Scientists have conducted a groundbreaking study that measures how common marine mammals react to military sonar, discovering that these creatures are affected by lower sound levels than previously believed.
For the very first time, a group of scientists from UC Santa Cruz has directly observed the responses of widely recognized marine mammals to military sonar. They found that these animals are sensitive to sound at much lower volumes than earlier expected.
In a recent study published on October 23 in the journal Royal Society Open Science, an international team examined 34 groups of dolphins—totaling thousands of individuals—under controlled conditions where they either played simulated and real military sonar or refrained from any sound as a control. By analyzing their responses to known sonar stimuli, the researchers uncovered some surprising behaviors.
“There is clear evidence of acoustic responses—detailed changes in movement including strong avoidance and shifts in group dynamics,” stated lead author Brandon Southall, a research associate at UC Santa Cruz and senior scientist at Southall Environmental Associates (SEA). “While these behavioral changes vary over time, it’s surprising to see them occur at sound levels much lower than those predicted in existing regulatory assessments, indicating these animals are more sensitive to noise than we had imagined.”
The researchers used an innovative combination of methods, including aerial drone imagery, underwater sound recorders, and visual observers from shore to study two social dolphin species. This integrated approach provided a comprehensive understanding of how these social animals react to sound disturbances, according to co-author Ari Friedlaender, an ocean sciences professor at UC Santa Cruz.
Utilizing advanced technology allowed researchers to track these fast and playful species more effectively, as observing their behavior in fine detail has often been considered challenging. This included the use of “drone photogrammetry,” a technique that gathers measurements from non-invasive drone photographs. John Durban, a senior scientist at SEA, highlighted that they have advanced this method over the last ten years to assess the health of whales and dolphins. “In this study, we’ve refined this technique to precisely pinpoint the location of dolphins, allowing us to objectively quantify changes in their behavior,” said Durban, who is also a co-author.
Social dolphins, capable of gathering in groups numbering hundreds or thousands, are frequently found off the coast of California and elsewhere, often coming into contact with powerful military sonar systems known to disrupt, harm, or even kill other species. Until now, there had been no direct evidence of how these sonar systems might impact these abundant dolphins, even though regulatory assessments had indicated that millions of marine animals could be affected each year.
Furthermore, recent instances of mass beachings of whales and dolphins have occurred alongside the deployment of sonar systems by navies globally, raising concerns that this noise exposure poses risks to cetaceans, including whales, dolphins, and porpoises. These events often coincided with the use of tactical sonars operating at “mid-frequency,” typically within the range of 1 to 10 kHz, while most operating in the 3-4 kHz band, as noted by the study.
“It is critical to understand how these animals respond to such acoustic signals to mitigate the potential effects these disturbances can have on social animals that rely on sound for communication, feeding, and other essential activities in their lives,” added study co-author Caroline Casey, a researcher at UC Santa Cruz.
