In a study published on October 23 in Royal Society Open Science, the international research team observed 34 dolphin groups, involving thousands of individuals, under controlled conditions using both simulated and operational sonar. They discovered that dolphins exhibited unexpected behaviors in response to sonar exposure.
"We see clear evidence of acoustic responses - fine-scale changes in movement including directed, sustained, strong avoidance, and changes in group configurations," said lead author Brandon Southall, a research associate at UC Santa Cruz and senior scientist at Southall Environmental Associates (SEA). He noted that the behaviors appeared at sound levels far below those predicted in current regulatory assessments. "These animals are clearly much more sensitive to noise exposure than we thought," Southall added.
The team used innovative techniques combining drone imagery, underwater recorders, and visual observation to monitor two dolphin species. Co-author Ari Friedlaender, a professor at UC Santa Cruz, explained that the integration of these tools gave the researchers a comprehensive view of how dolphins respond to acoustic disturbances.
Technological advances played a crucial role in the study. John Durban, another senior scientist at SEA, highlighted how drone photogrammetry, a method used to measure animals with high precision from aerial photographs, allowed them to track and quantify dolphin behavior with centimeter-level accuracy.
Dolphins are commonly found off the coast of California, where they frequently encounter military sonar. Despite previous regulatory estimates that millions of dolphins might be affected annually, this is the first study to provide direct data on how these sonar systems influence dolphin behavior.
Mass strandings of marine mammals, coinciding with the use of naval sonar, have raised concerns in recent years. Sonar systems typically operate at mid-frequency (1 to 10 kHz), with many in the 3-4 kHz range, posing potential threats to species such as whales, dolphins, and porpoises.
"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," said Caroline Casey, another UC Santa Cruz researcher and co-author of the study.
The study, titled Behavioural responses of common dolphins to naval sonar, also involved researchers from Oregon State University's Hatfield Marine Science Center, Cascadia Research Collective, Stanford University, SeaLife Response, Rehabilitation and Research, and several institutions in the Netherlands, including Kelp Marine Research and the Royal Netherlands Institute for Sea Research.
Research Report:Behavioural responses of common dolphins to naval sonar
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