5 - Linking predator foraging behaviour and diet with variability in continental shelf ecosystems: grey seals of eastern Canada  pp. 63-81

Linking predator foraging behaviour and diet with variability in continental shelf ecosystems: grey seals of eastern Canada

By W. D. Bowen et al.

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Upper-trophic-level marine predators are presumed to respond to environmental variability. However, the nature of these responses has been studied in few pinnipeds, particularly during the non-breeding season. Between 1992 and 2003, we measured a suite of behavioural, dietary and life-history variables in grey seals; variables which were expected to vary in response to changes in prey availability. We found significant inter-annual variation in some diving variables indicative of foraging effort and in the species composition of their diets. Postpartum body mass of adult females did not vary inter-annually, but duration of offspring investment (lactation length), total energy investment (offspring weaning mass) and the difference in weaning mass of male and female pups did. There was considerable inter-annual variation in the estimated biomass of grey seal prey species from summer bottom-trawl surveys; however, there was little correlation between grey seal response variables with those estimates. There could be several reasons for this result, but three stand out. First, grey seal numbers on the Scotian Shelf have increased exponentially over the past four decades, implying overall favourable environmental conditions. Grey seals may have adjusted their behaviour and diet to account for variability in prey characteristics other than biomass. Secondly, foraging grey seals and their prey were not sampled at the same time of year. Finally, trends in the biomass of many of the species eaten by grey seals are poorly estimated, thus limiting our understanding of predator responses to ecosystem state.

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