Scientists have recorded Cuvier's beaked whales changing their behavior in response to sonar use at a Naval base in California. It's the first long-term study of whale response to real-world sonar use.

Why it matters: Scientists have known for decades that military sonar use can cause mass strandings and even deaths in marine mammals. (The Navy has faced several lawsuits over their long-distance sonar use.) Although scientists have measured marine mammal's response to sonar in the past, those studies have only gathered data over a few hours and involved scientists using technology to mimic sonar. The research was published in the journal Royal Society Open Science.

"They dive down, putting vertical space between themselves and a sudden, debilitatingly loud sound," study author Erin Falcone, a marine biologist at the nonprofit Marine Ecology and Telemetry Research, tells Axios.

What they did: The scientists used tags attached to 16 Cuvier's beaked whales to monitor their behavior. The whales lived in an area used for Naval training exercises, so the researchers were able to cross-reference the whale's behavior with Navy-provided data on when, where, and what type of sonar (from boats or helicopters) they were using.

What they found: When the whales heard sonar, they dove deeper and longer, and spent less time foraging for food. Surprisingly, the whales had different responses depending on the source of the sonar. Even though sonar from helicopters is quieter, the whales acted more evasively. Falcone thinks this is because whales are more accustomed to the boat sonar and can hear the boats coming, while the helicopter sonar comes out of nowhere.

The takeaway: It's not just the volume of sonar that can have an impact on whales, it's also the context.

The big picture: The behavioral changes seen in the whales might seem mild compared to mass strandings, but researchers still don't know the longer term health impacts sonar is having on the whales. "If the sonar makes them miss one meal, it's not a big deal. But if it happens repeatedly it could have a cumulative effect," says Falcone. Her team and others are comparing the birth rates for whales that experience regular sonar exposure to those who rarely encounter it to see the effects.