Meeting Europe's renewable energy targets in harmony with - RSPB

36 MEETING EUROPE’S RENEWABLE ENERGY TARGETS IN HARMONY WITH NATURE
greatest during installation, especially from pile
driving. However, mitigation measures to attenuate
noise levels are estimated to reduce the distance at
which noise from pile driving could affect marine
mammals by at least 66% (Nehls et al., 2007), and so
presumably would also reduce any impacts on birds.
There is now evidence from operational offshore
wind farms that densities of some bird species
decline in the vicinity of offshore installations
(Garthe and Hüppop, 2004; Desholm and Kahlert,
2005). In particular, divers and sea ducks have been
shown to be displaced by up to 2–4 km from wind
farm areas. This may have implications for foraging
success, hence for individual survival and breeding
success, and, in breeding birds, provisioning rates
(the number of visits adults make to the nest with
food). However, there is evidence that, at least for
common scoter, birds displaced during the first few
years after construction may return to using the
wind farm area at similar densities to those present
outside the wind farm. It is unclear to what extent
food availability may have affected use of the wind
farm area (Petersen and Fox, 2007). Evidence for
divers so far does not indicate recovery of use of
vacated areas and further spatial analysis is
necessary to determine cause and effect. The
magnitude of impacts will be determined in part by
the extent and suitability of alternative habitat, and
so the cumulative impact of multiple developments
is an extremely important consideration.
Offshore wind farms may also cause “barrier
effects”, creating a physical or perceptual
disruption to functional links, for example, between
breeding and feeding areas or causing diversions
of migrating birds (eg, Desholm and Kahlert, 2005;
Madsen et al., 2010).
The installation of submerged fixed structures such
as support piles and anchor plinths and associated
underwater substations and power cables may
cause considerable disturbance to the seabed, for
example, trenching for cables could cause
disturbance from the turbines all the way to the
shore. The level of impact will depend to a large
extent on the method of installation, the sensitivity
of seabed substrates/habitats and the species
present in the area.
Collision risk
During installation and decommissioning, some
collision risk with vessels is possible, such as boats
and helicopters (McCluskie et al., unpubl.). While
collisions with installation vessels are likely to
occur to the same extent as with other marine
vessels, those involved in turbine construction are
more likely to be stationary or moving slowly in
comparison with other commercial vehicles.
Overall there is a lack of empirical data to evaluate
the risk (Wilson et al., 2006).
It can be assumed that “rafting” bird species (those
swimming on the surface, often in large groups),
particularly those that do so at night, are more at
risk of being hit by installation vessels than those
that roost overnight on land (Daunt, 2006).
Similarly, since the danger of collision with turbines
is potentially greater at night or during periods of
poor visibility, it is likely to increase with species that
spend a higher proportion of time flying at night.
There is evidence that in good light or weather
conditions there is a considerable degree of
avoidance of wind turbines at sea (eg, Garthe and
Hüppop, 2004; Desholm and Kahlert, 2005).
Experimental studies at the Danish Tunø Knob
offshore wind farm and the surrounding area
indicated that wintering common eiders reacted to
the visual presence of the wind turbines
independently of whether or not the turbines were
rotating (Larsen and Guillemette, 2007). Not all
species however, show such a large degree of
avoidance, and species such as gulls and terns,
which spend a high proportion of flight time at
turbine blade height, might be at considerable risk
of collision (Everaert and Stienen, 2007).
Disturbance of the seabed during construction may
result in an increase in suspended sediment levels
and a consequent increase in turbidity. Diving
birds’ risk of collision with installation machinery
may be raised by any increased turbidity
associated with the installation, although the
response to other non-visual cues, such as
vibration, may compensate for the lack of visibility
(McCluskie et al., unpubl.). Furthermore, the reduced
visibility caused by increased turbidity during
installation could have effects on foraging success;
marine birds are thought to have a high sensitivity
to reductions in visibility (Strod et al., 2008),
although in all but the largest developments any
such impacts are likely to be relatively short-lived.
Collision risk can also affect marine mammals.
Harbour porpoise and bottlenose dolphin are the
cetaceans most commonly encountered and
described as part of offshore wind farm projects