Population, productivity and DDT contamination trends of Northern Gannets (Sula bassanus) at Bonaventure Island, Quebec, 1967–1984

A comparison of the results of recent surveys of Northern Gannets on Bonaventure Island shows that the population increased from 16 400 pairs in 1976 to 21 100 pairs in 1984, which represents an annual increase of 3.2%. Hatching success was only 36–40% between 1966 and 1970, after which it increased to 58% in 1974 and then varied from 78 to 89% during 1976–1984. Net productivity now exceeds the level necessary to maintain zero population growth. The marked increase in breeding performance coincided with a significant decline in DDT and dieldrin residues in eggs, which suggests that toxic chemical contamination was responsible for the low hatching success between 1966 and 1970.