The diet of Atlantic PuffinFratercula arcticaand Northern GannetSula bassanachicks at a Shetland colony during a period of changing prey availability

Abstract

The food of young Puffins and Gannets was examined between 1973 and 1988 on Hermaness, Unst. No prey other than fish was found in any year; 10 species were taken by Gannets and at least 13 by Puffins during this study period. Sandeel Ammodytes marinus was found to be the dominant prey species for Puffins in every sampling year except the last 2, comprising over 90% of the diet by weight in many annual samples. Sanded formed 90% of the diet of young Gannets in 1981 but declined steadily in importance thereafter, falling to 6% in 1988 by which time Herring Clupea harengus and Mackerel Scomber scrombrus¸ were the 2 most common prey species. In a successful breeding season, the Hermaness colonies of Puffins and Gannets consume about 3000 tonnes and 2500 tonnes, respectively, during their attendance at the colony. The mean weight of food loads delivered by Puffins to their young declined significantly after 1980, as did the size of Sandeels captured. Observation of the colonies reinforces the evidence from diet sampling that Sanded had been less available to both seabird species in the latter years of the study. Gannets were able to switch to other prey with no loss of breeding success, but Puffins probably suffered a severe breeding failure between 1986 and 1988. Available data on Shetland Sanded stocks provide evidence of a probable link between this species' abundance within the seabirds' foraging range and the quantity of Sandeels brought ashore to chicks by adult Puffins and Gannets.