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Why the Atlantic cod (Gadus morhua) stock off eastern Nova Scotia has not recovered

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Abstract:

An age-structured population dynamics model, incorporating interactions between Atlantic cod (Gadus morhua), the fishery, and the grey seal (Halichoerus grypus) population, was applied to the cod stock off eastern Nova Scotia (Northwest Atlantic Fisheries Organization Divisions 4Vs and 4W, commonly abbreviated to 4VsW), a stock that has dramatically declined since the late 1980s. Mortality was modeled as having three components: fishing mortality (F), seal predation (Mp), and all other sources of natural mortality (M). Specifically, M was assumed to be distinct for immature cod (ages 1–4; Mi) and mature cod (age 5 and older; Mm), and respective annual variations were estimated. Parameters estimated also included recruitment (cod abundance at age 1; R), F, and Mp. Based on our estimates of F, Mp, and M, it is unlikely that the collapse of the 4VsW cod stock can be attributed to a sudden increase in M; fishing appears to have been the primary cause for the stock's decline. However, after the moratorium on commercial fishing in 1993, increasing Mp and Mm and low R may have contributed to the failure of the 4VsW cod stock to recover.

Un modèle de dynamique de population basé sur les structures d'âges qui incorpore les interactions entre la Morue franche (Gadus morhua), les pêches commerciales et la population de Phoques gris (Halichoerus grypus) a été appliqué au stock de morues du large de la côte est de la Nouvelle-Écosse (divisions 4VsW de l'Organisation des pêches de l'Atlantique nord-ouest), un stock qui a chuté de façon dramatique depuis la fin des années 1980. La modélisation de la mortalité a trois composantes, la mortalité due à la pêche (F), la prédation par les phoques (Mp) et les autres sources de mortalité naturelle (M). Plus précisément, une mortalité distincte est attribuée aux morues immatures (âges 1–4, Mi) et matures (âges 5 et plus, Mm) et, pour chacune, les variations annuelles sont estimées. Les variables, comme le recrutement (R, abondance des morues à l'âge 1), F et Mp, sont aussi estimées. D'après nos estimations de F, Mp et M, il est peu probable que le déclin du stock 4VsW soit attribuable à une augmentation soudaine de M; les pêches commerciales semblent avoir été la cause principale de la chute du stock. Toutefois, après le moratoire sur les pêches commerciales en 1993, des valeurs croissantes de Mp et de Mm et un R faible peuvent avoir contribué à l'incapacité du stock 4VsW de se rétablir. [Traduit par la Rédaction]

Document Type: Research Article

Publication date: 2001-08-01

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  • Published continuously since 1901 (under various titles), this monthly journal is the primary publishing vehicle for the multidisciplinary field of aquatic sciences. It publishes perspectives (syntheses, critiques, and re-evaluations), discussions (comments and replies), articles, and rapid communications, relating to current research on cells, organisms, populations, ecosystems, or processes that affect aquatic systems. The journal seeks to amplify, modify, question, or redirect accumulated knowledge in the field of fisheries and aquatic science. Occasional supplements are dedicated to single topics or to proceedings of international symposia.
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