Breeding seabirds are extreme central-place foragers, commuting long distances between colonies and feeding areas. Central-place foraging theory predicts that prey items close to the colony will be preferred over prey items distant from the colony, which can lead to prey depletion near the colony (“Storer-Ashmole's halo”). To investigate the relevance of these ideas to a single-prey loader, we equipped chick-rearing Thick-billed Murres (Uria lomvia) with time—depth recorders (1999–2007) and monitored prey deliveries (1993–2008). Because feeding rates were constant for chicks 3–15 days old, we restricted analyses to that age group. Between-date relationships were examined within individuals to avoid confounding effects of specialization and parental quality. The mass of prey items increased with foraging distance0,5, which suggests that large prey items were depleted by foraging pressure. Foraging distance for pelagic species increased through the season in years without spawning. After accounting for bathymetry, foraging effort decreased with distance from the colony for benthic fish and, in years without spawning, for pelagic fish. Within each season, Thick-billed Murres “fished down the food web”; they began by feeding on large fish, progressed to medium-sized prey (small fish), and finished feeding on small prey (invertebrates). We concluded that pelagic species responded to seabird foraging pressure by moving away from the colony, creating a three-dimensional halo. Benthic species, meanwhile, were depleted from a nearby shelf but remained abundant at a more distant offshore bench. We suggest that populations of central-place foragers are partially regulated by prey depletion.
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Vol. 126 • No. 3