338. Dredging in preparation for pipeline laying emits broadband noise, mainly in the lower frequencies, which could be similar to noise emissions during use of jetting tool or plough for cable laying (OSPAR 2009). There are no records for noise from rock dumping (OSPAR 2009, JNCC 2010) however it is considered likely to be the noisiest activity associated with cable laying activities. Considering the activity it is expected that the noise will be broadband in nature.
12.6.1.3.1 Sensitivity 339. A review of the impacts of underwater noise on marine mammals is provided in section 12.6.1.1.
340. The potential for TTS is only likely in very close proximity to cable laying or rock dumping activities, and noise generated should not sufficient to cause PTS or other injury to marine mammals. Committing an injury offence to EPS is therefore very unlikely (JNCC 2010). Disturbance is the only potential noise impact from vessels. However, there are limited observational data to support the level of response that harbour porpoise or seals may exhibit as a result of these types of activities.
341. As stated previously, harbour porpoise are considered to have medium sensitivity to disturbance from noise, and harbour seal and grey seal are considered to have low sensitivity to disturbance.
12.6.1.3.2Magnitude
342. The impacts from cable laying and protection are temporary in nature, and will be limited to part of the construction phase. The offshore cable corridor does not include areas of high marine mammal density for any species that occur within the southern North Sea. Disturbance responses are likely to occur at significantly shorter ranges than pile driving noise, but may be greater than vessel related disturbance.
343. The magnitude of effect in all species is negligible, with less than 1% of the reference population being likely to be affected.
344. In the case of harbour seal (at the UK level) the magnitude of effect is still considered to be negligible (less than 1% of the South-east England MU, or 36 seals) due to the very low at sea densities in this region.
12.6.1.3.3Impact significance
345. The significance of the impact on harbour porpoise, harbour seal (at the reference population and UK level) and grey seal is negligible.
346. The confidence in the data used in this assessment is medium.
Preliminary Environmental Information May 2014
East Anglia THREE Offshore Windfarm
Chapter 12 Marine Mammal Ecology Page 85
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