Researchers have discovered that military sonar can change blue whale behavior. They found that some blue whales located off the coast of California alter their behavior when exposed to sonar during U.S. military exercises. The study’s findings reveal that whales may change diving behavior or temporarily steer clear of certain feeding areas.

According to a Duke University news release, the Southern California Behavioral Response Study analyzed what happened when tagged blue whales were exposed to simulated mid-frequency sonar sounds much less intense that the U.S. military utilizes.

“Mid-frequency military (1–10 kHz) sonars have been associated with lethal mass strandings of deep-diving toothed whales, but the effects on endangered baleen whale species are virtually unknown,” the authors write in the study’s abstract.

According to lead author Jeremy Goldbogen of Cascadia Research, the responses to mid-frequency sonar sounds are complex and rely on several factors, including whether whales were feeding in deep water, shallow water or not at all.

“Despite using source levels orders of magnitude below some operational military systems, our results demonstrate that mid-frequency sound can significantly affect blue whale behavior, especially during deep feeding modes,” the researchers add.

According to the news release, the researchers tagged the whales with non-invasive suction cups, which document acoustic data and high-resolution activities as the blue whales were exposed to the mid-frequency sonar sounds.

Ari Friedlaender, a research scientist at the Duke University Marine Laboratory, notes that the non-invasive suction cups provide researchers with a rare look into the underwater behavior of whales that would normally be almost impossible to obtain.

The researchers discovered the some of the blue whales feeding in deep water quit eating and either swam faster or zoomed away from the source of the sonar. The researchers also found that not all of the whales reacted to the noise, and not all in the same way.

“When a response occurred, behavioural changes varied widely from cessation of deep feeding to increased swimming speed and directed travel away from the sound source,” the researchers point out. “The variability of these behavioural responses was largely influenced by a complex interaction of behavioural state, the type of mid-frequency sound and received sound level.”

According to John Calambokidis, one of the study’s lead investigators of Cascadia Research, blue whales show up regularly off the southern California coast, where they feed. National Geographic notes that blue whales reside in all the world’s oceans usually swimming alone or in pairs. They typically spend summers feeding in cold waters and migrate towards the Equator when the weather turns chillier.

The area off the southern California coast is the site of military exercises that can involve military-grade sonar sounds. Prior to this study, very little information existed on whether or not blue whales reacted to sonar in the same way that other whales and marine mammal species do.

According to Brandon Southall, an adjunct researcher at both Duke University and the University of California at Santa Cruz, these results help researchers learn more about the threats to blue whales from human sound and guide conservation efforts.

The study’s findings are described in greater detail in the journal Proceedings of the Royal Society B.