The impact of great skua predation on seabird populations at St Kilda: a bioenergetics model

1. The number of great skuas Catharacta skua Brunnich breeding at St Kilda, Outer Hebrides, has increased rapidly in recent years, making it currently the second largest colony outside Shetland and the fastest growing in the UK. In comparison with Shetland, where the diet consists mostly of sandeel Ammodytes marimus Raitt and discarded gadoid fish, and very rarely of birds, great skuas at St Kilda feed far more extensively upon seabirds. This paper incorporates metabolic, dietary and demographic data to estimate the total mass of different prey consumed at St Kilda and to assess the potential impact of this predation on other seabird populations. 2. On the basis of a bioenergetics model incorporating fundamental life-history parameters, the great skua population at St Kilda was estimated to require 141 x 10(6) kJ of energy per season, most of which (88.0%) was necessary for the maintenance and activity of breeding adults. Energy demand was considerably lower for non-breeders (2.5% of the total), and for chicks and fledglings (9.2%). 3. In addition to seabirds, great skua diet at St Kilda also included a considerable proportion of fish and goose barnacles Lepas sp. However, because of differences in mean meal mass and caloric density, meals of larger seabird prey were more important in terms of their energetic contribution in the diet than in terms of their relative abundance. 4. Combining the bioenergetics and prey consumption models, it was estimated that a total mass of 12.2 tonnes of fish, 1.6 tonnes of goose barnacles and 8.8 tonnes of seabirds was consumed by the great skua population at St Kilda to fulfil its total energy requirement in 1996. Overall seabird consumption was estimated to be 40,800 seabirds of seven different species. Although a proportion of birds killed are likely to be visiting non-breeders, the magnitude of this figure nonetheless suggests that great skuas may have a considerable impact on the internationally important populations of some seabirds at St Kilda.