The Acoustic Thermometry of Ocean Climate/Marine Mammal Research Program

by Pat Leonard last modified 2007-04-20 13:51

Sound waves move slightly faster in warm water than in cold water, so by measuring the time it takes sound to travel underwater between two points, scientists can figure out the average temperature along that path. In a swords-to-plowshares move, researchers from the Scripps Institution of Oceanography in La Jolla, California, hoped to measure global warming using formerly top-secret U.S. Navy underwater "listening posts" that once detected enemy submarines. Oceanographer Walter Munk was the principal investigator for the project, which was called Acoustic Thermometry of Ocean Climate project (ATOC). Funding came from the Advanced Research Projects Agency.

Ironically, the methods that were used to assess one environmental problem, global warming, might have created another problem. Some people criticized the project, claiming that the noises it used may be harmful to marine life. Bioacoustics Research Program (BRP) director Christopher Clark, an expert in marine mammal communication, said "Large whales use low-frequency sound to communicate with one another, and since the ATOC project involves broadcasting loud, low-frequency sounds underwater from transmitters off Point Sur, California, and Kauai, Hawaii, no one knew if these sounds would interfere with whale communication. Low-frequency noise pollution caused by humans might be a problem for whales, or it might not--it's time to find out."

Although marine mammal assessment has was part of the ATOC research plan from the beginning, further study was clearly needed. So in 1992 Scripps awarded Clark a grant to develop and direct a Marine Mammal Research Program (MMRP) for research on how the ATOC sounds might affect whales. He tapped Daniel Costa of the University of California at Santa Cruz to lead the marine mammal research effort at the California ATOC site. Adam Frankel, a postdoctoral research associate at the Lab, was field team leader for the Hawaiian studies.

ATOC experiments that were scheduled to begin in the winter of 1994 had to be delayed after environmentalists raised the alarm that the sounds might deafen whales. The protests led federal regulators to require a lengthy and complicated Environmental Impact Statement. Frankel and his team used the delay to good purpose, carrying out additional baseline studies of humpbacks on Kauai, before the transmissions began.

Their work had three components: observations of humpback whale behavior from the air and from shore; underwater recording to measure background ocean noise and normal humpback singing; and aerial surveys to document the abundance and behavior of marine mammals around the Hawaiian Islands. Whale watching is popular in Hawaii, so the researchers also measured how whales respond to the noise of helicopters and small boats. Says Frankel, "The animals affect one another's behavior, and boat noise can also affect their behavior. The question is, will the ATOC sounds affect whale behavior? And if so, how much? We'll have to pick apart what's responsible for what."

With baseline data under their belts, Clark, Frankel, and crew began, in the Spring of 1996, to measure how the whales respond to ATOC-like sounds. "Before the real ATOC sound sources get turned on full force, we're doing scaled-down 'playback' experiments, with sounds that are ten to a hundred times softer than the actual ATOC sounds," says Clark. "If there's no adverse response we'll work up to the full-scale sound." Frankel explains the experimental protocol: "From a hilltop shore site, we visually follow a pod of whales, using long-range binoculars and surveying instruments; document their behavior under normal sound conditions; then slowly turn up the experimental sound, and watch for any change in behavior."

Eighty five trials were conducted in 1996, fifty of which involved playing the low-frequency "m-sequence" sound from a boat moored offshore, and thirty-four of which were no-added-sound controls. The sound level, when it arrived at the whales being studied, ranged from ambient (approximately 90 dB) to 130 dB. The results were a bit surprising: there was no difference between the tracks the whales followed when the sound was being played and tracks when the sounds were not being played. Actually, vessel traffic in the area and whether the humpback pod included a calf had larger impacts and affected more behavioral variables than whether playbacks were underway.