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Marine Ecology Progress Series

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MEPS 442:149-167 (2011)  -  DOI:

Evaluating the potential for grey seal predation to explain elevated natural mortality in three fish species in the southern Gulf of St. Lawrence

Hugues P. Benoît1,*, Douglas P. Swain1, W. Don Bowen2, Greg A. Breed3, Mike O. Hammill4, Valerie Harvey4

1Gulf Fisheries Centre, Fisheries and Oceans Canada, Moncton, New Brunswick E1C 9B6, Canada
2Bedford Institute of Oceanography, Fisheries and Oceans Canada, Dartmouth, Nova Scotia B2Y 4A2, Canada
3Department of Biology, Dalhousie University, Halifax, Nova Scotia B3H 4J1, Canada
4Institut Maurice Lamontagne, Fisheries and Oceans Canada, Mont-Joli, Quebec G5H 3Z4, Canada

ABSTRACT: Despite 2 decades of very low fishing levels, numerous NW Atlantic demersal fish stocks have failed to recover from collapsed states or are presently collapsing. In the southern Gulf of St. Lawrence, adult natural mortality (M) appears to be the demographic rate that most limits population productivity in at least 3 species: Atlantic cod Gadus morhua, white hake Urophycis tenuis and winter skate Leucoraja ocellata. The causes of elevated M are not well understood, though there is indirect evidence consistent with an effect of predation by grey seals Halichoerus grypus. However, direct evidence is lacking due to uncertainty in the seal diet. Consequently, Monte Carlo simulations were undertaken using data on the spatial overlap between the seals and the fishes and a seal food-consumption model, to estimate the plausibility that different seal-diet compositions could explain observed M levels. Under the simulation assumptions, we find that predation could explain up to 20 to 50% of M in adult white hake and cod even if these species comprise a small percentage of grey seal diets (<25%). If seals consume some of these fish only partially, by selectively feeding on soft tissues, a predation effect becomes more plausible. Predation can also plausibly explain the observed elevated M in adult winter skate, even if they comprise a negligible (<0.1%) percentage of the grey seal diet. Though the simulations deal with the factors that shape the potential for predation, a greater understanding of prey selection is required to conclude whether grey seals are actually adversely impacting these fish populations.

KEY WORDS: Energetics model · Monte Carlo simulation · Generalist predator · Natural mortality · Belly-biting · Partial consumption

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Cite this article as: Benoît HP, Swain DP, Bowen WD, Breed GA, Hammill MO, Harvey V (2011) Evaluating the potential for grey seal predation to explain elevated natural mortality in three fish species in the southern Gulf of St. Lawrence. Mar Ecol Prog Ser 442:149-167.

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