SPATIAL AND TEMPORAL VARIATION IN THE DIET OF DOUBLE-CRESTED CORMORANTS (PHALACROCORAX-AURITUS) BREEDING ON THE LOWER GREAT-LAKES IN THE EARLY 1990S

We sampled 1573 pellets and 560 boli regurgitated at double-crested co rmorant (Phalacrocorax auritus) colonies on the three lower Laurentian Great Lakes (Lake Ontario, Lake Huron, and Lake Erie) during the bree ding season in 1992 and 1994. This constitutes the first extensive stu dy of cormorant diet in this region. We found significant spatial and temporal heterogeneity in diet among colonies within a lake. Differenc es in diet among colonies were usually consistent with knowledge of ch anges in fish behaviour during the cormorant breeding season, differen ces in the surrounding bathymetry among breeding colonies, and the pro ximity of colonies to the preferred habitat of prey species. Our resul ts indicate that temporal and spatial variation, as well as the techni que for diet determination, are very important factors that must be co nsidered when assessing the effects of double-crested cormorant predat ion on fisheries.