COLLEGE PARK, Maryland -- On land, the sound of jackhammers or passing semis might make it difficult for humans to hear, but not much is known about how anthropogenic noises in the ocean – from ship's sonar, pier pile drivers, or oil well operations, for example – affect marine species. Since some of these marine species, such as dolphins, rely on sound to navigate, researchers from the U.S. Navy and the National Marine Mammal Foundation set out to learn more about how anthropogenic noise might affect dolphins' echolocation ability. The scientists' initial results, which they will discuss at the 162nd Acoustical Society of America Meeting in San Diego, Calif., suggest continuous noise and noise at frequencies within a dolphin's echolocation range has the potential to negatively impact echolocation performance.
At the U.S. Navy Marine Mammal Program facility, the researchers used a hydrophone to detect a dolphin's clicks. They then fed the signal to a computer to be converted into an "echo" that was delayed and played back to the dolphin. By adjusting the delay, the scientists created echoes to simulate a physical object between 3 and 17 meters away, for both stationary and rotated objects. The dolphin was trained to make a buzzing sound when he detected the echo signal changed from a stationary object to a rotated object. While playing the echoes, the scientists also played different types of manmade noise and tested how each noise type affected the dolphin's ability to identify when the echo changed. The researchers tested seven different noise types at varying frequencies and durations.
"Preliminary results show that intermittent noise at frequencies outside of the echolocation range of the dolphin had little effect on his echolocation performance, while continuous noise and noise within the dolphin's echolocation range decreased performance at a farther distance," says Eryn Wezensky, a researcher on the project. One surprising result, she notes, is that Gaussian noise [broad spectrum noise whose amplitude distribution follows a normal curve] in the mid-frequency range, which is outside the echolocation range of the dolphin, still decreased the dolphin's ability to detect the echo change at distances from 13 to 16 meters. As a next step, the researchers plan to analyze the dolphin's click characteristics under different noise conditions, to test whether noise might prompt dolphins to compensate with louder clicks, or slow the animals' response time.
The presentation 5aAB5, "Performance of an echolocating bottlenose dolphin in the presence of anthropogenic masking noise," by Eryn M. Wezensky et al. will be at 9:15 a.m. on Friday, Nov. 4.
Views expressed in this article do not necessarily reflect those of UnderwaterTimes.com, its staff or its advertisers.