For the first time ever, scientists have directly measured the behavioral responses of some of the most common marine mammals to military sonar.
Social dolphins, which can occur in groups of hundreds or even thousands, are very common off California and many other coastal regions. They are regularly exposed to powerful military sonar systems known to disturb, harm, and even kill other species. Until now, there were no direct data on whether and how these sonars might impact these most abundant dolphins, despite regulatory assessments predicting that millions of animals might be impacted annually.
A unique integration of research methods focused on two social dolphin species provides some answers. A new paper in the journal Royal Society Open Science entitled “Behavioural Responses of Common Dolphins to Naval Sonar” uses a novel design that combines aerial drone imagery, underwater listening recorders, and shore-based visual observers.
“Many of our earlier studies of deep-diving, solitary beaked whales and large, endangered baleen whales involved deploying tags on individual animals, but asking these questions for highly gregarious species requires a totally different approach,” said lead author Dr. Brandon Southall, a Senior Scientist at Southall Environmental Associates (SEA) and a Research Associate at the University of California, Santa Cruz (UCSC). “This complementary combination of acoustic and visual sensors sampling different space and time resolution allows us to measure behavior at the correct level of analysis, namely groups of animals.”
“What makes this study unique is the combination of different tools and methodologies that are merged together to develop a holistic view of how these social animals behave and respond to acoustic disturbance” said Dr. Ari Friedlaender, co-author and Professor at UC Santa Cruz.
The study includes several substantial technological advances to measure these fast and gregarious species for which fine-scale aspects of behavior had often been deemed too difficult to study. “This is an exciting application of drone photogrammetry – the process of taking measurements from photographs collected non-invasively with drones – which we have pioneered in the study of whale and dolphin health over the last decade” said Dr. John Durban, a Senior Scientist with SEA. Durban notes that “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”.
By sampling 34 different dolphin groups with thousands of individuals in experiments where simulated as well as operational military sonars were activated in carefully controlled conditions (or deliberately not played in experimental ‘control’ conditions), the types and likelihood of responses to known sonar exposures could be determined. The responses were surprising.
“We see clear evidence of acoustic responses, fine-scale changes in movement including directed, sustained, strong avoidance, and changes in group configurations,” said Southall. “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.”
Study co-author Dr. Caroline Casey, a Research Associate at UCSC added: “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”.