When oceanographers in La Jolla wanted to use underwater sound to gauge ocean warming, they found themselves awash in protests from people worried the noise would deafen whales and other protected marine life. To allay fears, researchers dramatically revised their $35-million experiment and delayed it 18 months.
Then, last year, scientists from the Scripps Institution of Oceanography had barely started engineering tests of their revised system on the Pacific Seamount when three dead whales were discovered nearby. Although there was no evidence that the underwater sound experiment was responsible, the furor again forced scientists to temporarily suspend their research, which later resumed.
In each instance, people were worried about the effects of underwater noise on the health and safety of endangered whales, sea turtles, seals and fish that use sound to navigate, hunt and communicate with each other.
Now, in the first systematic look at how marine mammals react to the underwater broadcasts, researchers at Cornell University say humpback whales seem to show no response to the kind of low-frequency sounds produced by the Scripps climate-sensing system.
After 84 separate trials off the coast of Hawaii this year using their own sound source, the Cornell researchers concluded that pods of humpback whales under observation treated the low-frequency sounds as part of just another noisy day in the ocean. In all, they tested 115 groups of whales.
A detailed statistical analysis of the whales' response to the computer-generated rumble showed no discernible effect on feeding behavior, diving or swimming habits, the researchers said. Breathing patterns appeared unaltered. Mothers and calves seemed unperturbed.
Several whales who were singing at the time of the test did not stop when the underwater sounds were played.
"We saw no overt response," said Adam Frankel, the biologist at the Cornell Bioacoustics Research program who led the study. "We were worried about dramatic responses, but in all the work that has been done there has been no evidence of any dramatic response. . . . A barely detectable response is more like it."
Scientists gathered the information about humpback whale behavior as part of a $5.7-million marine mammal research program designed to assess the safety of an ambitious effort by Scripps to turn the world's oceans into a thermometer that measures rising temperatures caused by global climate change.
In the Acoustic Thermometry of Ocean Climate project (ATOC), the Scripps researchers want to use low-frequency underwater sound to monitor global warming across the entire Pacific Ocean basin by taking advantage of the fact that sound travels faster in warm water than in cold water.
When water temperature increases, as most computer models of global warming predict, the sound waves will travel faster and faster. By timing how long it takes a sound signal to travel from a loudspeaker on the ocean floor off California to submerged receivers near Alaska and New Zealand, they expect to keep track of changing water temperatures over a broad expanse of the planet.
At full power, the ATOC units broadcast periodically at 195 decibels--about the noise level of a passing supertanker or the full-throated song of a blue whale. The normal level of ambient noise in the open ocean is about 75 decibels.
Researchers set up the first ATOC speaker on the ocean floor of the Pacific Seamount about 55 miles off the California coast last December. They plan to set up a second speaker on the ocean floor in October, about three miles off the island of Kauai, near the calving and breeding grounds of the humpback whales.
"This first two-year period is a feasibility study, both to determine whether the transmissions have any effect on marine mammals and also to see if the [temperature] measurements are accurate enough," said ATOC program manager Peter Worcester.
"It is looking very good," he said.
Although little is known about the hearing of marine mammals, the humpback whales are believed to be especially sensitive to the 75-megahertz sound frequency used for the Scripps experiment, because the whales use that frequency to communicate with each other.
For their tests, the Cornell researchers used a low-power version of the sound generator lowered from a boat to ensure that the signal would encompass only those humpback whales in close proximity. Their sound source was only one-tenth as powerful and only one-hundredth as loud as the ATOC unit now broadcasting from the Seamount, but to the whales near the boat it would sound just as loud as the operational ATOC unit.
The Cornell findings have not convinced environmental activists that the ocean climate experiment is safe. They said it was too soon to draw any conclusions about how marine life will react to the underwater noises.