Eavesdropping on the largest animal on Earth

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 seasonally gather in the Monterey Bay region to feed on small shrimp-like crustaceans called krill, allowing MBARI researchers to better understand their complex lives.

But blue whales are elusive animals. They can quickly travel large distances underwater, making them challenging to track. MBARI researchers and collaborators have employed a natural technique for tracking blue whales: sound.

MBARI’s MARS (Monterey Accelerated Research System) observatory offers a platform for studying the ocean in new ways. Funded by the National Science Foundation, this cabled observatory provides continuous power and data connectivity to support a variety of instruments for scientific experiments, including a pair of hydrophones, or underwater microphones.

When blue whales dive out of sight beneath the ocean’s surface, scientists turn to the whales’ booming vocalizations to study their behavior. Image: William Oestreich (NMFS Permit #16111)

The ocean soundscape is a continuously changing mosaic of sounds that originate from living organisms, natural processes, and human activities. Listening to sound in the sea is a rich exploration of the marine environment. Sound travels powerfully through the sea, over great distances. Many animals use sound in their essential life activities—communicating, foraging, reproducing, socializing, and navigating a vast and largely dark world. Sound recordings capture the signatures of these life activities, thereby revealing animal presence and behavior.

Since 2015, a hydrophone on the observatory has been recording audio beneath the ocean’s surface. The trove of acoustic data from the hydrophone has provided important insights into the ocean and its inhabitants.

In 2019, MBARI and the Naval Postgraduate School installed a second directional hydrophone on the observatory. The directional hydrophone records sounds and identifies the direction from which they originate. This information can enable monitoring of habitat use by protected species that produce sound.

MBARI researchers and collaborators listened for the booming vocalizations of blue whales. By tracking the blue whales’ B call—the most powerful and prevalent vocalization among the regional blue whale population—researchers could follow the movements of individual whales as they foraged within the region. The team’s work revealed how these ocean giants respond to changes in the wind.

Along California’s Central Coast, spring initiates an annual period of coastal upwelling. From spring through fall, coastal winds episodically push the top layer of water out to sea, allowing the cold water below to rise to the surface. The cooler, nutrient-rich water fuels blooms of tiny phytoplankton, jumpstarting the food web in Monterey Bay, from small shrimp-like krill all the way to giant whales. When the winds create an upwelling event, blue whales seek out the plumes of cooler water, where krill are most abundant. When upwelling stops, the whales move offshore into habitat that is transected by shipping lanes.

A directional hydrophone on MBARI’s cabled observatory has allowed researchers to track the movements of blue whales by following the sound of their booming calls. Image: © 2019 MBARI

Previous MBARI research revealed that swarms of forage species—anchovies and krill—react to coastal upwelling. When coastal upwelling is strongest, anchovies and krill form dense swarms within upwelling plumes. This time, researchers combined satellite and mooring data of upwelling conditions and echosounder data on krill aggregations with the acoustic tracks of foraging blue whales logged by the directional hydrophone. The research team learned that blue whales track the dynamic upwelling plumes where abundant food resources are available.

Blue whales recognize when the wind is changing their habitat and identify places where upwelling aggregates their essential food, krill. For a massive animal weighing up to 150 tonnes (165 tons), finding these dense aggregations is a matter of survival.

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

Tracking many individual animals simultaneously is challenging in any ecosystem, especially in the open ocean. This new methodology has implications not only for understanding how whales interact with their environment and one another but also for advancing management and conservation.

By studying vocalizations from “supergroups” of blue whales while they feasted on krill in Monterey Bay, researchers observed that rather than keeping quiet about finding an abundance of food, individual whales called to others to share in the feast. Image: © Duke Marine Robotics and Remote Sensing (NMFS Permit #16111)

Support for this research was provided by the David and Lucile Packard Foundation. The National Science Foundation funded the installation and maintenance of the MARS cabled observatory through awards 0739828 and 1114794. Directional acoustic processing work was supported by the Office of Naval Research, Code 32. Tag work was funded in part by the National Science Foundation (IOS-1656676), the Office of Naval Research (N000141612477), and a Terman Fellowship from Stanford University.

