| Summary: | During the summers of 1987 and 1989, large scale transect surveys were conducted throughout the North Atlantic by several national agencies in Denmark (off Greenland), Faroe Islands, Iceland, Norway and Spain (North Atlantic Sightings Surveys, NASS-87 and NASS-89). This paper analyses the pilot whale (Globicephala melas) survey data collected by three Icelandic and one Faroese survey vessel in 1987, and four Icelandic, one Faroese and one Spanish vessel in 1989. Norwegian survey vessels operated north and east of this area in both years, but only five groups (three primary sightings) were observed in 1989 and none in 1987. Furthermore, no sightings were made in the area north and northeast of Iceland, thus indicating that the joint surveys covered the northernmost areas of pilot whale distribution east of 42°W. The area further to the west was not covered in either survey. The coastal European waters between 42-52°N were covered by the Spanish vessel in 1989. Sightings made in 1989 by the Icelandic vessels tended to be at the southernmost boundaries of the survey area. The present data were examined with respect to several potential stratification factors, namely geographic block, Beaufort (i.e. wind speed), vessel and school size, but sample size precluded stratification by all these factors simultaneously. The encounter rate was generally lower in the 1987 survey than in 1989, but the difference was not statistically significant. The total estimate for the 1989 survey, covering a wider area and further to the south than in 1987, was 778,000 (CV=0.295). This is regarded as the best available estimate of the total stock of long-finned pilot whales in the northeastern North Atlantic Ocean, although small numbers occur outside the NASS survey areas. The paper discusses potential biases in the abundance estimates, and the problems of estimating pilot whale abundance from sightings data.
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