The ocean is no longer the silent haven that it once was. Massive ships churn the sea; submarine explosions and ear-splitting construction can wreck the peace. Alexander Supin from the Russian Academy of Sciences explains that aquatic mammals are particularly vulnerable to noise pollution as they depend on their hearing for orientation, navigation and communication. Even a temporary loss of hearing could cause disoriented animals to beach in shallow water. ‘It is very important to know how severe the influence of noise is on cetacean hearing and which restrictions are necessary to minimise the impact and to speed recovery’, says Supin. However, sound can damage hearing in many different ways: a continual low-volume squeal can be as harmful as an explosion if the animal's hearing is particularly susceptible. Supin, Vladimir Popov and their colleagues explain that measuring the impact of multiple acoustic factors on cetaceans is challenging. They decided to measure the electrical activity in an animal's auditory system at the surface of the skin (auditory evoked potential) to rapidly assess the effect of various acoustic factors on the hearing of two captive belugas and to find out how quickly they recover from loud noise pollution (p. ).
Supin and colleagues travelled to the Utrish Marine Station, situated on the Russian Black Sea coast, to test the animals' hearing. ‘The belugas were very cooperative, otherwise it would hardly be possible to perform the study’, he recalls. Gently lowering each animal into a small tank, the team measured the electrical activity produced by the animal's auditory system as it listened to soft sounds over frequencies ranging from 8 to 128 kHz to establish the minimum sound intensity that the animal could hear. Then, having established the whale's hearing threshold, the team played a loud sound – to simulate the kind of noise pollution that the whale might encounter – before monitoring the temporary effect that it had on the whale's ability to hear and the animal's recovery.
Repeating the experiment while varying the pitch of the simulated noise pollution from 11.2 kHz up to 90 kHz, the intensity from 145 dB to 165 dB, and playing the sound for periods lasting from 1 to 30 min, the team found that the whales' hearing always recovered completely within a day. However, unexpectedly the whales' hearing took longer to recover after experiencing the lower pitched noises (11.2 and 22.5 kHz) than the higher pitched noises (45 and 90 kHz). ‘A commonly adopted point of view is that high frequencies are the most susceptible to any influence causing hearing loss’, says Supin, ‘but in our experiments, the middle frequency range [20–30 kHz] displayed the largest post-exposure loss of sensitivity’.
In addition, the whales found it harder to hear sounds that were half an octave higher than the noise that they had just experienced, and when the team tracked the whales' recovery, they found that the longer the noise exposure, the longer it took the animals' hearing to recover. Finally, the team compared the impact that noise had on the two animals and they were surprised to find that the hearing of one of the animals was much more sensitive to noise than the other's, even though they were the same age and their initial hearing was equally good.
Having discovered that the hearing of belugas is particularly sensitive to noise of specific frequencies, Supin is optimistic that the team's discoveries will help us to design better regulations for aquatic noise reduction in order to protect the hearing of marine mammals better.
