The usefulness of pellets for assessing the diet of adult ShagsPhalacrocorax aristotelis

Abstract

Each day Shags were fed known numbers of either Sandeel, Sprat, Herring or Cod and the contents of the pellets which the birds regurgitated were examined. Normally, each bird produced a single pellet each day which contained otoliths from the fish eaten during the previous 24 hr. The proportion of recovered otoliths varied greatly both from day to day and according to the species of fish. Otoliths from Cod had the highest recovery rate, those of Herring and Sprat were recovered less often and the recovery rate of Sandeel otoliths was intermediate. Measurements of otoliths from the pellets gave a very misleading estimate of the size of fish eaten. Fundamental questions need to be answered before pellets can be used in general dietary studies of seabirds.