Naval sonar affects dolphins at shockingly low levels – Here’s why that matters

October 25, 2024Environment

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Scientists have discovered that dolphins are far more sensitive to military sonar systems than previously believed, responding to sound levels far below what experts thought would disturb them. The alarming finding represents the first experimental investigation of how dolphins respond to military mid-frequency active sonar (MFAS) – the same type of sonar that has been linked to mass strandings of some whale species.

The research team, led by Brandon Southall from UC Santa Cruz and Southall Environmental Associates, conducted an extensive study off the California coast between 2017 and 2021. They observed 34 different dolphin groups, comprising thousands of individuals from two species: short-beaked and long-beaked common dolphins. These dolphins frequently encounter military sonar in their natural habitat, as they’re among the most numerous marine mammals in areas where naval operations occur.

The study’s findings, published in the Royal Society Open Science journal, challenge current regulatory guidelines about how sonar affects marine mammals. The dolphins showed clear behavioral changes at sound levels significantly lower than the threshold currently used by regulators to predict responses. This discovery is particularly important because military sonar systems, which typically operate in the 3-4 kHz frequency range, have been associated with mass strandings of some whale species in various locations worldwide.

Using a sophisticated combination of monitoring techniques, including drone photography, underwater microphones, and shore-based observers, the researchers tracked how dolphin groups ranging from 12 to 900 individuals responded when exposed to both simulated and real military sonar signals. This multi-method approach allowed them to capture different aspects of dolphin behavior simultaneously, from individual movements to group dynamics.

The results revealed distinct patterns of response between the two dolphin species. Short-beaked common dolphins typically increased their swimming speed and showed changes in group cohesion, with separate subgroups coming together after exposure. Long-beaked common dolphins often showed opposite reactions, slowing down during exposure and splitting into more subgroups afterward.

Importantly, these behavioral changes occurred at much lower sound levels than the threshold currently used by regulators to predict dolphin responses. The current guidelines assume dolphins won’t show significant responses until exposed to sonar at 160 decibels (underwater). However, this study found clear behavioral changes at substantially lower levels.

“We see clear evidence of acoustic responses—fine-scale changes in movement including directed, sustained, strong avoidance, and changes in group configurations,” says Southall, a UC Santa Cruz research associate and senior scientist at Southall Environmental Associates (SEA), in a statement. “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.”

The research has significant implications for naval operations and marine mammal protection. While there’s no evidence that these behavioral disruptions lead to the kind of mass strandings seen in some whale species, the study suggests that current guidelines may need revision to better protect these marine mammals from unnecessary disturbance.

“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,” says study co-author Caroline Casey, a researcher at UC Santa Cruz.

The research team’s findings, supported by the U.S. Office of Naval Research’s Marine Mammals and Biology Program, open new avenues for understanding and protecting marine mammals. As human activities in the ocean continue to increase, this knowledge will be crucial for developing more effective conservation strategies while maintaining essential naval operations.

Paper Summary

Methodology

The research team pioneered an innovative approach combining multiple observation methods. They used “drone photogrammetry” – taking precise measurements from drone photographs – which allowed them to track dolphin movements with centimeter-level accuracy.

Underwater microphones recorded dolphin vocalizations, while shore-based observers tracked overall group movements. Each experiment lasted 30 minutes total, with 10-minute segments before, during, and after sonar exposure. They also conducted control experiments without sonar to ensure any observed changes were indeed related to the sonar exposure.

Key Results

The study documented clear acoustic and behavioral responses in both dolphin species. These included fine-scale changes in movement, sustained avoidance behaviors, and changes in group configurations. Importantly, these responses occurred at sound levels far below what current regulatory assessments predict would cause disturbance. The behavioral changes varied between species and persisted for different durations.

Study Limitations

The researchers faced several challenges, including the dolphins’ naturally dynamic movement patterns and the potential influence of environmental factors like prey presence or other vessels. The relatively short duration of the experiments may have limited their ability to detect longer-term behavioral changes.

Discussion & Takeaways

This research provides crucial new information about how common dolphins respond to military sonar in their natural environment. It emphasizes the importance of understanding how acoustic disturbances affect social marine mammals that rely on sound for communication, feeding, and other vital activities. The findings suggest that current regulatory guidelines may need significant revision to better protect these animals.

Funding & Disclosures

The research was funded by the U.S. Office of Naval Research’s Marine Mammals and Biology Program and conducted under various federal permits. The study involved collaboration between multiple institutions, including UC Santa Cruz, and the authors declared no competing interests.

Source: Study Finds

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Image: Picture of long-beaked common dolphin off Southern California taken during research authorized by permit #19091 from the U.S. National Marine Fisheries Service. (Photo by John Durban)