(1984) and a number of studies reported by Worcester (2006) have also demonstrated avoidance response when close to the sound source. Blaxter et al. (1981) also showed that when ramping up the sound level from a lower level, the sound level required to elicit a response is higher than if they are exposed to the same level from the onset, even if the ramp is only over two cycles. This may have implications for the use of soft-start during piling where the sound level is expected to gradually increase with time.
79. Although no behavioural disturbance criteria have been established, Washington State Department of Transportation, who do specify a requirement for the FHWG interim criteria for fish injury to be followed, also provide guidance in their Biological Assessment manual (WSDOT 2011) for behaviour effects in the form of a disturbance threshold which is based on work by Hastings (2002). The recommended threshold is 150dB re 1 μPa RMS, where the RMS refers to the root-mean squared pressure over the duration of the pulse. The authors do not provide peak or peak-to-peak levels and it is not possible to convert any relationship between peak pressure level and RMS levels as this will depend on the propagation distance and will be heavily influenced by water depths, which are generally very shallow for the previous pile- driving studies in the United States of America.
80.
The most comprehensive study identified observing fish behaviour when exposed to an impulsive type noise source was carried out by Curtin University in Australia for the oil and gas industry by McCauley et al. (2000a and 2000b). This study exposed various fish species in large cages to a seismic airgun and assessed behaviour, physiological and pathological changes. Both the acoustic and biological methodologies are described in detail. The study made a number of observations:
•
• •
• •
A general fish behavioural response to move to the bottom of the cage during periods of high level exposure (greater than RMS levels of around 156- 161dB re 1 μPa).
A greater startle response by small fish to the above levels.
A return to normal behavioural patterns some 14 to 30minutes after airgun operations ceased.
No significant physiological stress increases attributed to air gun exposure.
Some preliminary evidence of damage to the hair cells when exposed to the highest levels, although it was determined that such damage would only likely occur at short range from the source (further discussed in McCauley et al. 2003).
Preliminary Environmental Information May 2014
East Anglia THREE Offshore Windfarm Appendix 9.1 Underwater Noise Modelling 29
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