Gentry made it clear, though, that naval sonar is a lethal hazard for whales and dolphins. In 1996, a dozen Cuvier’s beaked whales were killed by a sonar operation near Greece. Four years later, the US Navy reported the death of nine beaked whales from the use of sonar in the Bahamas.
But Gentry also pointed out that naval sonar represents a tiny fraction of the manmade acoustic energy generated in the oceans. He said offshore petroleum development creates a considerably larger share of noise than the military.
“Geophysical exploration by oil and gas companies at sea for reserves: They use large stainless steel tubes filled with compressed air. They release that air all at once with a loud thudding sound. There are thirty or forty guns in an array that all go off at the same time. Those arrays also have the ability to kill animals or alter their behavior.”
The combination of military sounds, geophysical exploration, and natural sounds made by wind and waves – and even other whales – still represents less than a quarter of the noise present in the oceans. Gentry said the remainder was produced by shipping.
“There are something like 80,000 commercial vessels operating worldwide. These are the largest ones, not the smaller ones. And their engines are going bank-to-bank, their propellers are cavitating, and they make this noise that is continuous. And this has far more capability of masking communication calls than any other.”
Outside of commercial fishing bycatch, which kills an estimated 300,000 cetaceans a year, it is acoustic “masking” that may be the greatest threat to whales and dolphins. Masking blocks both communication calls and echolocation, which cetaceans use to find food. Masking may lie behind the 41 documented mass strandings of marine mammals since 1960.
Gentry described two experiments designed to study the “dose response” of whales to controlled sound: One off the coast of Australia, and one in Southern California. In the California experiment, whales were tagged to allow researchers to record their movements underwater. Gentry said that even when exposed to mild sounds, Cuvier’s beaked whales abandoned their feeding patterns.
Gentry said it’s not damage to ear that causes whales to go ashore.
“Instead it’s much more likely that the animal hears the sound, and makes some kind of really unfavorable decision, and its behavior somehow affects its tissues and causes it to strand.”
Although cochlear tissue is among the most delicate in mammals, Gentry said that stranded whales typically do not have ear injuries. Instead, they’ll have brain hemorrhages, which suggest rapid surfacing.
The cetaceans that are killed by intense sound have to be very close – within a kilometer of the source. Gentry said that the phenomenon known as “spreading loss” reduces lethal sounds by 60 decibels beyond a kilometer.
“An analogy would be if you had a strawberry that size, and it has a certain amount of flavor. If you blow it up to the size of a basketball it’s not going to have nearly that much flavor. That’s kind of what happens to sound – it gets diluted over a very short distance.”
Gentry said that the two acoustic experiments conducted so far cost about $7-million dollars each, with funding from the petroleum industry and the US military. There were about 250 people in the audience for his Whalefest presentation, including high school students from as far away as Barrow.
Gentry said that despite this high-profile research, and some spectacular failures, scientists still knew very little about cetaceans and sound. He said science was well on its way to understanding behavioral response to acute sound, but as yet there was no measurement of the increase in background sounds. “We need more help and more researchers,” he said.
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