The importance of the lesser sandeel Ammodytes marinus in the diet of the shag Phalacrocorax aristotelis

Regurgitated food and pellets were collected from Shags on the Isle of May, Scotland. Young Shags were fed almost exclusively lesser sandeels. The relative contributions of 0-group and older sandeels varied markedly between years. Fish were the predominant prey of full grown birds which consumed a wider spectrum of fish than were fed to chicks. Otoliths from at least 14 species were identified but sandeels dominated the diet and were present in 93% of all pellets and accounted for 97% of otoliths. There was no evidence that clupeids were ever an important prey. Although most published data suggest that lesser sandeels spend most of the winter buried in the sand they were obviously still available to Shags during this time and their otoliths occurred in 90% of pellets and accounted for 90% of otoliths. The availability of sandeels, specifically when the fish emerge from the sand in spring is probably a major determinant of the timing of breeding of Shags on the Isle of May.