a depth of 891 metres. An instrument linked to the shore by cable allows continuous recording, without the interruptions for batteries or servicing that limit autonomous moorings.
The period covered runs from July 2015 to June 2021, with 2,111 days analysed out of the 2,192 in the interval. The authors divide their series into six offset years, running from 1 July to 30 June, so as not to split a feeding season between two annual bins.
The variable measured is the proportion of days on which the song of the species is detected. Three species are monitored: the blue whale, the fin whale and the humpback whale, whose songs occupy distinct frequency bands and can be told apart without ambiguity.
The chronological starting point matters: the series begins as the Blob was ending, the marine heatwave that affected the north-eastern Pacific from 2014 to 2016 and caused krill populations to collapse.
Results: a shared phase, then a divergence
The series reads in two stages, a distinction that several reports left out.
| Species | Years 1 to 3, coming out of the heatwave | After year 3 |
|---|---|---|
| Humpback whale | detection up by a factor of 1.4 (34% to 48% of days) | continues to rise, 47% to 76% of days |
| Blue whale | up by a factor of 1.5 | down in two comparisons out of three |
| Fin whale | up by a factor of 1.8 | down in two comparisons out of three |
All three species therefore increase during the recovery phase, then their trajectories separate. This is neither a silence, nor a continuous decline over six years, nor a 40% fall: the result is a relative decline of two species compared with a third, within the same ecosystem.
How the prey populations changed
The authors tracked prey abundance in parallel, by echosounder and by survey cruises, in the form of anomalies, that is, departures from the long-term mean.
- Years 1 to 3: krill moves from negative anomalies to positive anomalies; anchovy and sardine remain persistently low.
- Years 4 to 6: the reverse configuration. Krill swings towards strongly negative anomalies, while anchovy shows strongly positive anomalies three years running.
- Years 5 and 6: the echosounder measures a forage fish biomass higher by a factor of 1.7 in median value, and a number of aggregations higher by a factor of 4.4.
The timing of this switch coincides with that of the acoustic divergence.
Interpretation: specialist against generalist
The explanation proposed rests on a well established difference in diet. The blue whale is a krill specialist, krill making up very nearly all of its food. The humpback whale is a generalist and takes krill and forage fish alike. When krill collapses and anchovy proliferates, the first loses its resource while the second gains one.
Feeding still has to be linked to song. The authors propose that the change in prey populations acts on foraging efficiency, hence on search effort, habitat occupancy and energy reserves, and consequently on the time budget the animal can devote to activities other than feeding. Singing takes up time that is not spent feeding.
An independent observation supports this reading. In the humpback whale, the ratio of daytime song to night-time song rose by a factor of 1.9 over the period: as feeding conditions improved, the species began to sing at hours previously given over to the search for food. In the blue whale and the fin whale, this ratio remained stable.
Limits of the interpretation
Acoustic detection is not a population size. A fall in the number of days with song may reflect a decrease in the number of animals present, or an unchanged number of animals singing less. The protocol does not settle between these two causes, and the authors do not claim that it does.
A single listening point. The measurements come from a single hydrophone, in a single ecosystem, that of the California Current. They say nothing about the global state of the populations. A population finding less krill off Monterey may have moved elsewhere.
Song has more than one function. It also belongs to breeding behaviour and to social structuring, whose year-to-year variation is not controlled for in this analysis.
Scope: an acoustic indicator of trophic state
The title of the paper, for that matter, does not mention silence, but audible changes in marine trophic ecology. That is where the contribution of the work lies: establishing that mysticete song tracks variations in the abundance and the composition of prey, and can therefore serve as an indicator of the state of the ecosystem.
The practical interest is considerable. Estimating the abundance of krill and forage fish requires cruises that are expensive and necessarily occasional. A cabled hydrophone records continuously at marginal cost, and returns the information as the predators themselves perceive it, since they sample the ocean far more finely than any vessel can. Our pages on passive acoustic monitoring and hydrophones with live listening describe this approach, and our sound library allows the songs concerned to be heard.
The result finally carries a lesson about marine heatwaves: they do not stop producing effects when the temperature comes back down. The 2014-2016 event left a reorganised food web, in which specialist species and opportunist species are not affected in the same way. Our pages on ocean currents and plankton document the same mechanisms in European waters.
Reference: Ryan J. P., Oestreich W. K., Benoit-Bird K. J., Waluk C. M., Rueda C. A., Cline D. E. et al., “Audible changes in marine trophic ecology: Baleen whale song tracks foraging conditions in the eastern North Pacific”, PLOS One, 20(2), e0318624, 26 February 2025, open access. Press release: MBARI. French coverage: Science et Vie, National Geographic, Geo and 20 Minutes.
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