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Marine Ecology Progress Series

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MEPS 379:279-297 (2009)  -  DOI:

Role of predation by harp seals Pagophilus groenlandicus in the collapse and non-recovery of northern Gulf of St. Lawrence cod Gadus morhua

Emmanuel Chassot1,4,*, Daniel Duplisea1, Mike Hammill1, Amanda Caskenette1, Nicolas Bousquet2, Yvan Lambert1, Garry Stenson3

1Fisheries and Oceans Canada, Institut Maurice-Lamontagne, 850 route de la Mer, Mont-Joli, Quebec, G5H 3Z4, Canada
2Département de Mathématiques et Statistiques, Université Laval, Pavillon Alexandre-Vachon, Quebec, G1K 7P4, Canada
3Fisheries and Oceans Canada, Northwest Atlantic Fisheries Centre, PO Box 5667, St. John’s, Newfoundland, A1C 5X1, Canada
4Present address: IRD, CRH, Avenue Jean Monnet, BP 171, 34 203 Sète Cedex, France

ABSTRACT: A statistical catch-at-age model was developed to assess the effects of predation by the northwest Atlantic harp seal population on northern Gulf of St. Lawrence cod by estimating the relative importance of different sources of mortality that affected the stock during a period of collapse and non-recovery. Cod recruitment at age 1 is modeled via a non-linear stock-recruitment relationship based on total egg production and accounts for changes in female length-at-maturity and cod condition. Natural mortality other than seal predation also depends on cod condition used as an integrative index of changes in environmental conditions. The linkage between seals and cod is modeled through a multi-age functional response that was derived from the reconstruction of the seal diet using morphometric relationships and stomach contents of more than 200 seals collected between 1998 and 2001. The model was fitted following a maximum likelihood estimation approach to a scientific survey abundance index (1984 to 2006). Model results show that the collapse of the northern Gulf of St. Lawrence cod stock was mainly due to the combination of high fishing mortality rates and poor environmental conditions in the early to mid-1990s contributing to the current state of recruitment overfishing. The increase in harp seal abundance during 1984 to 2006 was reflected by an increase in predation mortality for the young cod age-groups targeted by seals. Although current levels of predation mortality affect cod spawning biomass, the lack of recovery of the NGSL cod stock seems mainly due to the very poor recruitment.

KEY WORDS: Cod · Harp seal · Functional response · Model · Predation · Recovery

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Cite this article as: Chassot E, Duplisea D, Hammill M, Caskenette A, Bousquet N, Lambert Y, Stenson G (2009) Role of predation by harp seals Pagophilus groenlandicus in the collapse and non-recovery of northern Gulf of St. Lawrence cod Gadus morhua. Mar Ecol Prog Ser 379:279-297.

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