Aarhus University Seal / Aarhus Universitets segl

No. 45: Assessment of the effects of underwater noise on marine organisms. Part 2 - Effects

Tougaard, J. 2014. Vurdering af effekter af undervandsstøj på marine organismer. Del 2 – Påvirkninger. Aarhus Universitet, DCE – Nationalt Center for Miljø og Energi, 51 s. - Teknisk rapport fra DCE - Nationalt Center for Miljø og Energi nr. 45. 

Summary

It is well known from humans that noise can have negative effects of various kinds. Likewise, there is increasing attention to the fact that underwater noise can affect marine organisms negatively and there are increasing requirements for assessment and monitoring of underwater noise generated by anthropogenic (man-made) activities at sea. A significant step towards actual regulation of this field was the review made by Southall et al. (2007), which works through the relevant literature for marine mammals and makes the first proposal for exposure criteria for marine mammals. As the origin of the review was to a high degree prompted by a demand from US legislation, it seems appropriate to review the recommendations with specific Danish conditions in mind.

Recommendations of Southall et al. (2007) and their limitations

Two types of effects of underwater noise were treated by Southall et al. (2007): injury and behavioural changes. To be able to generalise conclusions, the marine mammals were divided into four functional groups, based on differences in hearing abilities: ’Low frequency cetaceans’ (baleen whales), ’Medium frequency cetaceans’, (most odontocetes), ’High frequency cetaceans’ (porpoises and others) and ’Pinnipeds’. A major consequence of this division is the derivation of group-specific frequency weighting functions (M-weighting) for assessment of underwater noise.

As criterion for injury the temporary threshold shift (TTS) is adopted by Southall et al. (2007), based on an assumption that the hearing organs are the most acoustically sensitive physiological system in the marine mammals. Based on laboratory data from bottlenose dolphin, beluga, harbour seal and California sea lion, this leads to derivation of TTS-thresholds, expressed both as pressure and accumulated energy (SEL, sound exposure level).

Behavioural reactions are scored on a response severity scale 0-10 and the literature is reviewed in the light of this scale, however, without derivation of consistent thresholds for reactions.

The authors of Southall et al. (2007) deserve credit for proposing actual criteria and thus opening the discussion, even if based on limited data material. The major limitations in the conclusions relate to effects of noise which are not covered (among others masking and physiological effects); audibility as a prerequisite for effects; scaling of behavioural reactions with immediate severity of the response and not the long-term consequences for the animals; frequency weighting (M-weighting) not being based on experimental evidence and the limited number of species taken to be representative for marine mammals at large.

Recommendations for Danish species

Based on Southall et al. (2007) and the additional data obtained since 2007, a number of recommendations can be put forward relevant for Danish species of marine mammals.

Two important studies on TTS in harbour seals are available (Kastak et al., 2005; Kastelein et al., 2012a). Due to lack of data for grey seals, these data are also taken to be valid for this species. The threshold for eliciting TTS by low frequency noise is found to be in the range 169-182 dB re 1 µPa2s (sound exposure level). Seals are considered robust when it comes to disturbance of behaviour by sound, but very few actual studies are available.

Since 2007 results of several new studies on TTS in porpoises have become available (Kastelein et al., 2012b; Kastelein et al., 2013b; Lucke et al., 2009; Popov et al., 2011). Taken together these results indicate that the sound level required to induce TTS in porpoises is frequency dependent, roughly paralleling the shape of the audiogram. Expressed as sound exposure level, the threshold for TTS for a 1 s pure tone signal is 105 dB above the detection threshold.

Analogous to this, are the results of a range of studies on behavioural reactions towards pile driving noise, pingers and seal scarers (Culik et al., 2001; Carlström et al., 2009; Olesiuk et al., 2002; Johnston, 2002; Brandt et al., 2011, 2012, 2013, Tougaard et al., 2009, 2012; Thompson et al., 2010 and Dähne et al., 2013), which also display frequency dependence paralleling the shape of the audiogram. The threshold for negative phonotaxis, expressed as sound pressure and weighted with the integration time of the mammalian ear (125 ms) is roughly 55 dB above the detection threshold.

Very few data are available for assessment of impact on other species relevant for Danish waters, primarily white-beaked dolphin and minke whale. Until further data are available, TTS thresholds from bottlenose dolphins are the best available data. These studies have shown TTS induced at sound exposure levels in the range 190-210 dB re. 1 µPa2s, depending on stimulus frequency and duration. No firm data is available to base recommendations regarding behavioural reactions for both species. Likewise, the lack of data prevents any conclusions regarding effects on diving birds, fish and invertebrates.