The reasons for population and breeding success variations in seabirds are complex, vary between species and geographical location and include such factors as climate, sea-surface temperature, plankton biomass and sandeel stocks. Seabirds are an integral part of marine ecosystems, where they may consume vast amounts of fish. It has been estimated that seabirds consume 600,000 tonnes per year of food in the North Atlantic.

The sensitivity of the distribution patterns of the pelagic prey of fish-feeding and plankton-feeding seabirds imply that small changes in the ocean environment resulting from climate changes could affect seabird reproductive success. Seabirds are generally long-lived and can survive adverse short-term environmental events. However, small populations tied to restricted habitat, such as the Galapagos penguin (Spheniscus mendiculus), may be threatened by long-term climate warming.

Modelling studies have shown that in several marine ecosystems, seabirds eat 20-30% of the annual pelagic fish production. The dependence on some species of fish, particularly during breeding, and their large abundance make seabirds a good indicator of ecosystem change. Where changes in breeding success or mortality occur, however, distinguishing the climate impact from fishing impacts can be difficult. Very few decadal-scale studies of seabirds are available to assess the impacts of long-term variations in climate.

Changes in oceanographic conditions in the California Current between 1985 and 1994 (sea surface temperature, wind speed and thermocline depth all increased) were mirrored by seabird distributions. Numbers of sooty shearwaters (Puffinus griseus) and Cassin’s auklets (Ptychoramphus aleuticus) declined, but common guillemot (Uria aalge) numbers remained stable. This was attributed to the guillemot’s ability to feed throughout the water column. Cold-water perturbations on the Newfoundland Shelf, northwest Atlantic, during the 1990s caused a major dietary shift of northern gannets (Morus bassanus) from migratory warm-water fish and squid to cold-water fish.

Population trends in kittiwakes (Rissa tridactyla) in Britain and Ireland over the last 30 years have been linked to changes in the marine environment and in conequence key fish stocks that have impacted breeding success, adult body condition and survival. Breeding success and annual survival of kittiwakes in the northwest North Sea are for example, negatively correlated with winter sea temperature. Many seabird species in the North Sea rely on lesser sandeels (Ammodytes marinus), but recruitment of sandeels is reduced during warm winters. Kittiwakes depend heavily on sandeels in the breeding season for feeding young. Over a 30-year period the size of sandeels brought in by breeding puffins on the Isle of May declined. There was a widespread breeding failure of seabird colonies in northeast Britain in 2004. In this year guillemots brought in mainly sprats, rather than sandeels, to their young. This change in diet led to the poor breeding success an analysis of the sprats showed that they had a lower energetic value.

The Coastal Seas Ecology Group of the Centre of Ecology and Hydrography