Centurial and decadal oceanographic influences on changes in northern gannet populations and diets in the north-west Atlantic: implications for climate change

Millennial and centurial changes in oceanography influence the distributions and movement patterns of fish and invertebrates. These changes, in turn, determine the availability of food resources for higher trophic levels and, hence, affect the distributions and abundances of marine birds. A century-long population trend of northern gannets (Sula bassana) is correlated with warming surface water conditions and increased mackerel (Scomber scombrus) availability. On a decadal scale, a major dietary change of breeding gannets from migratory warm-water pelagic fish and squids to cold-water fish is associated with cold-water perturbations in the north-west Atlantic during the 1990s. Cold-water influences appear to have inhibited migratory pelagic fish and squid from moving into the region in recent years, causing a major shift in pelagic food webs on the Newfoundland Shelf. Such findings imply that slight changes in oceanographic conditions, possibly associated with climate warming, could have large-scale and pervasive effects on seabird distributions, feeding ecology, reproductive success, and populations. Such changes might be detected initially near the limits of seabird ranges and the margins of oceanographic regions. (C) 1997 International Council for the Exploration of the Sea.