Constancy of frontal aggregations of seabirds at the shelf break in the Skagerrak

The persistence of concentrations of seabirds in the frontal zone between inflowing and outflowing water masses in the Skagerrak was studied over a 5-year period by means of replicate counts along a fixed cross-shelf transect. Two species of seabirds, fulmar Fulmarus glacialis and little auk Alle alle, showed significant affinities for the part of the frontal zone located at the shelf break. The distribution of the two species differed slightly with respect to the position of peak abundances and range in relation to the location of the frontal zone, yet the main pattern of their distribution was remarkably stable with a high frequency of aggregations observed only along less than a quarter of the transect located over the shelf break. Fulmars were aggregated over the lower parts of the slope located centrally in the front zone, while the concentration of little auks spanned the entire width of the front zone. The stability of the border of the aggregation of both species is striking, as very few birds were observed in the water mass entering the Skagerrak along the Danish coast or in the Norwegian Coastal Current. The average distribution of trawlers across the shelf indicated that the concentration of fulmars at the shelf break was unrelated to the attendance of trawlers.