Effects of the Juan de Fuca Eddy and upwelling on densities and distributions of seabirds off southwest Vancouver Island, British Columbia

Abstract

I compared meso-scale averages of sea surface temperature (SST) and hydroacoustic indices of prey abundance with densities of seabirds measured year-round over the continental shelf off southwest Vancouver Island, British Columbia, Canada in 1993-1996. A fixed strip transect (total length 110 km; width 300 m) was divided into six legs (lengths 14-30 km) to sample different shelf habitats. Three foraging guilds were considered: divers (dominated by Common Murres Uria aalge and other alcids), surface-feeders (dominated by California Gulls Larus californicus in summer, and other gulls year-round), and shearwaters (mainly Sooty Shearwater Puffinus griseus). Mean SST, prey scores, and densities of most birds (all surface-feeders and most divers) were low and similar among the 6 transect legs during winter and spring (mid-December through mid-June), but these measures all increased and differed significantly among the legs during summer and autumn (mid-June through mid-December). In summer and autumn, cold SSTs, high prey scores, and high seabird densities were consistently associated with the effects of the seasonal eddy over the Juan de Fuca canyon, whose influence spilled over the adjacent shelf. SST alone, however, did not explain the observed patterns of prey and seabird dispersion. One leg characterized by cold, upwelled water supported low prey and bird abundance, while another leg adjacent to the outer canyon had high prey and bird abundance, but SST was not consistently low. These results suggest that SST alone (such as satellite imagery) cannot be used to predict seabird distribution in this area. The interactions of bathymetry, ocean currents, and physical conditions of seabirds and their prey need to be more clearly understood in this area before reliable predictions of seabird distributions based on satellite imagery are possible.