CENTURIAL AND DECADAL OCEANOGRAPHIC INFLUENCES ON CHANGES IN NORTHERNGANNET POPULATIONS AND DIETS IN THE NORTH-WEST ATLANTIC - IMPLICATIONS FOR CLIMATE-CHANGE

Millennial and centurial changes in oceanography influence the distrib utions and movement patterns of fish and invertebrates. These changes, in turn, determine the availability of food resources for higher trop hic levels and, hence, affect the distributions and abundances of mari ne birds. A century-long population trend of northern gannets (Sula ba ssana) is correlated with warming surface water conditions and increas ed mackerel (Scomber scombrus) availability. On a decadal scale, a maj or dietary change of breeding gannets from migratory warm-water pelagi c fish and squids to cold-water fish is associated with cold-water per turbations in the north-west Atlantic during the 1990s. Cold-water inf luences appear to have inhibited migratory pelagic fish and squid from moving into the region in recent years, causing a major shift in pela gic food webs on the Newfoundland Shelf. Such findings imply that slig ht changes in oceanographic conditions, possibly associated with clima te warming, could have large-scale and pervasive effects on seabird di stributions, feeding ecology, reproductive success, and populations. S uch changes might be detected initially near the limits of seabird ran ges and the margins of oceanographic regions. (C) 1997 International C ouncil for the Exploration of the Sea.