It is becoming increasingly difficult for whales and other mammals that echolocate and communicate by singing in the oceans to be heard above the volume of man-made noise. And many scientists are also concerned that our noisy activities in the oceans could be damaging the hearing of these sensitive creatures. However, Paul Wensveen, from the University of Iceland, and an international team of collaborators explains that many navies that depend on sonar for detection have developed a strategy where they gradually increase sonar volume, in the hope that the animals will move away calmly before the sound reaches full volume. But no one knew how effective these warning strategies were until Wensveen and his colleagues recorded the responses of 13 humpback whales to sonar in the Barents Sea north of Norway.
After attaching data logging tags – which recorded sound and movement – with suction cups to individual whales, the team then sailed toward the animals towing a sonar emitter and recorded the animals’ responses as they gradually increased the sonar to full volume. Wensveen says, ‘The risk of inducing hearing loss with an intense sound source like naval sonar depends on both the peak pressure and the sound energy received by the animal’, so he and his colleagues measured the pressure of the sound waves reaching the whales and the amount of sound that the animals were exposed to, in addition to their distance from the sonar, to estimate the risk to the animals’ hearing.
While five of the 11 animals veered away from the ship on the first occasion, the team found that the animals’ evasive action did reduce the chances of them encountering damaging levels of sound. However, the animals that had responded when the sonar first sailed toward them were more reluctant to veer away on the second occasion; only a mother and her calf avoided the increasing noise, although three animals that had been undisturbed by the first pass took evasive action on the second occasion. The team suggests that humpback whales may be reluctant to avoid sonar if they have heard it before or are distracted by food, and they suspect that the strategy of gradually increasing the volume may be more effective with other species that are more sensitive to sonar disruption.