Three-Dimensional Acoustic Tracking Sheds Light on Beaked Whale Dive Behavior and Acoustic Detection

A new peer-reviewed study in PLOS One sheds light on why some beaked whale species are more detectable than others using passive acoustic monitoring . Researchers examined how differences in species-specific diving and echolocation behaviors affect the ability to detect beaked whales using underwater listening devices. Beaked whales are one of the ocean's most elusive groups of whales.

The findings are particularly important for the Gulf of America (formerly the Gulf of Mexico), a heavily industrialized region where reliable beaked whale population estimates are needed. A recent study found that beaked whale populations in the Gulf may have declined by as much as 83 percent since the 2010 Deepwater Horizon oil spill. This highlights the need to better understand them to conserve them effectively.

Beaked whales are notoriously difficult to study because they spend little time at the surface to be seen during traditional infrequent visual surveys (vessel and aerial). Passive acoustic monitoring offers a powerful alternative as continuous data is recorded for months at a time. However, this method requires knowledge of whale behavior to understand how to interpret the detection of whale sounds for population monitoring.

Acoustic Detection is Species-Specific

The study shows that detectability varies among beaked whale species, as does their dive and echolocation behavior. Goose-beaked whales ( Ziphius cavirostris ), for example, were detectable for longer periods than Blainville's ( Mesoplodon densirostris) or Gervais' ( Mesoplodon europaeus) beaked whales. Goose-beaked whales performed deeper foraging dives, often close to the seafloor, and produced clicks with higher source levels. These behavioral differences significantly increase the likelihood of detecting goose-beaked whales compared to the other two species.

These conclusions are based on three-dimensional acoustic tracking data collected in 2021 using two multi-hydrophone tracking High-Frequency Acoustic Recording Packages . Researchers deployed the instruments off the coast of Louisiana at a depth of approximately 1,100 meters. They reconstructed the underwater movements of individual whales. By measuring tiny differences in the time it took a whale's click to reach each hydrophone, we estimated the direction the sound came from, both horizontally and vertically. When directions from two listening devices were combined, we could pinpoint the whale's three-dimensional position underwater and track its diving path over time for goose-beaked, Gervais', and Blainville's beaked whales. From these tracks, scientists measured echolocation click source levels, sound beam directionality, and diving characteristics. Using these values with simulations and trial-based methods, we estimated detection probabilities for each species.

Beyond answering the primary research question, the study delivers several important scientific advances:

• First estimates of echolocation source levels and directivity for Gervais' beaked whale clicks
• First regional estimates of echolocation source levels and directivity indices for goose-beaked whales
• Reveals consistent differences in dive kinematics among species, pointing to ecological specialization that may influence habitat use.

"Most notably, this study offers a detailed description of Gervais' beaked whale diving behavior." Dr. Héloïse Frouin-Mouy (bio-acoustician at Cooperative Institute for Marine and Atmospheric Studies and lead author) explained. " Because this species has never been tagged, these acoustic observations provide a unique and unprecedented window into its underwater ecology."

These findings advance our ecological understanding of beaked whales and pave the way to explore other aspects of beaked whale life history and behavioral ecology.

Partners

This research is part of the LISTEN: Long-term Investigations into Soundscapes, Trends, Ecosystems, and Noise project, an ongoing project led by NOAA's Southeast Fisheries Science Center and University of California San Diego's Scripps Institution of Oceanography. Partners include:

Funding

This research was funded in part by the NOAA RESTORE Science Program under a grant to the Southeast Fisheries Science Center. It was funded in part by the Deepwater Horizon Open Ocean Trustee Implementation Group's " Reduce Impacts of Anthropogenic Noise on Cetaceans " project to restore natural resources injured by the oil spill.

Under the Marine Mammal Protection Act and the Endangered Species Act , NOAA Fisheries studies marine mammals, including beaked whales. We find ways to reduce risks posed to them by human activities and environmental change to recover and protect species.