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Longitudinal gradients in threshold sizes for alternative male life history tactics in a population of Atlantic salmon (Salmo salar)

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Atlantic salmon (Salmo salar) males may mature early in life in freshwater, rather than maturing after a migration to sea, if their size is above a threshold value. We analyzed the spatiotemporal variation in size and incidence of the early maturity tactic among males over an 8-year period in six subpopulations on two branches of a river and collected environmental data on each site and across the river scape. A positive longitudinal trend in the frequency of early maturing males that was stable over the 8-year period occurred from the mouth to the head of the river. Threshold sizes for early maturation varied among subpopulations; size thresholds for male parr to mature were higher in downstream habitats and lowest upstream. This pattern was consistent in both river branches over the 8-year period and was not related to either the density of parr or site-specific abiotic habitat characteristics. However, the cumulative incidence of habitat features that could impede migration of large individuals increased with increasing upstream distance. Migration costs may contribute to the observed variation in threshold sizes.

Les mâles du saumon atlantique (Salmo salar) peuvent atteindre la maturité sexuelle tôt dans la vie en eau douce au stade de tacon, plutôt qu'après une migration en mer, si leur taille est supérieure à une valeur seuil. Nous avons analysé la variation spatio-temporelle de la taille et de l'incidence de la stratégie de maturité hâtive chez les tacons mâles sur une période de 8 ans dans six sous-populations de deux branches d'une rivière et récolté des données sur l'environnement à chaque site et sur le paysage fluvial. Il existe un gradient positif longitudinal stable sur la période de 8 ans dans la fréquence de tacons mâles matures de l'embouchure à la tête de la rivière. La taille seuil pour la maturation des mâles tacons variait entre les sous-populations; la taille seuil pour la maturation des tacons mâles est plus élevée dans les habitats en aval et plus basse en amont. Ce patron est uniforme dans les deux branches de la rivière durant la période de 8 ans, mais n'est pas en corrélation avec la densité des tacons, ni les caractéristiques physiques des habitats spécifiques à chaque site. Cependant, l'incidence cumulative de caractéristiques de l'habitat pouvant empêcher la migration vers l'amont des grands individus anadromes augmente avec la distance vers l'amont. Les coûts de migration contribuent possiblement aux variations de la taille seuil pour la maturation.

Document Type: Research Article

Publication date: 2006-09-01

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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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