Two scientists, Mandy L.H. Cook and David Mann, from the University of South Florida College of Marine Science, and colleagues at Harbor Branch Oceanographic Institution, have investigated the issue of whether sonar can be correlated with the stranding of beaked whales, as some reports have claimed.
“Several mass strandings of beaked whales have been correlated with military exercises involving mid-frequency sonar, yet there are unknowns about their hearing sensitivities,” said Cook. “Because it has been hypothesized that some strandings are sonar-induced, our study measured hearing abilities of a beaked whale by measuring auditory evoked potentials (AEP), used commonly to measure hearing in human infants, birds, fish and other animals. We wanted to see if they had particular auditory sensitivity to mid-range sonar in wide use by the U.S. Navy and others.”
In July, 2004, when a young male beaked whale in poor health stranded near St. Lucie Inlet on the east coast of Florida, it came under the care of Harbor Branch Oceanographic Institution. The event afforded marine biologist Cook and colleagues to test its hearing by measuring its response to auditory signals of varying frequencies.
“A live stranding of a beaked whale is a relatively rare event, and this was an opportunity to learn about the hearing abilities of this little-known group of whales,” explained Cook. “We found that the hearing of beaked whales was similar to other echolocating dolphins and whales.”
According to Cook, beaked whales are capable of detecting sounds between five and 80 kHz, which is in the range of frequencies used for echolocation. The highest frequency humans can hear is 20 kHz. The U.S. Navy’s mid-frequency tactical sonars operate between 2.6 and 8.2 kHz.
“Our measurements did not support the hypothesis that beaked whales have a particularly high auditory sensitivity at the frequencies used in mid-range sonar,” concluded Cook. “However, these data are important for understanding the range over which beaked whales can detect mid-frequency sonar.”