How tolerant are harbour porpoises to underwater sound?

Abstract

The planned construction of offshore wind turbines in the North and Baltic Seas involves the emission of high numbers of intense impulsive sounds when turbine foundations are driven into the ground by pile driving. Based on knowledge about other toothed whales, it can be assumed that the source levels, which will on average exceed 225dB re 1μPa at 1m, will create a risk of at least temporary threshold shift (TTS) in the auditory system of harbour porpoises which inhabit these waters. These animals are vitally dependent on their hearing system. An auditory study was conducted to base the definition of noise exposure criteria on information on the acoustic tolerance of this species to single impulses. The measurements were conducted on a male harbour porpoise held at the Fjord- & Bælt Centre in Kerteminde, Denmark, in a semi-natural enclosure. All hearing data were collected by measuring the evoked auditory potentials (AEP method). After achieving baseline hearing data over the animal’s functional hearing range, the animal was subsequently exposed to single airgun stimuli at increasing received levels in a controlled exposure experiment. Immediately after each exposure the animal’s hearing threshold was tested again for any significant changes at three selected frequencies. The received levels of the airgun impulses were increased until TTS was reached at one of the frequencies. The animal’s hearing thresholds were elevated in comparison to published data from other studies.A systematic electrophysiological masking due to the active positioning of the animal at its underwater station and an acoustic masking due to the high background noise level in the enclosure are likely reasons for these elevated hearing thresholds. The acoustic characteristics of the auditory stimuli may also account for a systematic difference in the hearing sensitivity. The achieved harbour porpoise’s hearing sensitivity does therefore not represent absolute but masked hearing threshold levels. This, however, has no implication on the tolerance of the animal’s hearing system for intense impulsive sounds. At 4kHz the TTS-criterion was exceeded when the animal was exposed to an impulse at a received peak level of 200dB re 1μPa and a sound exposure level of 164dB re 1μPa2/Hz. The documented masked TTS level of the harbour porpoise is considerably lower than levels found in other toothed whale species tested so far, thus supporting the hypothesis of size dependant differences in the tolerance of the auditory system in toothed whales. Modelling the impact range of multiple exposures reveals a risk for auditory effects in harbour porpoises over larger distances as compared to single exposures. The results are likely to have implications for regulatory procedures regarding the construction of offshore wind turbines in German waters as well as the use of other impulsive sound sources.