Research Document - 2010/131

Summer Overlap Between a Central-place Forager And Its Prey in the Southern Gulf of St. Lawrence

By V. Harvey, M. Hammill, and D. Swain

Abstract

Predators affect prey populations by consuming individuals and inducing spatial changes in their behaviour. For the prey population, indirect effects of predation may be more important than predation itself by inducing reduced survival, growth or reproduction. The biomass of large demersal fishes of northwest Atlantic collapsed in early 1990s and failed to recover whereas the abundance of grey seals (Halichoerus grypus) is currently at historically high levels which could amplify their predation pressure on the fish stock. We followed 14 grey seals equipped with satellite transmitters in the southern Gulf of St. Lawrence during the summer period. Overall, seals were concentrated at <50 km from haulout place with a decreasing probability of foraging in areas away from haulout sites. Small white hake, small herring, and winter flounder were more abundant at distances of less than 50km from the haulout sites , whereas cunner, capelin, herring were more abundant further offshore. In August, seals overlap more with small cod near the haulout-sites compared to the overlap observed at 50-100km offshore, whereas the reverse was observed for sandlance. During September, the probability of having a foraging patch in areas of high density of cod was highest in between 50-100 km from the haulout site than in closer areas. The distribution of fish near haulout sites suggests that they try to minimize exposure to predation from grey seals, which could on a longer term affect recovery.

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