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Form and uncertainty in stock-recruitment relations : observations and implications for Atlantic salmon (Salmo salar) management

Gurney, W.S.C. and Bacon, P.J. and McKenzie, E. and McGinnity, P. and Mclean, J. and Smith, G. and Youngston, A. (2010) Form and uncertainty in stock-recruitment relations : observations and implications for Atlantic salmon (Salmo salar) management. Canadian Journal of Fisheries and Aquatic Sciences, 67 (6). pp. 1040-1055. ISSN 1205-7533

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Abstract

This paper reports an investigation of stock-recruitment relations for Atlantic salmon (Salmo salar). We regard these relations as stochastic functions characterized by an expected stock-recruitment relation and deviations from this expectation driven by observational error and uncharacterised environmental variability. We estimate model parameters by standard Bayesian methods. Analysis of the input-output characteristics of segments of the salmon life cycle in the Girnock Burn (Northeast Scotland) reveals two independent regulatory processes, one between ova and fry and the other between fry and smolts. Comparison of stock-recruitment relations for Atlantic salmon in Scotland, Ireland, and Canada, reinforced by an extended series of simulation studies, shows that even when comparatively long time series of high quality data are available, it is frequently difficult to exclude the possibility of low stock depensation - an effect whose implication of enhanced extinction risk implies that precautionary management policy would pay special attention to the posssibility of its occurence. A particular feature of our simulation results is their demonstration that inappropriate combination of distinct subpopulations both increases process noise and distorts the expected stock-recruitment relation, thereby greatly reducing the accuracy of any system identification process. Nous étudions les relations stock-recrutement chez le saumon atlantique (Salmo salar). Nous considérons ces relations comme des fonctions stochastiques caractérisées par une relation stock-recrutement attendue et des déviations de cette attente causées par des erreurs d'observation et une variabilité environnementale non caractérisée. Nous estimons les paramètres du modèle par des méthodes bayésiennes standard. Une analyse des caractéristiques d'entrée et de sortie des segments du cycle biologique des saumons de Girnock Burn (nord-est de l'Écosse) révèle l'existence de deux mécanismes indépendants de régulation, l'un entre l'œuf et l'alevin et le second entre l'alevin et le saumoneau. Une comparaison des relations stock-recrutement chez les saumons atlantiques d'Écosse, d'Irlande et du Canada, appuyée par une longue série d'études de simulation, montre que, même lorsque des séries chronologiques comparativement longues de données de qualité sont disponibles, il est souvent difficile d'exclure la possibilité d'un effet de dépensation sur les stocks bas - un effet qui implique un risque accru d'extinction, une possibilité dont une politique prudente de gestion doit tenir un compte spécial. Un aspect particulier de nos résultats de simulation est la démonstration qu'une combinaison inappropriée de sous-populations distinctes augmente le bruit de traitement et fausse la relation stock-recrutement - ce qui réduit grandement la précision de tout mécanisme d'identification des systèmes.