Ashmole's halo: direct evidence for prey depletion by a seabird

Abstract

population densities were measured at various distances from 2 large colonies of double-crested cormorants Phalacrocorax auritus to test the hypothesis that seabirds deplete prey close to their colonies. Fish densities were significantly lower in bays used by cormorants for feeding than in those outside their foraging range. Our findings provide direct evidence for prey depletion, and support Ashmole's (1963) hypothesis that seabird populations are limited by food supplies during the breeding season.