The sound shows giant blue whales dancing with the wind to find food

A study by MBARI researchers and collaborators, published today in the Environmental letters sheds new light on the movements of mysterious and endangered blue whales. The research team used a directional hydrophone at MBARI’s underwater observatory, integrated with other advanced technology, to listen for the booming calls of blue whales. They used these sounds to track the movements of blue whales and learned that these ocean giants respond to changes in the wind.

Along California’s central coast, spring and summer are upon us coastal upwelling. From March to July, seasonal winds push the upper layer of water out to sea, allowing cold water to rise to the surface. Cooler, nutrient-rich water feeds blooms of tiny phytoplankton, giving rise to Monterey Bay’s food web, from tiny shrimp-like krill to giant whales. When the wind creates upwelling, blue whales look for plumes of cooler water where krill are most abundant. When the upwelling ceases, the whales move away from the shore into habitat crisscrossed by shipping lanes.

“This research and the technology behind it open new windows into the complex and beautiful ecology of these endangered whales,” said John Ryan, a biological oceanographer at MBARI and lead author of the study. “These findings provide a new resource for managers looking for ways to better protect blue whales and other species.”

A directional hydrophone is a special underwater microphone that records sounds and determines the direction they come from. To use this technology to study blue whale movements, the researchers needed to confirm that the hydrophone was reliably tracking the whales. This meant matching acoustic bearings with a calling whale that was tracked by GPS. With confidence in established acoustic methods, the research team examined two years of acoustic monitoring of the regional blue whale population.

This research built on previous research led by MBARI senior scientist Kelly Benoit-Bird that showed schools of forage species—anchovies and krill—respond to coastal upwelling. This time, the researchers combined satellite and mooring data of upwelling conditions and sonar data of fin clumps with the acoustic tracks of foraging blue whales recorded by a directional hydrophone.

“Previous work by the MBARI team showed that when coastal upwelling was strongest, anchovies and krill formed dense swarms within the upwelling plumes. We now know that blue whales track these dynamic plumes where abundant food resources are available,” explained Ryan.

Blue whales recognize when the wind is changing their habitat and identify the places where the upwelling deposits their important food, krill. For a massive animal weighing up to 165 tons, finding these dense clumps is a matter of survival.

While scientists have long recognized that blue whales seasonally inhabit Monterey Bay during the upwelling season, this study found that the whales closely track the upwelling process on a very fine scale of both space (kilometers) and time (days to weeks).

“Tracking many individual wild animals simultaneously is a challenge in any ecosystem. It is especially difficult in open oceanthat is often opaque to us as human observers,” said William Estreich, formerly a graduate student at Stanford University’s Hopkins Marine Station and now a postdoctoral fellow at MBARI.

“Integrating technologies to measure the sounds of these whales has made this important discovery about how groups of predators find food in a dynamic ocean. We are very excited about the future discoveries we can make by eavesdropping on blue whales and other noisy ocean animals.”

Background

Blue whales (Balaenoptera musculus) are the largest animals on Earth, but despite their large size, scientists have many unanswered questions about their biology and ecology. These gentle giants congregate seasonally in the Monterey Bay region to feed on small shrimp-like crustaceans called krill.

Blue whales are elusive animals. They can travel long distances underwater very quickly, making them difficult to track. MBARI researchers and staff used a new technique to track blue whales – sound.

MBARI’s MARS (Monterey Accelerated Research System) observatory offers a platform for exploring the ocean in new ways. In 2015, MBARI researchers installed a hydrophone, or underwater microphone, at the observatory. The treasure trove of acoustic data obtained from the hydrophone has provided important information about the ocean’s soundscape, from the migratory and feeding behavior of blue whales to the impact of noise from human activities.

In 2019, MBARI and the Naval Postgraduate School installed a second hydrophone at the observatory. A directional hydrophone determines the direction the sound is coming from. This information can reveal the spatial patterns of sounds underwater, determining where the sounds are coming from. By tracking the B call of blue whales—the loudest and most common call among the regional blue whale population—researchers were able to follow the movements of individual whales as they searched for food in the region.

The researchers compared the directional hydrophone recordings with data recorded by tags Stanford University scientists had previously deployed on blue whales. Validation of this new acoustic tracking method opens up new possibilities for simultaneously recording the movements of multiple whales. It could also enable animal tagging research, helping researchers find whales to tag.

“The integrated set of technologies demonstrated in this paper represents a transformative toolkit for interdisciplinary research and mesoscale ecosystem monitoring that can be deployed at scale across protected marine habitats. This is a game-changer and takes cetacean biology and biological oceanography to a new level,” said Jeremy Goldbogen, assistant professor at Stanford University’s Hopkins Marine Station and co-author of the study.

This new methodology has implications not only for understanding how whales interact with their environment and with each other, but also for advancing management and conservation.

Despite protection, blue whales remain endangered, primarily due to the risk of collisions with ships. This study found that blue whales in the Monterey Bay National Marine Sanctuary regularly occupy habitat crossed by shipping lanes. Acoustic whale tracking can provide real-time information to resource managers to reduce risk, for example by reducing vessel speed or changing route during critical periods.

“Such integrated tools could allow us to spatially and temporally monitor and eventually even predict ephemeral biological hotspots. This promises to be a breakthrough in adaptive risk management for protected and endangered species,” said Brandon Southall, president and senior fellow at Southall Environmental Associates Inc. and co-author of the research study.