Ocean / Robin Baird, a marine mammal scientist who’s been studying Hawaii whales for 11 years, wondered why sonar used during naval exercises around Hawaii never produced a single mass stranding of beaked whales, the extreme divers of the ocean and the most vulnerable to sonar. Similar exercises by the Spanish Navy in the Canary Islands, off the coast of West Africa, have produced at least six strandings, leaving a total of 43 beaked whales dead on beaches and rocks.

In all, out of 40 mass strandings recorded since a new, more powerful sonar was introduced worldwide in the early 1960s, 28 were simultaneous with such exercises, according to a 2007 study by Lindy Weilgart of Dalhousie University in Halifax, Nova Scotia. They killed 206 beaked whales and eight members of other species.

Baird and graduate student Meghan Faerber of the University of Wales in the United Kingdom examined the evidence. Last month, they published a paper that showed that the reason we aren’t seeing mass strandings here isn’t necessarily that the whales aren’t affected; rather, they found half a dozen reasons why we wouldn’t see the dead whales if they were. “The absence of evidence is not evidence of absence,” they wrote in the journal Marine Mammal Science.

Weilgart, in her study, noted that the fact that it has taken 30 years to discover a link between naval exercises and strandings (the first paper to do so, in the journal Nature, dates from 1991) underscores how easy it is to miss such impacts from human activities.

A whale of a creature

Whales and dolphins that have teeth and hunt other animals (as opposed to baleen whales, which scoop up plankton or schools of small fish) tend to stick to the upper 1500 feet of the water column, as do predatory fish like tiger sharks and bluefin tuna. But beaked whales, which have snouts that are called beaks, have evolved in a unique way. They developed collapsing lungs and other traits that enable them to routinely dive to 5,000 feet, where the water pressure is 1,500 pounds per square inch and not much above freezing, according to Andreas Fahlman, a marine mammal physiologist at the Woods Hole Oceanographic Institution in Massachusetts. Despite their speed of 5 miles per hour, it still requires them to spend up to 90 minutes under water, though most dives last about an hour.

At that depth, the water contains so little oxygen, the main fuel of marine life, that everything moves around in slow motion. Using their bat-like clicking sonar locators (it’s pitch dark down there), they are able to scoop up squid and fish with ease without competition.

The down side: to avoid nitrogen bubbles, known as the bends, they need to come up very carefully. Autopsies of stranded whales suggest they were so disturbed by mid-range sonar during the naval exercises that they surfaced too fast, dying of the bends. “It seems they may live with nitrogen levels right at the brink of getting the bends and a small behavior change can put over the top and kill them,” Falhman adds.

Examining six cases between 1985 and 2004 in the Canary Islands, Baird and Faerber found that in most cases, juveniles were overrepresented, though not the majority of victims. “It looks like some adults tend to get used to the sonar but juveniles and some adults might panic when they hear it for the first time,” says Baird.

Explanations

Baird and Faerber listed the following reasons why we probably won’t see dead whales on our beaches anytime soon, regardless of how often the Navy uses its sonar.

• Hawaii has a lot of very aggressive tiger sharks, which prey on sick humpback whales. In the Canaries, in contrast, reports of sharks feeding on whales are rare.

• Hawaii has gentler slopes than the Canaries, so the beaked whales spend most of their time further away from the coastline than they do in the Canaries.

• Hawaii has stronger currents than the Canaries, so a sick whale is likelier to be carried out to sea.

• Hawaii, with half the coastline human population density of the Canaries, also has large swaths of coastline that are either high sea cliffs or utterly unpopulated or both, so the chances of anyone seeing a beaked whales–they average 20 feet–is more remote.

Hawaii has two groups of subspecies of beaked whales: Cuvier’s (55 individuals) and Blainville’s (120). Neither commonly ventures beyond 30 miles of the Big Island.

“It would make sense to at least ban sonar within 30 miles of the Big Island,” argues Baird. The population of Cuvier’s has fewer juveniles than normal, but it’s not clear whether either population is declining, he adds.

Farther offshore, in the 927,000 square miles that form Hawaii’s Exclusive Economic Zone, the federal government estimates that there are at least 7,000 Cuvier’s and about at least 1,200 Blainville’s. “As far as we know, most Navy exercises take place off Oahu and Kauai, and some of these whales are probably being affected,” said Baird.

A second measure the Navy could take is, at the start of exercises, to use their sonars on a low-power setting, which would allow the whales to leave or get used to the sound rather than being startled.

A third involves the Barking Sands Underwater Range Expansion, which consists of 18 hydrophones covering 880 square nautical miles. “These hydrophones are going to be upgraded this year and will be able to pick up the sound beaked whales make when they use their own sonar to find food,” says Baird. “Once that’s done, if the hydrophone shows the whales are feeding in the range, the Navy could delay using their sonar until the whales have moved away, or they could use a different part of the range. These are all measures that would reduce the impact on whales without too much cost to the Navy.”