Estimating Harbour Seal Abundance and Status in an Estuarine Habitat in North-East Scotland

Abstract

1. Harbour seals are the most widespread of pinniped species, coming ashore onto a variety of different habitats to rest, moult and breed. Estimates of their abundance and status rely on counts of animals in terrestrial haul-out groups but it is not clear to what extent current techniques are appropriate for all habitats. 2. This study aimed to determine the most appropriate methods for estimating the abundance and status of harbour seals in an estuarine habitat in north-east Scotland. Regular low-tide counts were conducted to identify the best time for annual counts. Survey data for 1993 were then combined with telemetry data on seal activity to produce an estimate of abundance. Finally, simulations using data on the variability of counts within a single year were used to determine the power of these techniques to detect trends in abundance. 3. The results suggest that annual counts conducted during the pupping season (mid-June to mid-July) provide the best estimates of abundance in this habitat. These results contrast with those from studies in rocky-shore habitats where counts made during the August moult provided more reliable abundance estimates. 4. In 1993, an average of 1007 seals were hauled out at low tides during the period 15 June-15 July. There were significant differences in haul-out frequency for males (0.521 of low tides) and females (0.698 of low tides) but no within-sex variation was detected during this period. Combining the telemetry data with the results of counts from the Moray Firth produced an abundance estimate of 1653 (95% confidence limits 1471-1836). 5. The potential effects of within and between-year variation in haul-out behaviour are discussed in relation to identifying suitable indices of abundance for trend analysis. In view of likely seasonal changes in the sex structure of haul-out groups, we recommend that programmes to monitor trends in harbour seal abundance should conduct annual counts at two different points in the annual cycle.