THE IMPORTANCE OF PREY AGGREGATIONS TO THE DISTRIBUTION OF BRUNNICHS GUILLEMOTS IN STORFJORDEN, SVALBARD

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 numbers of Brunnich's gullemots f rom these colonies foraged within the area, as well as to the south, o utside 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 so uth, 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 obse rvations of prey were differentiated into two classes of signals, whic h we interpreted as originating from aggregated and dispersed organism s. The numbers of foraging guillemots were strongly correlated with th e strength of echoes of the aggregated type, whereas correlations with dispersed echoes were consistently weaker. The distribution of foragi ng guillemots showed no significant correlations with either horizonta l or vertical gradients of physical properties of the water column. Ou r finding that guillemots respond differently to aggregated and disper sed prey has important implications both for the interpretation of pas t work on the foraging ecology of marine birds, and for the management of fisheries.