1. The foraging ecology of Brunnich's guillemots Uria lomvia was studied during the breeding season in south-eastern Svalbard. In the region of Storfjorden there are two large breeding colonies comprising a total of about 540 000 individuals. These birds forage in the western part of Storfjorden and further to the south. Their main prey: are polar cod Boreogadus saida, pelagic amphipods Parathemisto spp. and euphausiids Thysanoessa inermis. 2. A ship-based transect survey was used to record bird abundance and the acoustically determined biomass of presumed prey. The five transects were divided into 33 segments, each 8-11 km in length. The resolution of the survey was 150 m, and analyses of correlations between predators and prey were performed at length scales: from 150 m to 9 km. We differentiated acoustic signals into aggregated and dispersed categories according to the estimated horizontal distribution of presumed prey. 3. Foraging guillemots were consistently more strongly correlated with the aggregated prey than with dispersed prey over scales ranging from 150 m to 9 km. Correlations were weak at small scales (150 m - 1 km) and increased and stabilized at scales of 23 km. The spatial scale at which we obtained a shift from weak to strong correlations between guillemots and their prey was similar to the scale at which the spatial variances in both guillemot and prey abundance were high. 4. Guillemots showed low correlations with prey at low prey densities. Similarly, correlations between guillemots and prey were low at low bird densities. The data support the hypothesis that the birds associate with prey patches with densities above a certain threshold, and that 'regional' prey abundances affect local use of patches. 5. The numerical aggregative response curves between guillemot and prey density were classified as being neither hyperbolic (type II) or sigmoidal (type III) within the range of prey densities observed in this study. The aggregative response curves were sensitive to spatial scales, which suggest that studies of response curves should be conducted at a range of spatial scales.

We studied the influence of the distribution of prey and hydrographic fronts on the spatial distribution of foraging Brunnich's guillemots (Uria lomvia) in Storfjorden, southeastern Svalbard in late July 1992. Two large breeding colonies, comprising a total of 540,000 individuals, were located adjacent to the study area, and large numbers of Brunnich's gullemots from these colonies foraged within the area, as well as to the south, outside of Storfjorden. Within the study area, most guillemots foraged on the west side of the fjord, coincident with a weak subsurface front between warm Atlantic water, which penetrated Storfjorden from the south, and cold Arctic water. Food samples from the guillemots collected in the study area contained primarily crustaceans (Parathemisto spp. and Thysanoessa inermis) and polar cod Boreogadus saida. Acoustic observations of prey were differentiated into two classes of signals, which we interpreted as originating from aggregated and dispersed organisms. The numbers of foraging guillemots were strongly correlated with the strength of echoes of the aggregated type, whereas correlations with dispersed echoes were consistently weaker. The distribution of foraging guillemots showed no significant correlations with either horizontal or vertical gradients of physical properties of the water column. Our finding that guillemots respond differently to aggregated and dispersed prey has important implications both for the interpretation of past work on the foraging ecology of marine birds, and for the management of fisheries.