Population dynamics of Northern Gannets in North America, 1984–2009

Northern Gannet (Morus bassanus) colonies (N = 6) in North America have been regularly censused since the late‐1960s. Monitoring populations of a top‐predator like gannets provides insight into possible changes in the marine environment and contributes to an understanding of their demography. We report the results of aerial censuses of gannet colonies conducted from 1984 to 2009. Standard methods were used and involved making high‐quality photographs of colonies from a fixed‐wing aircraft, and subsequently counting apparently occupied sites (AOS) in the photographs. As of 2009, the breeding population of gannets in North America was estimated to be 116 825 pairs, or ～27% of the world population. From 1984 to 2009, the population grew at an average rate of 4.4% per annum and this growth was likely the result of continued population recovery after acute persecution in the past. Growth rates began to slow during the latter part of our study, likely due to density‐dependent effects and reductions in food availability. Despite the growth of the North American gannet population, gannets have not established new colonies, possibly because populations had been so depressed that growth could be accommodated within existing colonies.