A spatial comparison of common guillemots and sandeels in Scottish waters

Abstract

The lesser sandeel (Ammodytes marinus) is an important prey of many seabirds and is the subject of the largest single-species fishery in the North Sea. Owing to their dependence on suitable sediment types, the distribution of post-settled sandeels (late O-group and older age classes) is restricted and constant through time. Consequently, it is possible to compare the presence/absence of post-settled sandeels with the distribution of predators without the requirement for simultaneous collection of data on predator and prey distribution. Post-settled sandeels are an important prey of guillemots (Uria aalge), particularly during the breeding season. This paper examines the extent to which the spatial distribution of guillemots, sandeels, and fisheries overlaps. A recently collated database on the distribution of post-settled sandeels together with data from the European Seabirds at Sea database is used to compare the seasonal distributions of guillemots and sandeels. Spatial analyses were conducted at a single scale using two approaches: a non-parametric test which determines whether the distributions of predator and prey are the same; and generalized additive modelling. These analyses indicate that sandeel distribution is an important influence on guillemot distribution in the breeding season, but is less important during other times of the year. As such, the seasonal changes in association appear to reflect changes in diet and prey availability. Other factors that appeared to be important in influencing guillemot distribution were distance from colony and latitude (colony location). Data on fishery landings per ICES rectangle were used to consider the degree of common utilization of sandeel grounds by fisheries and guillemots in recent years. This comparison indicated that there were some areas important for both guillemots and fisheries, but that most concentrations of guillemots were found in unexploited sandeel areas.