In the last 15 years, there have been 187 recorded cetacean strandings around the main Hawaiian islands. None of those animals survived. Determining the cause of a stranding is difficult. Beached carcasses quickly decay, which often interferes with scientists’ ability to determine a cause of death. Globally, there have only been a handful of strandings that are attributed to active sonar use, and most have been mass events associated with naval training. In Hawaii the closest link between sonar and marine mammal distress is the near stranding of the 200 melon-headed whales in Hanalei Bay.

Over the last five years, the Navy has committed about $160 million to help scientists — and the military — better understand which cetacean species react to Navy sonar exercises, what types of reactions occur and the duration and magnitude of these responses. More than $2.5 million in Navy-funded marine mammal monitoring research has been conducted off Hawaii and Southern California so far this year.

Still, not much is known about how different species react to one-time sonar exposure versus repeated exposure. As the Navy adjusts where and how often it blasts sonar in the Pacific, this area of study is becoming increasingly critical.

Robin Baird, a marine biologist who leads Navy-funded studies on the interplay between marine mammals and underwater noise for the Cascadia Research Collective in Olympia, Washington, said last month’s settlement may have unintended consequences. It’s feasible, he said, that by agreeing to reduce the frequency of sonar events in some Hawaiian waters, the Navy could cause local dolphins and whales more harm than good.

He speculated that stationary whale and dolphin populations in Hawaii that are repeatedly exposed to sonar throughout their lifetime gradually become less startled by sonar the more they hear it. The corollary to this theory is that marine mammals that pass through areas where sonar is being blasted might be more likely to startle in reaction to the foreign noise. If the frequency of sonar events in certain Hawaiian waters is significantly reduced, he reasoned that resident animal populations that appear to have grown accustomed to the noise might begin to act out.

“If you reduce the use of sonar, you’re going to have more calves who have never heard sonar who are effectively going to have greater risk than if they were exposed [more frequently] and had developed some understanding of it,” Baird said.

It's a complicated situation. Until more is known about the relationship between marine mammals and noise pollution, he said, it’s feasible that numerous small resident whale and dolphin populations off Hawaii might suffer under the new sonar limitations that seek to protect the animals.

“The only way you’re going to protect those populations, in my opinion, is if you completely exclude sonar from the range of those populations,” Baird said. “I’m not convinced that limiting it is going to provide any meaningful protection.”