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

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mained higher in the period after shooting, while longline catches of Greenland
halibut, on the other hand, decreased during seismic shooting.
Based on these contradictory results and the fact that the offshore fishery of
Greenland halibut has not been studied it is difficult to assess the effect of
seismic activity. However, if catches are reduced by a seismic survey, the effect
is most likely temporary and will probably only affect specific fisheries
for a few days. The offshore fishery of Greenland halibut in the assessment
area is large in relation to the total catch in Greenland and the trawling
grounds are restricted to specific depths at approximately 1,500 m. Alternative
fishing grounds would therefore be limited if Greenland halibut are
displaced by seismic activity. Another potential impact is the risk of scaring
spawning fish away from the spawning grounds. These are assumed to be
situated on the slope of the sill between Greenland and Baffin Island, but as
spawning is assumed to take place in early winter the seismic activity would
probably be absent or very low. In Norway, some spawning grounds for
herring and cod are closed for seismic surveys in the spawning period.
It should be mentioned that there are other examples where fisheries have
increased after seismic shooting, which is assumed to be an effect of changes
in the vertical distribution of the fish (Hirst & Rodhouse 2000).
The few studies available on seismic impacts on crustacean fisheries did not
find any reduction in catchability (Hirst & Rodhouse 2000, Christian et al.
2003, Andriguetto-Filho et al. 2005, Parry & Gason 2006), indicating that the
shrimp and crab fisheries within the assessment area (Fig. 5.1.1 and 5.1.2)
will not be affected by seismic surveys.
Impact of seismic noise on birds
Seabirds are generally not considered to be sensitive to seismic surveys, because
they are highly mobile and able to avoid the seismic sound source.
However, in inshore waters, seismic surveys carried out near the coast may
disturb (due to the presence and activity of the ship) breeding and moulting
congregations.
Next to nothing is known about underwater hearing in diving sea birds and
no-one has attempted to assess the possible impact of exposure to airgun
sounds during diving. Their hearing ability underwater is likely to be inferior
to marine mammals and in any case restricted to lower frequencies, not
extending into the ultrasonic range. Diving birds are not known to use hearing
underwater, but may do so. Diving birds may potentially suffer damage
to their inner ears if diving very close to the airgun array, but unlike in the
case of mammals, the sensory cells of the inner ear of birds can regenerate
after damage from acoustic trauma (Ryals & Rubel 1988) and hearing impairment,
even after intense exposure, is therefore temporary.
Impact of seismic noise on marine mammals
Responses of marine mammals to noise fall into three main categories: physiological,
behavioural and acoustic (Nowacek et al. 2007). Physiological responses
include hearing threshold shifts and auditory damage. Behavioural
responses include changes in surfacing, diving and heading patterns, and
may result in avoidance of the area or reduced feeding success. Low frequency
sounds may effectively mask the calls of baleen whales, thus interfering
with their social activities and/or navigation and feeding activities.
Acoustic responses to masking by anthropogenic noise include changes in