Whales are the largest animals on Earth, yet there are still many unanswered questions about their biology, behavior, and ecology. In a new paper published in the scientific journal PLOS One, MBARI researchers and an interdisciplinary team of collaborators share results from a detailed analysis of MBARI’s unique archive of ocean acoustic data, demonstrating what whale songs can reveal about ocean health. 

“Analyzing baleen whale songs has revealed year-to-year variations that reflect changes in the availability of the species they forage on. These findings offer vital clues about how resilient different whale species may be in the face of changing ocean conditions,” said John Ryan, a biological oceanographer who leads MBARI’s Ocean Soundscape Team and was the lead author of this new study.

Baleen whales migrate vast distances each year between foraging habitat off California and breeding habitat off Mexico and Central America. While offshore of California, whales must consume a lot of food to build up energy stores that carry them through the winter breeding season. However, their food resources can vary tremendously from year to year, as phenomena like marine heat waves and El Niño change the productivity of their foraging grounds. MBARI researchers and their collaborators wanted to know if changes in whales’ acoustic behavior may be caused by changes in the whales’ sources of food.

Since 2015, MBARI researchers have been using underwater microphones, or hydrophones, to record the underwater soundscape in the Monterey Bay National Marine Sanctuary, including whale vocalizations. Prior research with these recordings revealed new dimensions of baleen whale ecology. For example, studying patterns in MBARI’s acoustic data showed that blue whales (Balaenoptera musculus) change the timing of their southward breeding migration by as much as four months from year to year, depending on corresponding changes in the conditions of their foraging habitat.

Baleen whales, including blue whales and humpback whales (Megaptera novaeangliae), produce rhythmic, structured arrangements of sound—a behavior described as singing. The songs of each whale species have their own unique features. Blue whales sing with a very low pitch and require the use of special speakers to be heard by humans. Humpback whales have a large vocal range, and humans can hear most of their sounds. Because sound travels so efficiently through water, these vocalizations can be detected if the singing whale is anywhere within an area thousands of square kilometers around the hydrophone. The powerful propagation of sound in the sea greatly extends the reach of scientific sensing.

MBARI began recording whale sounds off Central California in summer 2015, at the height of a marine heat wave, providing an opportunity to examine how whales respond to major environmental changes. For this study, MBARI researchers looked at variations in the intensity and persistence of blue and humpback whale songs over six years. Past MBARI research focused on individual whale species, and this was the team’s first comparative study across multiple whale species. 

The team found that the lowest levels of both blue and humpback whale songs were detected during the 2015 climax of a major marine heat wave. While song detection for both whale species increased similarly over the next few years, it diverged after the third year of the study—humpback whale song detection continued to rise while blue whale song detection fell for two consecutive years. 

Blue whales and humpback whales have different foraging strategies. Blue whales only eat krill, while humpback whales readily switch between swarms of krill and schools of small pelagic fishes, including anchovies and sardines.

NOAA fisheries data revealed that abundances of krill and fish changed dramatically during the study period. In the first year of the study, both krill and forage fish abundance were low as a result of the heat wave. These poor foraging conditions would have negatively impacted both blue and humpback whales. By the third year of the study, krill were relatively abundant, while forage fish were less abundant. Both blue and humpback whales would have benefited from increased availability of krill. However, by the fifth year of the study, krill abundance was lower than average, while forage fish were highly abundant. Humpback whales would have uniquely benefited from increased availability of fish.

To examine the diets of the whales, the MBARI team collaborated with researchers at Cascadia Research Collective, NOAA Fisheries, and the University of Wisconsin to measure stable isotopes in tiny samples of whale skin. The ratios of carbon and nitrogen isotopes in these tissue samples allowed scientists to examine what the whales had been eating. The team focused on comparing isotopes across the third and fifth years of the study period, when there were the largest shifts in forage species. This analysis confirmed that the diets of both whale species had substantial changes. Blue whales continued to feed on krill, but had to forage over a larger geographic area when krill populations became depleted. In contrast, humpback whales switched to a fish-dominated diet.

Using a community-supported photo ID database capable of recognizing individual humpback whales, the research team examined other factors that could account for the changes in song detection, including changes in local population abundance and the timing of their annual migration. However, changes in foraging conditions remained the most consistent factor explaining the observed changes in song detection.

“As the ocean changes, some species will be more affected than others,” said Ryan. “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. These findings can help scientists and resource managers predict how marine ecosystems and species will respond to climate change.”

Taken together, the extensive observational and theoretical perspectives of this research underscore that listening to whale songs is much more than a rich sensing experience, it is a window into their lives, their vulnerability, and their resilience. 

This work was funded as part of the David and Lucile Packard Foundation’s longtime support of MBARI’s work to advance marine science and technology to understand a changing ocean. The U.S. National Science Foundation funded installation and maintenance of the MARS cabled observatory through awards 0739828 and 1114794. Whale biopsy sampling and scientific surveys by Cascadia Research were conducted under NOAA National Marine Fisheries Service scientific research permit 21678 issued to J. Calambokidis.

MBARI makes its trove of acoustic data available for researchers and community scientists around the world via the Pacific Ocean Sound Recordings project. Immerse yourself in a symphony of humpback whale songs on MBARI’s YouTube channel.

Research Publication:
Ryan, J.P., W.K. Oestreich, K.J. Benoit-Bird, C.M. Waluk, C.A. Rueda, D.E. Cline, Y. Zhang, T. Cheeseman, J. Calambokidis, J.A. Fahlbusch, J. Barkowski, A.H. Fleming, C.N. Turner Tomaszewicz, J.A. Santora, T. Margolina, J.E. Joseph, A.S. Friedlaender, and J.A. Goldbogen. 2025. Audible changes in marine trophic ecology: Baleen whale song tracks foraging conditions in the eastern North Pacific. PLOS One, 20(2): e0318624. https://doi.org/10.1371/journal.pone.0318624