Underwater noise can affect the hearing and behavior of marine mammals, as shown in previous research studies. These highly mobile and fast-moving animals are difficult to study in their natural surroundings, so most research has tested the effects of loud noises on the hearing and behavior of captive individuals. The results have informed U.S. National Marine Fisheries Service guidelines and restrictions for activities that produce underwater noise, but these guidelines are now seven years old and may need revisiting in order to protect the species affected.
Scientists from Aarhus University in Denmark have now undertaken a review of the more recent experiments on the ways in which underwater noise affects marine mammals. The findings, published today in The Journal of the Acoustical Society of America, suggest that the current noise regulations may need to be changed to protect porpoises, seals, and other sea-dwelling mammals.
Regulations are issued by government agencies, such as the Danish Energy Agency and the U.S. National Marine Fisheries Service. The guidelines put limits on the intensity of noise from marine activities to prevent temporary and permanent hearing loss by marine mammals. These mammals depend on auditory input for communication, as many of them live in social groups or locate each other using long-distance calls.
The noise restrictions imposed by government agencies, such as the Danish Energy Agency and the U.S. National Marine Fisheries Service, are based on measurements made for captive individuals. These animals are exposed to noise levels that induce a temporary threshold shift (TTS) in hearing, meaning that hearing is impaired briefly after exposure to the noise. The TTS onset level is the lowest noise exposure capable of inducing a small temporary reduction of hearing sensitivity, also known as auditory fatigue, before the animal recovers its hearing fully.
Although these experiments indicate the lowest intensities at which hearing is affected, the results can be extrapolated to determine the higher noise levels at which hearing would be damaged permanently. It is these levels that are used to regulate activities that produce loud underwater noises, such as pile driving for wind turbines, seismic surveys and navy sonars.
The authors of the review found that current guidelines for seals and porpoises are based on very few measurements, and only in a limited frequency range. More recent experimental results have indicated that different animal groups have different hearing sensitivities and these variations are not taken into account by the current government guidelines. For example, seals were found to be affected particularly by loud noises in the low frequency range, while porpoises were sensitive to high frequency noise pollution.
Since different species have different hearing sensitivities, the authors of the review felt it necessary to recommend adjustments to the guidelines for underwater noise regulation. Their results were sent to the Danish Energy Agency and formed the basis for the most recent update of Danish guidelines regarding assessment of impact of offshore wind energy development on marine life.
“Harbor porpoises and harbor seals are of particular interest with respect to pile driving, because they are acoustically sensitive and among the most common marine mammals in shallow western European waters, a center of the rapidly expanding offshore wind farm industry,” said study author Jakob Tougaard.
Offshore wind turbines, as well as other offshore and port constructions, are founded on driving piles (structural columns) into the ocean floor. Impact pile drivers essentially hammer the pile into the sediment without first excavating a hole or foundation, and this process produces strong impulsive underwater noise that is potentially damaging to the hearing of marine mammals. The authors highlight the need for more experiments to determine the possible impacts of such noises on hearing sensitivity in these animals.
“Retaining the current guidance has the possible consequence of over-regulating future activities rather than endangering the animals,” Tougaard said. “The observed discrepancies between predicted and measured TTS onset thresholds can only be resolved through new and dedicated experiments.”
By Alison Bosman, Earth.com Staff Writer