Delayed capelin (Mallotus villosus) availability influences predatory behaviour of large gulls on black-legged kittiwakes (Rissa tridactyla), causinga reduction in kittiwake breeding success

During 1998 and 1999, the impact of predation by herring gulls (Larus argen tatus) and great black-backed gulls (Larus marinus) on breeding success of black-legged kittiwakes (Rissa tridactyla) at Gull Island, Witless Bay, sou theastern Newfoundland, was quantified in relation to the timing of the ann ual arrival of capelin (Mallotus villosus) to spawn. The frequency of preda tion attempts by large gulls on kittiwakes was compared among three periods : before the mean hatching date for herring gulls, between the mean hatchin g date for herring gulls and the arrival of the capelin, and following cape lin arrival. The frequency varied significantly among the three periods, be ing highest after gull chicks hatched but before the capelin arrived. The f requency of gull predation was significantly correlated with the percentage of kittiwake eggs and chicks that disappeared each week. We estimated that 43 and 30% of kittiwake eggs and chicks at Gull Island were taken by gulls in 1998 and 1999, respectively. Kittiwakes have been indirectly (through i ncreased predation by gulls) affected by the delayed arrival and lower abun dance of capelin in recent years, which underlines the need to understand m ultispecies interactions when interpreting the effects of human alteration of the marine environment.