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are very tolerant (Moriyasu et al. 2004). The
diversity of invertebrates is large and thus potential
responses may vary greatly, and little is known
about the potential effects on differing life cycle
stages.
Conclusions
It is reasonable to assume that the impacts of noise
on invertebrates during construction will be local.
and thus have small effects on assemblages as a
whole. Little research on this is, however, available.
Certainty: 2.
7.5 Mitigation of construction noise effects
For marine species avoiding the area under construction,
this inevitably results in loss of habitat,
which could include feeding, spawning, nursing and
resting (migrating species) grounds. In the prospecting,
planning and permitting processes, key species,
key life cycle stages and seasonal habitat use, as
well as approaches (e.g. mitigation measures) used,
need to be taken into careful consideration (Anderson
1990, David 2006). To mitigate adverse effects,
construction activities can be timed with regard to
the seasonal behaviour of key marine organisms
using the area. For instance, for temperate species,
reproduction generally takes place during spring
and summer, and the abundance of juveniles then
increases near shore. It has, also, been noted that
in many areas the highest ambient noise environment
occurs during the winter (temperate areas,
US DIMMS 2007). Construction activities during
wintertime may therefore cause less disturbance
impacts. This unfortunately concurs with the most
unfeasible (technically and logistically) and unsafe
period for sea work. Also, by developing floating
wind turbines for shallow areas, the size of piles
used for anchoring of turbines could be significantly
reduced (Henderson et al. 2004).
Several methods are used to reduce the damage
caused by noise from pile driving. A standard
approach is to gradually increase the strength of
the pile-driving hammer to give mammals and
larger fish a chance to escape the most dangerous
area (Joint Nature Conservation Committee 2004,
Dong energy et al. 2006). It should be noted, however,
that this method is controversial as it may
lead to gradual habituation and even attraction to
the initially week sounds (Compton et al. 2008).
Another method is to surround the pile driving area
with a curtain of bubbles or wrap the piles in sound
damping material, although the implementation is
very limited, as it is dependent on calm waters and
weak currents. Such bubble protection can reduce
the sound volume by 3-5 dB, i.e. half of the sound
intensity (Würsig et al. 2000). A bubble curtain
significantly reduced mortality of caged bluegill
(Lepomis macrochirus) during demolition work on
the Mississippi River (Keevin et al. 1997). Scaring
devices (seals) and pingers (porpoises) have been
used to clear the vicinity of pile driving activities
from marine mammals (Frank 2006). See Nehls et
al. (2007) for details on sound mitigation measures.
7.6 Operational noise and fish
During operation, vibrations in the tower of the
wind turbine generated by the gearbox mesh and
the generator cause underwater noise with frequencies
in the range of 1 – 400 Hz, and of 80 –
110 dB re: 1µPa, and this is likely to increase as a
function of the number of turbines (e.g. Nedwell &
Howell 2003). In the frequency range above 1000
Hz emitted noise is generally not higher than ambient
noise. The towers have a large contact surface
with the water that transmits sound effectively.
The tower will also transmit vibrations to the sea
floor but this effect is in most cases highly local
and therefore considered of minor importance.
Airborne noise from the blade tips is effectively
reflected away from the water surface and is unlikely
to significantly add to the underwater noise level
(Ingemansson 2003).
Estimates of how far fish can detect a wind turbine
vary from a few hundred meters to 50-60 km,
depending on environmental conditions and species
(Nedwell & Howell 2003, Wahlberg & Westerberg
2005, Thomsen et al. 2006). The spread of
noise, including both sound pressure and particle
motion, in the water depends largely on the type
of wind turbine, local hydrography and geological
conditions, depth, the ambient noise level caused
by both natural and anthropogenic sources, and
weather conditions (Nedwell & Howell 2003, Wahlberg
& Westerberg 2005, Madsen et al. 2006, Sigray
et al. 2009). Since it is very complicated to model
noise distribution around wind power plants in a
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