Abundance of Atlantic white-sided dolphin (Lagenorhynchus acutus) during summer off northwest Scotland

Résumé

A shipboard cetacean survey was conducted in July/August 1998 within an area to the west of Scotland, UK, commonly known as the Atlantic Frontier. The aim of the survey was to document the distribution and abundance of cetaceans to provide baseline population data for an area that is being increasingly explored and developed by oil companies. A double platform ‘independent observer’ (IO) method was used to estimate the abundance of the Atlantic white-sided dolphin (Lagenorhynchus acutus) using standard line-transect and distance sampling methodology. Previously, uncorrected Atlantic white-sided dolphin abundance was estimated as 27,194 (CV = 0.29) from this survey. This paper presents abundance estimates corrected for g(0) < 1 using a direct duplicate method. The value of g(0) was estimated to be 0.61 (CV = 0.09). The abundance in two strata was estimated as 21,371 (CV = 0.54) to the west of the Outer Hebrides and 74,626 (CV = 0.72) in the Faroe Shetland Channel. The high CVs are the result of small sample sizes, particularly of the duplicate data set. However, the abundance estimates represent the first for this species to the northwest of Scotland and adds to existing baseline abundance estimates for small cetaceans in UK waters. The results could be useful for planning future surveys that aim to calculate more precise abundance estimates. These results, together with opportunistic sightings data collected during other surveys, suggest that the waters to the west of Scotland are an important habitat for the Atlantic white-sided dolphin. Presently, threats to this species in the area are relatively unknown but a baseline population estimate will be an integral part of any management regime should there become a need in future.