Page 99 - Alderney Regional Environmental Assessment of Renewable Energy: Environmental Report

Alderney Regional Environmental Assessment of Renewable Energy:
Environmental Report
noise, the noise emitted during pile driving is understood to have the greatest potential effects
on marine wildlife (Thomsen et al., 2006). This is due to the fact that pile driving generates
very high sound pressure levels over a relatively broad frequency range (20 Hz to >20 kHz).
Fish typically respond strongly to lower frequencies of noise, as opposed to marine mammals
that are sensitive to a broader bandwidth of sound (see Section 5.5.2.3). Fish that have
specialist structures (e.g. Weberian ossicles, swimbladder diverticulae and gas filled bullae)
that enhance hearing have been referred to as hearing ‘specialists’, whereas fish that do not
have such structures are referred to as hearing ‘generalists’. Those species that have a low
hearing threshold over a wide spectrum of frequencies and are most sensitive to noise are the
hearing specialists. The impacts of noise can broadly be split into lethal and physical injury,
auditory injury and behavioural response. Richardson et al. (1995) defined four zones of noise
influences, depending on the distance between the source and receiver. These are as follows:
ƒ Zone of hearing loss, discomfort or injury, the zone within which hearing or other
severe damage results;
ƒ Zone of masking, the region within which noise is strong enough to interfere with
detection of other sounds, such as communication or echolocation clicks;
ƒ Zone of responsiveness, the region in which the animal reacts; and
ƒ Zone of audibility, the area within which the animal is able to detect the sound.
At very high exposure levels, such as those close to piling operations, fatality may occur in
marine species. The likelihood of fatality is also related to the time period of exposure. With
respect to auditory injury and particularly where there are repeated high level exposures from
activities such as impact pile driving underwater sound has the potential to cause hearing
impairment in marine species. This can take the form of a temporary loss in hearing sensitivity,
known as Temporary Threshold Shift (TTS), or a permanent loss of hearing sensitivity, known
as Permanent Threshold Shift (PTS). TTS occurs more frequently whereby an individual’s
ability to hear is temporarily decreased. This short-term reduction in hearing ability returns to
pre-exposure levels soon after (perhaps a few days), although persistent levels of noise can
lead to PTS.
At lower noise levels, it has been reported that behavioural responses may be observed in
marine species. Behavioural responses include leaving the source area for a period of time,
either temporarily or permanently, or a startle reaction to the noise.
Nedwell et al. (2007) have developed a generic decibel (dB) scale, which enables better
estimates of the effects of sound on marine species to be made and allows the likelihood of
behavioural effects and damage to hearing to be assessed for a wide range of species (Table
14). Of significance for this assessment, is that at 90 dBht (species) and above there will be a
strong avoidance reaction by all individuals of that species, and that below 50 dBht (species)
there will be a mild reaction by a minority of individuals.
It should be noted that these criteria reflect the initial response and do not reflect the complexity
of behavioural, physiological and auditory impacts over the medium and long-term.
Furthermore, this criterion has not been validated by experimental study. The potential effects
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