New research by scientists in South Florida indicates that a high percentage of stranded dolphins were almost deaf at the time of stranding. The cause of the near-deafness is unknown. It might be age-related, or attributed to birth defects, disease, or a complex of several causes. One potential cause that cannot be ruled out is anthropogenic (man-made) noise in the ocean, which, in specific instances has already been proven to cause animals to strand and die. If such noise is also having a chronic effect on marine animals, then the long-term impacts could be devastating. Anthropogenic ocean noise of greatest concern is that caused by shipping, military sonar, explosions, and seismic airguns used by the oil and gas industry. In some parts of the oceans anthropogenic noise levels have doubled every decade for the past 60 years. Scientists are finding that some species of whale are making louder vocalizations, presumably in an effort to overcome the background din. The impacts of this behavioral change are not known - although there are obvious concerns with any impairment in marine mammals’ ability to use sound to detect prey, avoid predators, and communicate with mates and offspring.
A total of 34 animals were tested, including seven Risso’s dolphins, two pygmy killer whales, one Atlantic spotted dolphin, one short-finned pilot whale, one juvenile Gervais’ beaked whale, one spinner dolphin, seven bottlenose dolphins, and 14 rough-toothed dolphins. The study was conducted using sensors attached to the dolphins’ heads with suction cups, on animals who had live-stranded and were being rehabilitated. The sensors were able to detect changes in electrical activity in the brain in response to tones. The short-finned pilot whale, four of the bottlenose dolphins, and five of the rough-toothed dolphins were found to be suffering from severe to near-total hearing loss. With this limited data it is difficult to draw definitive conclusions. But with such an alarming result, it is important to test more animals and make efforts to reduce anthropogenic ocean noise.