In April 1999 seismic investigations started in an area off western Norway as part of an ordinary three-dimensional survey, using a vessel with two seismic sources, each of 20 air guns and 10 hydrophone streamers. The seismic sources, towed at a depth of 8 m, were alternatively fired every 25 m along 51 transects, each 51525 m long, separated from adjacent transects by 500 m. The possible influence of this seismic activity on pelagic fish (herring, blue whiting and mesopelagic species) was investigated in two ways. First, the distribution and abundance within the seismic area and the surrounding waters up to 30–50 km away were mapped acoustically three times. In all three surveys the acoustic abundance of pelagic fish was higher outside than inside the seismic shooting area, indicating a long-term effect of the seismic activity. Secondly, the acoustic abundance was recorded directly prior to and after shooting along some of the seismic transects. In these comparisons no differences were found, indicating that the shooting had insignificant short-term scaring effects. However, both blue whiting and mesopelagic species were found in deeper waters in periods with shooting compared to periods without shooting, indicating that vertical movement rather than horizontal movement could be a short-term reaction to this noise.

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