Dolphins display strong avoidance behavior in response to naval sonar systems

For the first time, researchers from UC Santa Cruz have directly measured how common marine mammals react to military sonar.

This study reveals that dolphins are sensitive to sonar sounds at much lower levels than previously thought. The research team observed 34 dolphin groups, totaling thousands of individuals.

They conducted experiments using both simulated and real military sonar in controlled conditions, alongside control conditions where sonar was not played. This allowed the scientists to analyze how these animals respond to known sonar exposures, leading to unexpected findings.

Surprising behavioral responses

Lead author Brandon Southall, a research associate at UC Santa Cruz and senior scientist at Southall Environmental Associates (SEA), stated, “We see clear evidence of acoustic responses—fine-scale changes in movement including directed, sustained, strong avoidance, and changes in group configurations.”

He continued, “While these behavioral changes occur and persist on variable time scales, they are surprising in that they collectively demonstrate responses at sound levels that are orders of magnitude lower than predicted in current regulatory impact assessments. These animals are clearly much more sensitive to noise exposure than we thought.”

These findings are significant because social dolphins often gather in large groups, especially along the California coast, where they regularly encounter powerful military sonar systems . Such sonar is known to disturb, harm, and even kill various marine species.

Until now, there had been no direct data on how these systems affect dolphins, despite predictions that millions of animals might be impacted annually.

Innovative research methods

To conduct this groundbreaking study, the team combined multiple research methods. They utilized aerial drone imagery, underwater listening devices, and shore-based visual observers to monitor the behavior of two social dolphin species.

In a press release, co-author Ari Friedlaender, a professor of ocean sciences at UC Santa Cruz, explained , “The merging of methodologies and tools presented a holistic view of how these social animals behave and respond to acoustic disturbances.”

The team employed significant technological advances to observe these fast and playful species , which were previously challenging to study in detail. One such method was “drone photogrammetry,” which involves taking measurements from photographs collected non-invasively using drones.

John Durban, another senior scientist at SEA, highlighted the importance of this technique. “In this study, we have been able to further develop this technique to geolocate dolphins with centimeter-level precision, enabling changes in behavior to be quantified in an objective way,” said Durban.

Addressing the impact of sonar

The concern surrounding the impact of sonar on marine life has grown, especially after mass strandings of cetaceans coincided with naval sonar use worldwide. These incidents typically occurred when tactical sonars operated at mid-frequency levels ranging from 1 to 10 kHz, with many operating in the 3–4 kHz range, as noted in the study.

Caroline Casey, a researcher at UC Santa Cruz and co-author of the study, emphasized the significance of understanding how dolphins respond to these acoustic signals.

She said, “Understanding how these animals respond to these types of acoustic signals is important for mitigating the impacts that this type of disturbance can have on social animals that rely on acoustics for communication, feeding, and other critical facets of their lives.”

This study was published in the journal Royal Society Open Science .