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 Aberdeen, says.
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 (anthropogenic) noise.
''It seems likely that stronger responses may be expected in populations that have previously had little exposure to anthropogenic noise.''
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 variation.
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.