Dolphins hear military sonar over vast distances: new study

.In the new study, published 23 October in the journal Royal Society Open Science, the team studied 34 groups of dolphins, numbering thousands of individuals. The experiments used both simulated and real military sonars, which were activated under carefully controlled conditions or not activated in control situations. The researchers determined the types and likelihood of responses to known sonar exposures, revealing unexpected behavioural changes.

'We observe clear signs of acoustic responses - small-scale changes in movement, including directed and sustained avoidance, and changes in group configuration,' said lead author Brandon Southall, a UC Santa Cruz researcher and senior scientist at Southall Environmental Associates. - These behavioural changes occur and persist over different time scales, but what is surprising is that they occur at sound levels that are orders of magnitude lower than current regulatory estimates predict. These animals are clearly much more sensitive to noise than we thought.

For the study, the team used a novel approach, combining aerial drone photography, underwater acoustic recorders and ground-based visual observations to monitor two social dolphin species. This combination of methods provided a holistic view of how these social animals behave and respond to acoustic disturbances.

Social dolphins, which can congregate in groups of hundreds or even thousands of individuals, are common off the coast of California and in many other regions where they regularly encounter powerful military sonars known to disturb, harm and even kill other species. Until now, there has been no direct data on how these sonars may be affecting these most common dolphins, even though regulatory estimates predict that millions of animals may be impacted each year.

In recent years, mass cetacean landings coinciding with sonar use by naval forces around the world have raised concerns about the potential threat such noise impacts pose to whales, dolphins and porpoises. These incidents occurred when tactical sonars were operating at "medium frequencies" between 1 and 10 kHz.

Understanding how these animals respond to such acoustic signals is important to mitigate the impact of this type of disturbance on social animals that rely on sound for communication, feeding and other critical aspects of their lives, said study co-author Carolyn Casey of UC Santa Cruz.

The study, titled"Behavioural responses of common dolphins to naval sonar," was conducted with the participation of scientists from several international institutions. They hope that the findings will help develop measures to protect these animals from the negative effects of acoustic noise.