Associations between wintering avian predators and schooling fish in the Skagerrak-Kattegat suggest reliance on predictable aggregations of herring Clupea harengus

Abstract

We investigated the distributional dynamics of piscivorous seabirds in relation to the distribution of young fish outside the breeding season in the Skagerrak and Kattegat, using synoptic bottom trawl and seabird surveys. Kittiwake Rissa tridactyla, Common Guillemot Uria aalge and Razorbill Alca torda were significantly correlated with herring Clupea harengus abundance at coarse spatial scales (> 20 km). The association between the three seabird species and other abundant fish species like sprat Sprattus sprattus, whiting Merlangius merlangus and Norway pout Trisopterus esmarki was considerably weaker. In the Skagerrak and Kattegat, the location of aggregations of immature herring during winter is relatively stable and predictable. The correlation with herring and the lack of correlation with the abundant, but less stable occurrence of sprat of similar caloric value suggest that the tendency among piscivorous seabirds to depend on predictable concentrations of prey during the breeding season is a common feature during the non-breeding season as well. The results are discussed in relation to ecological and winter habitat characteristics of wintering piscivorous seabirds in the eastern North Atlantic, and in relation to the management and conservation of nursering herring in the Skagerrak and Kattegat.