Diet and breeding performance of black-legged kittiwakes Rissa tridactyla at a North Sea colony

The lesser sandeel Ammodytes marinus is a key prey species for many avian predators in the North Sea, and is the target of the largest single species fishery in the area. This has led to concern about the potential impact of the fishery on seabirds. The most vulnerable predatory species are small and surface-feeding, characteristics that are exemplified by the black-legged kittiwake Rissa tridactyla. This paper reports on the first assessment of seasonal changes in prey composition and prey size for the kittiwake in the North Sea during 4 breeding seasons (1997 to 2000) in which breeding success varied dramatically. Kittiwake diet showed little inter-annual variation, with a well-defined seasonal change from planktonic crustacea in early spring, to 1+ group sandeels in April and May, to 0 group sandeels in June and July. However, there was evidence that temporal differences in sandeel life history events were well reflected in both kittiwake diet and breeding success. Thus, the most successful year (2000) was characterised by the earliest appearance of 0 group sandeels, while the least successful season had the latest appearance. There was also a link between annual variations in breeding success and sandeel size such that success was lower when 0 group sandeels were smaller and hence of lower energy value. Our study included 3 seasons (1997, 1998, 1999) during which the industrial fishery was operating within 50 km of the study colony and 1 (2000) in which the fishing grounds were closed. The higher breeding success in 2000 than in the other years suggests that the closure of the fishery might have had an immediate and positive effect on kittiwake productivity. However, as the dietary data indicated, the earlier appearance and rapid growth rates of 0 group sandeels in 2000 suggests that the enhanced breeding success was more likely to have been a response to environmental factors influencing the growth and timing of life history transitions of this prey.