Working with collaborators from around Monterey Bay, MBARI researchers have opened the door to understanding the behavioral context of patterns in our acoustic recordings of whales.

One study led by graduate students at Stanford University’s Hopkins Marine Station leveraged biologging tags, acoustic prey mapping, hydrophone recordings of social cues, and remote sensing of ocean currents to investigate the ecosystem dynamics underlying unusually dense aggregations of blue whales—up to 40 of the giants within a one-kilometer radius area.

The combination of oceanographic conditions and seafloor terrain (bathymetry) concentrated large numbers of shrimp-like crustaceans called krill, which are the primary food of blue whales. The immense size of the krill swarms allowed these “supergroups” of blue whales to forage together without exhausting the food supply.

In the hours immediately preceding these remarkable aggregations, MBARI’s hydrophone recorded anomalously dense clusters of a specific blue whale call type associated with foraging.

Small shrimp-like crustaceans called krill are an important part of the diet of blue whales. Seasonal upwelling drives dense krill aggregations in Monterey Bay, attracting foraging whales. Image: © 2003 MBARI

The hydrophone recordings revealed that, counterintuitively, the whales exhibited a social foraging strategy. Rather than competing for food, blue whales called to other whales to signal food was present. The blues’ bellows invited others to join the feast.

Modeling social interactions indicated that using social information from other whales reduced the time required for individual whales to discover and exploit the dense patches of food they need to survive. The whales’ foraging became more efficient without any apparent costs to the caller who first found the patch of food.

A second study expanded upon previous work by researchers at MBARI and Stanford University that documented distinct seasonal changes in blue whale vocalizations that reveal when these gentle giants begin their annual migration. During summer and early fall, blue whales sing more during the night. Later in the fall and into winter, the whales begin singing more during the day. This change coincides with the time of year when the whales reduce feeding and begin their annual southward migration. Data from biologging tags confirmed that the acoustic signature detected by the hydrophone reflected changes in the whales’ behavior.

Now, MBARI hydrophone data have revealed how blue whales change the timing of their migration back to breeding areas from year to year.

“This research and its underlying technologies are opening new windows into the complex and beautiful ecology of these endangered whales. Our findings demonstrate a new resource for managers seeking ways to better protect blue whales and other species.” —MBARI Senior Research Specialist John Ryan

We have long known that whales time their migratory movements with natural cycles in their marine habitat, especially seasonal changes in productivity. But how populations adjust the timing of their migrations in response to year-to-year environmental variability has remained unclear.

The data, collected from summer 2015 through spring 2021, recorded the bellowing vocalizations of blue whales in the Monterey Bay region. Sound signaled when whales stopped foraging on the local abundance of krill to begin their southward breeding migration. To the team’s surprise, the start of the whales’ migration could vary up to four months from year to year.

Considering that the blue whale breeding season itself spans only approximately four months, this large variation in the timing of migration was initially puzzling. Here, data about annual ecosystem changes offered important clues.

Migration timing closely followed conditions within the whales’ foraging habitat. Specifically, blue whales lingered longer off Central California when the ecosystem provided more opportunity for them to build energy stores. A later transition from foraging to migration occurred in years with an earlier onset, later peak, and greater accumulation of biological productivity.

These findings suggest that in years of the highest and most persistent biological productivity, blue whales wait to begin their southward migration. Researchers believe the whales do not simply depart toward their southern breeding grounds as soon as sufficient energy reserves are accumulated. Rather, the whales delay their migration when food is plentiful to maximize their energy intake on their foraging grounds.

Exploration of the ocean soundscape is essential to stewardship.

Despite protections, blue whales remain endangered, primarily from the risk of collisions with ships. The work of the ocean soundscape team and their collaborators has found that blue whales in Monterey Bay National Marine Sanctuary regularly occupy habitat transected by shipping lanes. Acoustic tracking of whales may soon provide real-time information for resource managers to mitigate risk, for example, through vessel speed reduction or rerouting during critical periods. The ability to monitor and ultimately predict whale movements could open the door to adaptive management for protected and endangered species.

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