Influence of hydrography, fishing activity, and colony location on summer seabird distribution in the south-eastern North Sea

Abstract

Possible determinants of the distribution of 11 species of seabird in the south-eastern North Sea were investigated in July 1993 and 1994. During ship transects in both years, seabird distribution, trawling activity, and hydrographic parameters (salinity, temperature, stratification/mixing, water depth, water clarity) were measured. Analysis of environmental and seabird density variables by principal component analysis revealed seven components, explaining 71% of the variance found in all variables. The most important components were: (1) a land-sea gradient combining hydrographic properties and distance to land/nearest colony; (2) trawler abundance; and (3) a factor linked to water stratification and surface temperature. Distance to colony had a significant influence on the densities of all nine breeding species considered. Flight radii differed considerably between the species. Densities in the vicinity of trawlers were particularly high in black-headed gulls (Larus ridibundus), herring gulls (L. argentatus), and great black-backed gulls (L. marinus). Black-headed gulls and herring gulls had the highest proportion of ship-followers of the total number of seabirds at sea; no ship-followers at all were found in Sandwich terns (Sterna sandvicensis), common/ Arctic terns (S. hirundo/paradisaea), or guillemots (Uria aalge). Surface salinity, transparency, and thermal stratification were apparently the most important hydrographic properties, characterizing distinct water masses. Cluster analysis revealed a group of four offshore species (Fulmarus glacialis, Uria aalge, Rissa tridactyla, Larus fuscus) and seven inshore species (Larus ridibundus, L. canus, L. marinus, L. argentatus, Sterna sandvicensis, S. hirundo/paradisaea). Relations between important abiotic parameters and seabird diet are discussed.