More studies are needed before the true impact of seismic
surveys on small marine mammals such as porpoises and
dolphins is known, Prof Paul Thompson, of the University of
His paper, presented at the 20th Biennial Marine Mammal
Conference in Dunedin yesterday, comes as Shell New Zealand
prepares to start a two-dimensional seismic survey off
Otago's coast next month.
The survey was reported to be similar to an ultrasound. Air
under high pressure would be released to provide sound waves
down to the ocean floor.
Behind the survey vessel, an 8km ''streamer'' sat below the
surface, measuring the sound waves. A computer program was
then used to create an image of the sea floor.
Prof Thompson's study was based on how harbour porpoises in
the North Sea were affected by a commercial two-dimensional
seismic airgun survey.
The results suggested the survey noise did not lead to
broader scale displacement, but he warned it could be
different in other areas which did not have a long history of
exposure to impulsive noise and other human generated
''It seems likely that stronger responses may be expected in
populations that have previously had little exposure to
The study used passive acoustic monitoring and digital aerial
surveys to study changes in the numbers of porpoises across a
2000sq km study area during the study.
Results from the data showed evidence of group responses to
airgun noise over 5km to 10km but the animals were typically
detected again at affected sites within a few hours and the
level of response declined through the 10-day survey.
Overall, acoustic detections decreased significantly during
the survey period in the impact area compared with the
control area, but this was small in relation to natural
The study highlighted the need for longer-term individual
species-based studies to assess the consequences of seismic
surveys, he said.
The Otago Daily Times reported recently that Shell was a
signatory to the original voluntary Department of
Conservation code to minimise disturbance to marine mammals,
which meant the seismic vessel would have a team of four
independent marine observers on board who would maintain a
24-hour visual and passive acoustic watch.
If marine mammals were detected within specific zones, as
defined by the code, the survey would be shut down until they
had moved out of the area.