Dolphins Are Far More Sensitive to Military Sonar Than Previously Thought

For the first time, scientists, including a team from UC Santa Cruz, have directly measured how common marine mammals react to military sonar. The surprising finding? Dolphins are much more sensitive to these sounds than previously predicted, reacting at significantly lower levels than current regulatory impact assessments anticipated.

Groundbreaking Research Unveils New Insights

The study, published on October 23 in Royal Society Open Science, involved an international team that conducted experiments with 34 dolphin groups, comprising thousands of individuals. The experiments tested the animals’ responses to both simulated and operational military sonar under carefully controlled conditions, as well as in situations where no sonar was played as a control measure. The results uncovered some unexpected behaviors in response to the noise.

"We see clear evidence of acoustic responses—fine-scale changes in movement, including directed, sustained, strong avoidance, and changes in group configurations," explained Brandon Southall, the study's lead author and a senior scientist at Southall Environmental Associates (SEA). "These behaviors occur on variable time scales, but they collectively show that dolphins respond to sound levels much lower than what regulatory guidelines predict. It’s clear that these animals are far more sensitive to noise exposure than we had thought."

Innovative Techniques Bring New Precision to Dolphin Research

The research utilized a novel approach by integrating aerial drone imagery, underwater listening recorders, and shore-based visual observers to monitor dolphin behavior. This combination of methods provided a comprehensive picture of how these social animals react to acoustic disturbances, says co-author Ari Friedlaender, an ocean sciences professor at UC Santa Cruz.

One key technological advancement used was "drone photogrammetry," a non-invasive technique for measuring animal behavior from drone-captured photographs. John Durban, a senior scientist at SEA and co-author of the study, noted that SEA has pioneered this approach over the past decade for monitoring whale and dolphin health. "In this study, we were able to develop this technique further to geolocate dolphins with centimeter-level precision, allowing us to quantify behavioral changes in an objective way," Durban stated.

Why Understanding Dolphin Sensitivity to Sonar Matters

Social dolphins, which often gather in groups of hundreds or even thousands, are found off California’s coast and in many other regions. These areas frequently see the use of powerful military sonar systems, known to disturb and even harm various marine species. Before this study, there had been no direct data on how such sonar might impact dolphins, despite predictions that millions of these animals could be affected annually.

Concerns about sonar's impact on marine life have increased due to recent incidents where mass strandings of cetaceans, including dolphins, coincided with naval sonar operations. These strandings typically occurred when mid-frequency tactical sonars (1-10 kHz) were used, with most systems operating in the 3-4 kHz range.

"Understanding how these animals respond to these acoustic signals is vital for mitigating the effects of this kind of disturbance," said Caroline Casey, a UC Santa Cruz researcher and study co-author. "These disturbances can significantly impact social animals that rely on sound for communication, feeding, and other critical aspects of their lives."