The distribution of sea birds relative to their fish prey off Vancouver Island: Opposing results at large and small spatial scales

We investigated the distribution of common murres, Uria aalge, and shearwaters, Puffinus griseus and P. tenuirostris, relative to their fish prey at two spatial scales, one of hundreds of kilometres and the other of kilometres. Data on oceanographic conditions, the distribution of sea birds and the distribution of fish in the upper water column were collected during three research surveys off Vancouver Island (British Columbia, Canada). At the large spatial scale, we found that both murres and shearwaters were more abundant inshore than offshore of the shelf break and that their fish prey were likewise more abundant in the inshore habitat. At the smaller scale, we investigated the relative importance of fronts and fish density. In contrast to what we expected based on our finding at the large spatial scale, there was a significant negative relationship between fish and both murre and shear-water density. We found that fronts, independent of fish density, did not have an effect on small-scale murre density and had only a weak effect on small-scale shearwater density. We suggest that the negative relationship between fish and sea bird density at the scale of kilometres may have been due to fish diving to escape foraging sea birds and thus avoiding capture in the near-surface trawl used in this study. If this avoidance behaviour is common in pelagic fish, then sea birds may affect fish populations by driving fish away from plankton-rich surface waters and thus negatively affecting the feeding and growth rates of their fish prey.