Interactive effects of food shortage and predation on breeding failure of black-legged kittiwakes: indirect effects of fisheries activities and implications for indicator species

We investigated the roles of food shortage and predation in anticipated breeding failure of black-legged kittiwakes Rissa tridactyla at Great Island. Witless Bay, Newfoundland, Canada in 1992 and 1993, following widespread failure in 1991. Reduced and delayed availability of capelin Mallotus villosus, plus elimination of previously plentiful fish offal and discards following an eastern Canadian groundfish moratorium in 1992, induced food-stress on kittiwakes and on large predatory Larus gulls. Breeding was late, a low proportion of pairs laid eggs, and egg and clutch sizes were small, indicating that feeding conditions for kittiwakes were poor early in the season. Extreme food-stress resulted in low breeding success of Larus gulls that turned to kittiwake eggs as an alternate food source. Forced egg predation by great black-backed gulls Larus marinus was the primary cause of low hatching success (10 to 11%) in both years. Durations of incubation and chick-rearing shifts decreased following the inshore arrival of capelin, and chicks that hatched after capelin arrival survived longer than earlier hatched chicks. Unattended nests were uncommon, probably in response to predation. In 1992, earlier hatching, later capelin arrival, and apparent lack of alternate prey resulted in many chicks starving and poor fledging success (7%). In 1993, food appeared to be more available both before and after capelin arrival, parents were able to provision chicks, and fledging success was normal (68%). The interactive effects of food shortage and predation have important implications for the use of kittiwakes as indicator species and for the indirect effects of fishery activities on seabird interactions. Kittiwake productivity also reflects the resource base of their predators and caution is required when linking kittiwake breeding success to prey abundance. Moreover, fishery activities, such as the eastern Canadian groundfish moratorium that eliminated fish discards and offal for large scavenging gulls, can have profound second-order effects on other seabird species that are preyed on by scavengers.