Military Sonar Alters Whale Behavior

Some blue whales (Balaenoptera musculus) off the coast of California change their behavior when exposed to the sort of underwater sounds used during U.S. military exercises. The whales may alter diving behavior or temporarily avoid important feeding areas, according to new research by the Southern California Behavioral Response Study.

Researchers exposed tagged blue whales in the California Bight to simulated mid-frequency (3.5-4 kHz) sonar sounds significantly less intense than the military uses. "Whales clearly respond in some conditions by modifying diving behavior and temporarily avoiding areas where sounds were produced," said lead author Jeremy Goldbogen of Cascadia Research. "But overall the responses are complex and depend on a number of interacting factors," including whether the whales were feeding deep, shallow or not at all.

The scientists tagged the whales with non-invasive suction cups, which recorded acoustic data and high-resolution movements as the animals were exposed to the controlled sounds (Figure 1). 

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Figure 1. Examples of behavior changes of tagged blue whales during exposure experiments. The sound exposure periods are highlighted in blue on each track line. The location of the sound source is indicated by the large red circle. (From Goldbogen et al. 2013)

"The tag technology we use offers a unique glimpse into the underwater behavior of whales that otherwise would not be possible," said Ari Friedlaender, a research scientist at the Duke Marine Laboratory.

The scientists found that some of the whales engaged in deep feeding stopped eating and either sped up or moved away from the source of the noise. Not all of the whales responded to the noise, and not all in the same way.

"Blue whales are the largest animals that have ever lived. Populations globally remain at a fraction of their former numbers prior to whaling, and they appear regularly off the southern California coast, where they feed," said John Calambokidis, one of the projects lead investigators.

That area of the ocean is also the site of military training and testing exercises that involve loud mid-frequency sonar signals. Such sonar exercises have been associated with several unusual strandings of other marine mammal species (typically beaked whales) in the past. Until this study, almost no information was available about whether and how blue whales respond to sonar. 

"These are the first direct measurements of individual responses for any baleen whale species to these kinds of mid-frequency sonar signals," said Brandon Southall, chief scientist from SEA, Inc. "These findings help us understand risks to these animals from human sound and inform timely conservation and management decisions."

A related paper published by the same research team in
Biology Letters has shown clear and even stronger responses of Cuvier’s beaked whales (Ziphius cavirostris) to simulated mid-frequency sonar exposures. Beaked whales showed a variety of responses to both real, military sonar in the distance and nearby simulated sonar. What the beaked whales were doing at the time appeared to be a key factor affecting their reactions.

The research was funded by the U.S. Navy Chief of Naval Operations Environmental Readiness Division and the U.S. Office of Naval Research, and appears in the Proceedings of the Royal Society B. 

Source: Modified from materials provided by Duke University.

References
Goldbogen, JA et al. (2013). Blue whales respond to simulated mid-frequency military sonar Proceedings of the Royal Society B DOI: -10.1098/rspb.2013.0657

DeRuiter, S. et al. 2013. First direct measurements of behavioural responses by Cuvier's beaked whales to mid-frequency active sonar. Biology Letters, DOI – 10.1098/rsbl.2013.0223