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The impact of underwater noise on marine life calls for identification of exposure criteria to inform mitigation. Here we review recent experimental evidence with focus on the high-frequency cetaceans and discuss scientifically-based initial exposure criteria. A range of new TTS experiments suggest that harbour and finless porpoises are more sensitive to sound than expected from extrapolations based on results from bottlenose dolphins. Furthermore, the results from TTS experiments and field studies of behavioural reactions to noise, suggest that response thresholds and TTS critically depend on stimulus frequency. Sound exposure levels for pure tones that induce TTS are reasonably consistent at about 100 dB above the hearing threshold for pure tones and sound pressure thresholds for avoidance reactions are in the range of 40–50 dB above the hearing threshold. We propose that frequency weighting with a filter function approximating the inversed audiogram might be appropriate when assessing impact.

Rapporten omfatter en vurdering af konsekvenserne ved at udvide muligheden for at sejle med vandscootere, jetski og lignende fartøjer i Natura 2000-områder og vildtreservater. Det vurderes, hvilke konsekvenser en eventuel lempelse vil have på fugle og havpattedyr, med særlig fokus på Natura 2000-områdernes udpegningsarter. Rapporten gennemgår den biologiske baggrund for forstyrrelser af vandfugle og havpattedyr ved fritidssejlads samt konsekvensen heraf. Desuden gives en oversigt over de forhold, som må tages i betragtning ved en eventuel vurdering af muligheden for, at der kan drives sejlads med vandscooter og lignende fratøjer i Natura 2000-områder. Det konkluderes, at det er vanskeligt at vurdere, hvilket forstyrrelsesniveau denne form for sejlads vil afstedkomme i danske Natura 2000-områder. Det vurderes dog, at det på baggrund af specifikke undersøgelser kan være muligt at lempe reglerne for sejlads med vandscooter og lignende fartøjer i visse Natura 2000-områder uden, at det påvirker forekomster af fugle og havpattedyr negativt.

Underwater noise was recorded from the Wavestar wave energy converter; a full-scale hydraulic point absorber, placed on a jack-up rig on the Danish North Sea coast. Noise was recorded 25 m from the converter with an autonomous recording unit (10 Hz to 20 kHz bandwidth). Median sound pressure levels (Leq) in third-octave bands during operation of the converter were 106–109 dB re. 1 μPa in the range 125–250 Hz, 1–2 dB above ambient noise levels (statistically significant). Outside the range 125–250 Hz the noise from the converter was undetectable above the ambient noise. During start and stop of the converter a more powerful tone at 150 Hz (sound pressure level (Leq) 121–125 dB re 1 μPa) was easily detectable. This tone likely originated from the hydraulic pump which was used to lower the absorbers into the water and lift them out of the water at shutdown. Noise levels from the operating wave converter were so low that they would barely be audible to marine mammals and the likelihood of negative impact from the noise appears minimal. A likely explanation for the low noise emissions is the construction of the converter where all moving parts, except for the absorbers themselves, are placed above water on a jack-up rig. The results may thus not be directly transferable to other wave converter designs but do demonstrate that it is possible to harness wave energy without noise pollution to the marine environment.

Ships and wind turbines generate noise, which can have a negative impact on marine mammal popu- lations by scaring animals away. Effective modelling of how this affects the populations has to take account of the location and timing of disturbances. Here we construct an individual-based model of har- bour porpoises in the Inner Danish Waters. Individuals have their own energy budgets constructed using established principles of physiological ecology. Data are lacking on the spatial distribution of food which is instead inferred from knowledge of time-varying porpoise distributions. The model produces plausible patterns of population dynamics and matches well the age distribution of porpoises caught in by-catch. It estimates the effect of existing wind farms as a 10% reduction in population size when food recovers fast (after two days). Proposed new wind farms and ships do not result in further population declines. The population is however sensitive to variations in mortality resulting from by-catch and to the speed at which food recovers after being depleted. If food recovers slowly the effect of wind turbines becomes negligible, whereas ships are estimated to have a significant negative impact on the population. Annual by-catch rates ≥10% lead to monotonously decreasing populations and to extinction, and even the esti- mated by-catch rate from the adjacent area (approximately 4.1%) has a strong impact on the population. This suggests that conservation efforts should be more focused on reducing by-catch in commercial gill- net fisheries than on limiting the amount of anthropogenic noise. Individual-based models are unique in their ability to take account of the location and timing of disturbances and to show their likely effects on populations. The models also identify deficiencies in the existing database and can be used to set priorities for future field research.