When 16 whales and a dolphin washed up on the beaches of the northern Bahamas back in March 2000, the U.S. Navy was the immediate suspect. A naval fleet using sonar was maneuvering through the islands, and all of the beached animals were spotted within 24 hours of the ships’ passage. Was it simply a coincidence? Even the Navy didn’t think so, since sonar blasts had been at least tentatively linked to whale injuries for decades.
In a later report on the mass stranding, which left seven of the animals dead, the Navy acknowledged that sonar was probably to blame. The animals suffered “acoustic or impulse trauma that led to their stranding and subsequent death,” the Navy wrote in its joint report with the National Oceanic and Atmospheric Administration, or NOAA.
But that was the last time the Navy and its environmentalist critics agreed on just about anything related to whales and sonar. Their increasingly heated scientific and legal battles have even reached the U.S. Supreme Court, which sided with the Navy in a November 2008 case involving limitations on its sonar use off the California coast. The Court’s decision, however, has done little to quiet the controversy: the Navy continues its legal underwater noisemaking and scientists struggle to confirm sonar’s relationship to whale and dolphin strandings.
“It’s very difficult to say how many animals have been affected,” says Tim Ragen, executive director of the Marine Mammal Commission, a federal agency that monitors efforts to protect whales and dolphins. “In order to come to a definitive number about how many animals are actually injured or killed [by sonar] – either directly through physiological processes or indirectly through behavior processes – you have to make a lot of assumptions.”
Looking for clues about how sonar may harm whales and dolphins, researchers have noticed hemorrhaging in the acoustic region of the brain and gas-bubble lesions in vital organs of stranded animals. Those injuries may be the direct result of sonar’s loud pulse, or they may be due to pressure changes from unusually fast climbs to the surface to escape the noise. Either way, many researchers suspect that sonar is damaging the animals’ extremely sensitive hearing. For marine mammals that rely on their own sonar to see and communicate, hearing loss can severely hinder hunting, mating, orientation and pod cohesion.
Human-created ocean noise can also mask healthy whales’ navigation and conversation signals. While less pervasive than commercial shipping’s contribution to ambient ocean noise, sonar blasts are exponentially louder and can send up to 235 decibels into the ears of whales at close range. Since decibel levels are based on a nonlinear scale, this would be perceived as nearly 3,000 times as loud to the human ear as the 120 decibels scientists have found frightens whales. No one is certain, however, how a whale’s sound perception differs from that of a human. A disoriented or noise-fleeing mother and calf may become separated in response to sonar, and stresses from cumulative exposures may lead to suppression of reproduction, explains Andrew Wright, a marine mammal scientist formerly with NOAA.
“A lot of the effects would not be lethal, or at least not immediately lethal,” says Wright, adding that significant impacts could remain hidden.
But even if researchers understand the basics of how sonar blasts may be affecting whales and dolphins, proving the case is far more difficult. A key problem is that many mass strandings seem to occur for completely unrelated and natural reasons, including disease, algal blooms, weather and collisions with ships.
“We had records of strandings centuries before we had motorized ships or sonar,” says Peter Tyack, a senior scientist at the Woods Hole Oceanographic Institution in Massachusetts.
He adds, however, that the strandings that occur in close proximity to Navy training often follow unnatural patterns: multiple groups of animals “spread over tens of kilometers of coastline.” In addition to the Bahamas, stranded whales and dolphins have been found in this unusual pattern along the coasts of the Canary Islands, Hawaii and North Carolina, among other places.
In all, there have been nine stranding incidents involving approximately 300 whales and dolphins near ongoing naval exercises between 1996 and 2006, according to the International Ocean Noise Coalition, a network of more than 150 environmental groups. The Navy recognizes their sonar was involved in five of these incidents, which killed an estimated 40 marine mammals.
Forty is a tiny number when compared to the more than 3,500 whales that naturally strand on U.S. beaches every year, according to the Navy. They also note that fisheries are a greater threat to marine mammals. Over 6,000 whales and dolphins were accidentally caught in the U.S. annually between 1990 and 1999, according to a 2006 paper in the journal Conservation Biology.
“You often hear that more animals die with fishing gear than with sonar,” says Chris Parsons, a marine biologist at George Mason University in Virginia. But this fact doesn’t diminish his concern about mass strandings.
Even if only a small number of whales strand in a sonar-related incident, Parsons points out that the consequences could be tragic for specific populations. For example, beaked whales, one of the least understood families of mammals, have been disproportionately represented in these events. They are also among those least commonly seen in natural strandings. “Many of these species are in small, isolated areas with small population sizes,” says Parsons. “If you see 10 stranded animals, the likelihood is more animals died in the water and haven’t actually stranded. And these population sizes, for all we know, could only be a couple of hundred animals. You could, very easily, have wiped out the entire population.”