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Pronounced long-term juvenation in the spawning stock of Arcto-Norwegian cod (Gadus morhua) and possible consequences for recruitment

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Abstract:

The oldest and largest individuals are disappearing from many fish stocks worldwide as a result of overexploitation. This has been suggested to impair recruitment through decreasing the reproductive capacity of the spawners and increasing the mortality rate of the offspring. By using a time series on spawners biomass by age class for Arcto-Norwegian cod (Gadus morhua) from 1913–2004, I have documented pronounced changes in the spawning stock, including a trend towards younger fish, a less diverse distribution across ages, and a declining proportion of repeat spawners. Despite the total spawning stock biomass (SSB) being at similar levels now as in 1933, the mean age in the SSB has declined from 10–12.5 years to 7–8 years during the study period, and the percentage of fish of age 10 or above in the SSB has decreased from ~97% to ~10%. Contrary to earlier theoretical and experimental studies, no clear link between age structure and recruitment was found here. Recruitment to the Arcto-Norwegian cod stock may thus be more robust towards spawner juvenation than expected, possibly because of strong recruitment compensation.

Les individus les plus âgés et les plus grands sont en train de disparaître de plusieurs stocks de poissons à l’échelle du globe à cause de la surexploitation. On croit que ce phénomène nuit au recrutement en diminuant la capacité reproductive des reproducteurs et en augmentant le taux de mortalité des rejetons. Des séries chronologiques de données sur la biomasse des reproducteurs en fonction de la classe d’âge chez des morues (Gadus morhua) arcto-norvégiennes de 1913 à 2004 mettent en évidence des changement dans le stock des reproducteurs, en particulier une tendance vers les poissons plus jeunes, une distribution moins diversifiée des classes d’âge et une diminution de la proportion des itéropares. Même si la biomasse totale du stock des reproducteurs (BTSR) est de même niveau actuellement qu’en 1933, l’âge moyen dans la BTSR a décliné de 10–12,5 ans à 7–8 ans, durant la période d’étude, et le pourcentage de poissons d’âge 10 ou au delà dans la BTSR est tombé d’approximativement 97 à 10 %. Contrairement à certaines études antérieures théoriques et expérimentales, la présente étude ne trouve aucun lien clair entre la structure en âges et le recrutement. Le recrutement du stock arcto-norvégien de morues peut ainsi être plus robuste vis-à-vis le rajeunissement des reproducteurs qu’on ne l’avait prévu, peut-être à cause d’un fort recrutement compensatoire.[Traduit par la Rédaction]

Document Type: Research Article

Publication date: March 1, 2008

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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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