Seabird distribution in relation to hydrography in the Skagerrak

During the SKAGEX II and SKAGEX III expeditions in September 1990 and January 1991 it was, for the first time, possible to undertake a detailed study of the relationships between the distribution of planktivorous seabirds and the complex hydrography of the Skagerrak. Counts along the transect from Oksoy (58°02′ N, 08°30′ E) to Hanstholm (57°14′ N, 08°33′ E) showed that fulmar Fulmarus glacialis dominated the bird fauna in September, whereas little auk Alle alle dominated in January. During five replicate transects these two species were highly aggregated, with at least 75% of the totals being counted near one or two of the 12 hydrographical stations. These two stations were located along the middle segment of the transect in the transition zone between in-flowing and out-flowing surface water masses. Areas within the flow gradient zone, characterised by a shallow halocline supported more individuals of both species than areas with a deeper halocline, whereas areas with a mixed water column did not support any of these seabirds in appreciable numbers. The area of the domed halocline, where most birds were observed, coincided with the doming of the nitricline and increased sub-surface concentrations of chlorophyll a. The aggregations of fulmar and little auk could not be related to particular coarse-scale surface fronts. Independent historic data on the average spatial variation of the studied species and the depth of the high saline water collected throughout the Skagerrak were used to test the general application of the findings. The historic data emphasised the affinity of fulmar to the centre of the region influenced by upwelling of Atlantic water, and the affinity of little auk to both the centre and periphery of the domed region.