Aerial and visual surveys to estimate harp seal pup production in the Greenland Sea

Abstract

In March-April 1991 a combined expedition using fixed-wing aircraft and ship-borne helicopter was carried out to estimate harp seal pup production in the Greenland Sea (the West Ice). Photographs were taken from the fixed-wing aircraft and analysed from counts made on negatives. Visual surveys were made fiom helicopter using funnel-shaped shades to limit the search strip. Three separate breeding patches of harp seals were sweyed by either or both of the methods, making direct comparisons between the methods possible. Abundance estimates calculated from the basic data are given for these patches. Bearing in mind that neither survey covered all known patches, the photographic estimate of surveyed areas was about 40,000 pups, and the visual estimate of surveyed patches about 60,000 pups. Both these estimates have an inherent negative bias caused by the fact that known patches were not included (which my account for at least -10%); neither have scattered pups between patches nor the temporal distribution of births been taken into consideration. A specific problem of the photographic survey are the errors made during reading and interpretation of the photographs. These aspects were investigated by reading negatives under slightly different conditions, and parallel reading of negatives and printed copies, which apparently have differing properties with respect to readability definitions of harp seal pups. By these two procedures it was demonstrated that counts increased by 3.2% and 5.5%, respectively. Conclusively, it is recognized that the aerial survey results in general are consistent with pup production estimates based on mark-recapture experiments conducted over the last 15 years.