The complexity of cormorants: stable isotopes reveal multiple prey sources and feeding site switching

Conflict between cormorants (Phalacrocorax spp.) and humans over fisheries is currently one of the most widespread wildlife management issues in the world. Cormorant impact assessments typically assume a single source of prey near the breeding colony. However, cormorants can fly long distances (>20 km), resulting in fish removal from multiple areas. Knowledge of the source of cormorant prey is critical for fisheries impact assessments. Carbon and nitrogen stable isotopes values from fish in double-crested cormorant (Phalacrocorax auritus) regurgitations revealed several prey sources for breeding birds in north-central Saskatchewan, Canada. Cormorants also switched feeding locations on a daily and seasonal basis. Foraging patterns inferred by stable isotope analysis closely paralleled observations of relative cormorant densities on different lakes. Up to 80% of prey came from areas well removed from the breeding colony lake (≤30 km). Cormorant foraging ecology can be complex and may be driven by factors independent of breeding colony site selection. The approach of estimating biomass removal from breeding colony lakes as the guideline for management decisions may be invalid in many locations.