Fishing Down a Halo

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Science  04 Sep 2009:
Vol. 325, Issue 5945, pp. 1182
DOI: 10.1126/science.325_1182a

Breeding seabirds are among the many animals that return to a central location after obtaining food. Such central-place foragers are expected to favor prey captured nearby. This preference could deplete prey in the area around the colony (forming a “Storer-Ashmole's halo”) through the removal of benthic species or the departure of mobile pelagic species. For marine predators, tests of this hypothesis must also consider water depth, which might have a greater impact than distance on foraging efficiency. Breeding thick-billed murres (Uria lomvia) usually return to their chick with a single easily identifiable item of food, and the birds are large enough to carry time-depth recorders that have little effect on their behavior. Thus, by monitoring prey deliveries of chick-rearing murres equipped with recorders, Elliott et al. were able to attribute individual prey items to specific dives. Drawing on data gathered over many years at a colony numbering roughly 100,000 individuals on Coats Island in northern Hudson Bay, they restricted their analyses to the interval of constant energy-delivery rates to chicks. Prey mass, and hence energy content, increased with distance from the colony, and dive depths became shallower as distance increased. As the breeding season progressed, murres flew farther for a given prey. Within each season, birds fished down the food web: taking larger fish first, then smaller fish, and even smaller invertebrate prey last. The prey depletion demonstrated by these four foraging patterns can lead to density-dependent population regulation and the life-history strategies of delayed maturity, low fecundity, and high adult survival found in many seabirds.

Auk 126, 613 (2009).

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