The Davis Strait - DCE - Nationalt Center for Miljø og Energi

190
have a negative impact on the energy uptake of the rorquals that are in West
Greenland to feed before their southward migration. Given the extent of oil
exploration in Greenland, there is a risk of cumulative effects if multiple
surveys occur at the same time in adjacent areas. In this case marine mammals
could be excluded from key habitats and unable to use alternative foraging
grounds.
The US National Marine Fisheries Service defines the radii about a seismic
ship with received sound levels of 160 dB (re 1µPA) as the distance within
which some cetaceans are likely to be subject to behavioural disturbance
(NMFS 2005). Actual distances would depend on the source levels of the airgun
array, the salinity and temperature layers of the water, and the depth of
the observation. A few studies have observed lack of measurable behavioural
changes by cetaceans exposed to the sound of seismic surveys taking place
several kilometres away. For instance, Madsen et al. (2002) found no reaction
of sperm whales to a distant seismic survey operating at tens of kilometres
away. More recently, Dunn & Hernandez (2009) did not detect changes in
the behaviour of blue whales that were at 15-90 km from operating airguns.
The authors estimated that the whales experienced sounds of less than 145
dB (re 1µPA) and concluded that, while their study supports the current US-
NMFS guidelines, further studies with more detailed observations are warranted
(Dunn & Hernandez 2009).
An acoustic effect widely discussed in relation to whales and seismic surveys
is the masking of communication and echolocation sounds. There are,
however, very few studies which document such effects (but see Castellote
et al. 2010, Di Iorio & Clark 2010), mainly because the experimental setup is
extremely challenging. Masking requires overlap in frequencies, overlap in
time and sufficiently high sound pressures. The whales and seals in the assessment
area use a wide range of frequencies (from < 10 Hz to > 100 kHz,
Fig. 4.8.6), so the low frequency sounds of seismic surveys are likely to overlap
in frequency with at least some of the sounds produced by these marine
mammals.
Masking is likely to occur as a result of the continuous noise from drilling
and ship propellers, as documented for beluga whales and killer whales in
Canada (Foote et al. 2004, Scheifele et al. 2005). Due to the low frequency of
their phonation, baleen whales followed by seals would be the marine
mammals most affected by auditory masking from seismic surveys (Gordon
et al. 2003). It has furthermore been shown that blue whales increase their
calling rate during seismic surveys, probably as a compensatory behaviour
to the elevated ambient noise (Di Iorio & Clark 2010). Similarly, changes in
the acoustic parameters of fin whale calls in the presence of airgun events
indicate that fin whales also modify their acoustic behaviour to compensate
for increased ambient noise (Castellote et al. 2010).
Sperm whales showed diminished foraging effort during airgun emission,
but it is not clear if this was due to masking of echolocation sounds or to behavioural
responses of the whales or the prey (Miller et al. 2005).
The most noise-vulnerable whale species in the assessment area belong to
the baleen whales – minke, fin, blue and humpback whale – and the toothed
whales – sperm whale and bottlenose whale (probably) – all of which all are
present in the area during the ice free months when seismic surveys usually
take place. At the time of writing this assessment we were not aware of any