Sound reveals giant blue whales dance with the wind to find food

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

Spring and summer bring coastal upwelling along California’s central coast. From March to July, seasonal winds push the top layer of water out to sea, allowing the cold water below to rise to the surface. The cooler, nutrient-rich waters fuel the blooms of tiny phytoplankton, kick-starting the food web in Monterey Bay, from tiny shrimp-like krill to giant whales. When the wind creates an upwelling, blue whales seek out the plumes of cooler waters, where krill are most common. When the upwelling stops, the whales move offshore to a habitat intersected by shipping lanes.

“This research and the underlying technologies 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 this study. “These findings provide a new resource for managers looking for ways to better protect blue whales and other species.”

The directional hydrophone is a specialized underwater microphone that picks up sounds and identifies which direction they are coming from. To use this technology to study blue whale movements, researchers needed to confirm that the hydrophone was reliably tracking whales. This meant matching the acoustic soundings to a calling whale being tracked by GPS. Confident in the established acoustic methods, the research team investigated two years of acoustic tracking of the regional blue whale population.

This study built on previous research led by MBARI Senior Scientist Kelly Benoit-Bird, which found that swarms of forage crops — anchovies and krill — responded to coastal upwelling. This time, researchers combined satellite and docking data from upwelling conditions and sonar data on krill aggregations with the acoustic trails of foraging blue whales captured by the directional hydrophone.

“Previous work by the MBARI team showed that when upwelling along the coast was strongest, anchovies and krill formed dense swarms in upwelling plumes. Now we’ve learned that blue whales follow these dynamic plumes where abundant food sources are available.” explains Ryan.

Blue whales recognize when winds change their habitat and identify places where upwelling gathers their vital food – krill. For a huge animal weighing up to 165 tons, finding these dense collections is a matter of survival.

While scientists have long recognized that blue whales seasonally occupy Monterey Bay during the upwelling season, this research has revealed that the whales closely monitor the upwelling process on a very fine scale of both space (miles) and time (days to weeks).

“Tracking many individual wildlife simultaneously is a challenge in any ecosystem. This is especially difficult in the open ocean, which is often opaque to us as human observers,” said William Oestreich, a former graduate student at the Hopkins Marine Station of the United States. Stanford University and now a postdoctoral researcher at MBARI.

“The integration of technologies to measure the sounds of these whales enabled this important discovery about how groups of predators find food in a dynamic ocean. We are excited about the future discoveries we can make by targeting blue whales and other noisy ocean creatures. to listen.”

Background

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

Blue whales are elusive animals. They can travel great distances underwater very quickly, making them difficult to track. MBARI researchers and collaborators used a new technique for tracking the sound of blue whales.

MBARI’s MARS (Monterey Accelerated Research System) observatory provides a platform to study the ocean in new ways. In 2015, MBARI researchers installed a hydrophone, or underwater microphone, at the observatory. The hydrophone’s wealth of acoustic data has provided important insights into the soundscape of the ocean, 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. The directional hydrophone indicates the direction from which a sound originates. This information can reveal spatial patterns for sounds underwater and identify where sounds come from. Tracking the B-call of blue whales — the most powerful and common vocalization among the regional blue whale population — allowed researchers to track the movements of individual whales as they forage in the region.

Researchers compared the recordings from the directional hydrophone with data captured by tags that scientists at Stanford University had previously used on blue whales. Validating this new acoustic tracking method opens up new possibilities for simultaneously recording the movements of multiple whales. It could also enable animal-borne tag research by 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 in protected marine habitats. This is a game-changer, bringing both whale biology and biological oceanography to a higher level,” said Jeremy Goldbogen, an associate 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, mainly due to the risk of collision with ships. This study showed that blue whales in Monterey Bay National Marine Sanctuary regularly occupy habitat crossed by shipping lanes. Acoustic whale tracking can provide real-time information for resource managers to mitigate risk, such as reducing vessel speed or diverting during critical periods.

“These kinds of integrated tools could enable us to spatially and temporally track and ultimately even predict ephemeral biological hot spots. This promises to be a turning point in adaptive risk management for protected and endangered species,” said Brandon Southall, president. and senior scientist for Southall Environmental Associates Inc. and a co-author of the study.