Foraging under the risk of predation in juvenile Atlantic salmon (Salmo salar L.) : effects of social status and hunger

Abstract

Following exposure to a predator, socially dominant individuals may reduce their risk of predation by waiting until subordinates have resumed foraging before doing so themselves. Although such status-related ordering in the resumption of foraging activity has been observed in several bird species, the underlying mechanism(s) facilitating such a delay remains unknown. Social status per se and status-related foraging benefits prior to a threat of predation (i.e., individual hunger level) have both been suggested as possible mechanisms. We tested between these two alternative suggestions using pairs of stream-dwelling juvenile Atlantic salmon, for which the dominant-subordinate relationship was known. Fish were tested at equal and unequal hunger levels. Fish were presented with drifting prey, followed by a predation threat in the form of an aerial predator model. Which fish (i.e., dominant or subordinate) initially resumed foraging activity after exposure to the predator model was recorded. When both fish were at an equal hunger level, the dominant fish was more likely to resume foraging first. When the dominant and subordinate fish differed in their hunger level, the hungrier fish was the first to resume foraging regardless of social status. These results support the conclusion that hunger level, rather than social status per se, determines the order in which juvenile Atlantic salmon resume foraging after exposure to a predator.