Our underwater noise pollution may harm marine mammals, but we cannot be sure without more data on their behaviours
The underwater world is a noisy place, filled with a cacophony of animal sounds from fish, whales and even shrimp. Recently, a new animal has begun to inject significant amounts of sound into the ocean: humans.
Much of this human sound is noise pollution, a by-product of shipping, dredging and construction. Other sounds are put in the water intentionally, to detect fish, submarines and the ocean floor. Our input of sound into the ocean has risen dramatically, but our understanding of how animals use sound or how noise from our activities might affect them has not. This is, in part, because we cannot hear sounds well underwater.
New technologies have opened our ears, and recent advances using hydrophone arrays and acoustic tag devices promise more breakthroughs. However, a wide gap in our familiarity with underwater sounds compared with those in the terrestrial world remains. Most people are unaware that their boat's depth-sounder is polluting the waters or that, underwater, their boat engine sounds like a car without a silencer.
In a recent study published in Nature , my colleagues and I reported the results of an experiment in which we exposed singing humpback whales to low-frequency sonar. In the winter breeding season, male humpbacks sing complex songs that are thought to attract females and to play a role in male-male competition. We followed individual singers for at least 30 minutes before asking a US Navy vessel to transmit a set of 10 to 42-second sonar "pings" at a controlled level. We detailed the whales' behaviour through the hour-long transmission cycle and afterwards. Roughly a quarter stopped singing, apparently in response to the sonar. Those that sang continuously lengthened their songs, increasing the songs' redundancy apparently to compensate for interference.
I was surprised by many of the singers' lack of response to the sonar. One whale did not miss a beat even when the vessel passed within 500m. At this range, the whale received 150dB of sonar, masking its song on our hydrophone array, yet it seemed unfazed - we had the impression that it was competing against the sonar and expected to win.
I am concerned about the consequences of the noise we add to the ocean. My PhD research on the acoustic behaviour of wild killer whales helped me develop technologies and skills that aid in evaluating the effects of noise on marine mammals. I feel a responsibility to work with noise-polluters such as the US Navy that are willing to explore the consequences of their activities.
The parties in the noise-pollution debate have become so polarised that there is little room for rational discussion or scientific research. Most mainstream environmental groups approved of our study, but some tried to stop it. The short-sighted tactics of these extreme groups to oppose scientific research on the effects of noise seems to be motivated more by political concerns than a commitment to understand and improve the environment of these remarkable animals.
Our findings with singing humpback males show that reactions to noise depend largely on an animal's behaviour. The breeding season may be the only time humpback males can mate, so singing may be a crucial behaviour affecting reproductive success. In such critical acts, animals may be less likely to exhibit overt reactions to disturbance because of the increased cost of those reactions.
We will be able to understand how noise pollution is affecting marine mammals only when we understand the behaviour and ecology of the marine mammals themselves.
Patrick Miller is a scientist at the Woods Hole Oceanographic Institution, Massachusetts, United States.