Systematic bias in estimates of reproductive potential of an Atlantic cod (Gadus morhua) stock: implications for stock–recruit theory and management

Authors: Marshall, C T.; Needle, Coby L; Thorsen, Anders; Kjesbu, Olav Sigurd; Yaragina, Nathalia A

Source: Canadian Journal of Fisheries and Aquatic Sciences, Volume 63, Number 5, May 2006 , pp. 980-994(15)

Publisher: NRC Research Press

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Abstract:

Stock–recruit relationships that use spawning stock biomass (SSB) to represent reproductive potential assume that the proportion of SSB composed of females and the relative fecundity (number of eggs produced per unit mass) are both constant over time. To test these two assumptions, female-only spawner biomass (FSB) and total egg production (TEP) were estimated for the Northeast Arctic stock of Atlantic cod (Gadus morhua) over a 56-year time period. The proportion of females (FSB/SSB) varied between 24% and 68%, and the variation was systematic with length such that SSB became more female-biased as the mean length of spawners increased. Relative fecundity of the stock (TEP/SSB) varied between 115 and 355 eggs·g–1 and was significantly, positively correlated with mean length of spawners. Both FSB and TEP gave a different interpretation of the recruitment response to reductions in stock size (overcompensatory) compared with that obtained using SSB (either compensatory or depensatory). There was no difference between SSB and FSB in the assessment of stock status; however, in recent years (1980–2001) TEP fell below the threshold level at which recruitment becomes impaired more frequently than did SSB. This suggests that using SSB as a measure of stock reproductive potential could lead to overly optimistic assessments of stock status.

Les relations stock–recrues qui utilisent la biomasse du stock reproducteur (SSB) pour représenter le potentiel reproductif présupposent que la proportion de SSB représentée par les femelles et que la fécondité relative (nombre d'oeufs produits par unité de masse) sont toutes deux invariables dans le temps. Afin d'évaluer ces deux présuppositions, nous avons estimé la biomasse des reproducteurs femelles seuls (FSB) et la production totale d'oeufs (TEP) chez un stock de morues franches (Gadus morhua) de l'Arctique sur une période de 56 ans. La proportion de femelles (FSB/SSB) varie de 24 à 68 % et elle change systématiquement en fonction de la longueur de telle manière que SSB favorise de plus en plus les femelles à mesure que la longueur moyenne des reproducteurs augmente. La fécondité relative du stock (TEP/SSB) varie de 115 à 355 oeufs·g–1 et elle est en corrélation positive significative avec la longueur moyenne des reproducteurs. FSB et TEP fournissent toutes deux une interprétation différente de la réaction du recrutement à la réduction de la taille du stock (surcompensation) par comparaison à la réaction du recrutement obtenue à partir de SSB (compensation ou bien effet d'Allee). Il n'y a pas de différence entre SSB et FSB pour ce qui est de l'évaluation du statut du stock; cependant, ces dernières années (1980–2001), TEP est tombée sous le seuil sous lequel le recrutement se détériore plus fréquemment que SSB. Cela laisse croire que l'utilisation de SSB comme mesure du potentiel reproductif du stock pourrait mener à des évaluations trop optimistes du statut du stock.

[Traduit par la Rédaction]

Document Type: Research Article

Publication date: May 1, 2006

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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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