Case closed. Sonar appears to have driven melon-headed whales dangerously close to Hawaiian shores.

(c) Robin W. Baird/

Whale Stranding: Sonar or Lunar?

On the morning of 3 July 2004, more than 150 melon-headed whales rushed into Hanalei Bay off the Hawaiian island of Kauai, apparently bent on beaching themselves. The whales milled about for most of the day and night in an agitated manner, tail-slapping and vocalizing. A rescue team organized by the National Oceanic and Atmospheric Administration (NOAA) herded the whales back to sea the next day, though a calf died. One study blamed the incident on U.S. Navy sonar; another blamed the moon. Now, researchers believe they've finally gotten to the bottom of this attempted mass stranding.

NOAA was the first to probe the incident. In 2006, the agency concluded that naval sonar was the most plausible cause, as U.S. and Japanese submarines and ships had been conducting training exercises nearby that same morning. Robert Brownell, a cetacean biologist at NOAA's Southwest Fisheries Science Center in Pacific Grove, California, says that although scientists aren't sure how ships' sonar harms whales, he suspects that the noise "forms an acoustic barrier, and they want to escape." Because the ships involved in these exercises are normally moving parallel to the coast, the whales cannot flee out to sea, and in desperation they end up stranding. The U.S. Navy argues that the risk to whales from sonar is not as great as many scientists and conservationists believe, and it has fought to continue training exercises.

The Navy's case got a boost when researchers reported that another group of melon-headed whales (Peponocephala electra) had nearly stranded themselves on the same day as the Kauai incident but almost 6000 kilometers away on the island of Rota in the western Pacific Ocean. Some scientists concluded that the two events were related and fingered lunar cycles, particularly the full moon, as the culprit.

After taking a fresh look at the two mysterious events, Brownell and colleagues argue that they were not similar. The researchers compared the melon-headed whales' behavior in Hawaii and Rota and analyzed other recent, unpublished sightings of the cetaceans in the islands of French Polynesia and Palymra, where the animals come close to shore to rest. The team's conclusion, says Brownell, is that, unlike the Kauai whales, the Rota whales were not trying to strand themselves but were behaving like those in French Polynesia and had traveled into Rota's shallower waters for a rest. "That is their normal behavior," he says. In contrast, the whales' actions in Hawaii were "identical to those that precede mass-strandings" of beaked whales, with pods swimming agitatedly in tight circles, spy-hopping (rising vertically out of the water), tail-slapping, and vocalizing.

Brownell's group also studied 21 other mass-stranding events involving melon-headed whales to determine at what point in the lunar cycle they occurred. There was no connection with the full moon or any lunar phase. "You can't blame the moon for what happened in Hawaii," says Brownell, whose team published its findings in the July issue of Marine Mammal Science.

Figuring out what caused the Kauai whales to strand is important, Brownell says, because so far only mass strandings of beaked whales have been conclusively linked to the Navy's use of sonar. "It's a big debate," says Brownell. "Why are only beaked whales affected and not others? Well, other species are." Brownell says that melon-headed whales probably aren't affected as often by naval sonar exercises because, unlike beaked whales, they usually hang out far from shore. He thinks "it was purely a coincidence" that melon-headed whales were near Kauai the morning the Navy ran its sonar test. "If any other cetacean species had been going by that morning, the same thing would have happened," he says.

The U.S. Navy disputes Brownell's paper. Chip Johnson, a Navy marine scientist, says that the new study only "contributes more uncertainty" about what happened to the melon-headed whales that morning. "No one may ever really know the underlying causes or if contributing factors were natural or manmade."

Other experts, however, say Brownell's study seals the deal. "It's a solid refutation of [the lunar] explanation," says Robin Baird, a marine biologist with the Cascadia Research Collective, a nonprofit research organization in Olympia, Washington. "It's difficult to have conclusive evidence in most of these events, but the weight of the evidence points to the Navy's [midfrequency] sonar as the causal factor." Michael Jasny, a policy analyst with the Natural Resources Defense Council, which has fought the Navy in court over its use of sonar, agrees. "Everything in 2004 and in the 2006 study points to the Navy, as does this close reanalysis," he says. "It's really no longer an issue. The question is now, 'What can be done to fix the problem?'"

  • Correction:

The original version of this article stated that the sonar from submarines harms whales. However, it is the sonar emitted by surface ships, such as destroyers, as they are searching for submarines that causes problems for whales.

Brownell's new paper also reports previously unpublished sightings of melon-headed whales swimming and resting near Palymra, in addition to such sightings from French Polynesia.