The effects of quantity and quality of prey on population fluctuations in three seabird species

Abstract

Capsule Population sizes of Common Guillemots Uria aalge, Razorbills Alca torda and Lesser Black‐backed Gulls Larus fuscus were associated with prey abundance but not prey quality. Aims To examine how the abundance and quality of prey fish affects seabird population size and to test the ‘junk‐food’ or nutritional stress hypothesis. Methods Analysis of long‐term seabird population size data and Sprat Sprattus sprattus biomass and age‐related weight data using a correlative approach. Results De‐trended seabird and Sprat population data showed that the abundance of Sprat, the main prey species, was associated with the abundance of seabirds, while no effect of age‐related size of prey on seabird population size was found. Conclusion As the Sprat population increased so did the seabird populations, regardless of decreases in ‘quality’ of Sprats, implying that more prey fish simply seem to mean more food in this marine ecosystem. No support for the ‘junk‐food’ hypothesis was found and the results contradict suggestions from earlier studies that prey quality is important to top‐predators in the Baltic Sea.