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Age structure and morphology differ among Pacific salmon (Oncorhynchus spp.) populations. Sexual selection and reproductive capacity (fecundity and egg size) generally favor large (old), deep-bodied fish. We hypothesized that natural selection from physical access to spawning grounds and size-biased predation by bears, Ursus spp., opposes such large, deep-bodied salmon. Accordingly, size and shape of salmon should vary predictably among spawning habitats. We tested this hypothesis by measuring the age composition and body depth of sockeye salmon, Oncorhynchus nerka, and the intensity of predation in a range of breeding habitats in southwestern Alaska. Stream width was positively correlated with age at maturity and negatively correlated with predation level. However, salmon spawning on lake beaches were not consistently old, indicating that different factors affect age in riverine- and beach-spawning populations. Body depths of male and female salmon were positively correlated with water depth across all sites, as predicted. However, the mouths of some streams were so shallow that they might select against large or deep-bodied salmon, even in the absence of bear predation. Taken together, the results indicated that habitat has direct and indirect effects (via predation) on life history and morphology of mature salmon.

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Connectivity of river networks and the movements among habitats can be critical for the life history of many fish species, and understanding of the patterns of movement is central to managing populations, communities, and the landscapes they use. We combined passive integrated transponder tagging over 4 years and strontium isotopes in otoliths to demonstrate that 25% of the mountain whitefish (Prosopium williamsoni) sampled moved between the Methow and Columbia rivers, Washington, USA. Seasonal migrations downstream from the Methow River to the Columbia River to overwinter occurred in autumn and upstream movements in the spring. We observed migration was common during the first year of life, with migrants being larger than nonmigrants. However, growth between migrants and nonmigrants was similar. Water temperature was positively related to the proportion of migrants and negatively related to the timing of migration, but neither was related to discharge. The broad spatio-temporal movements we observed suggest mountain whitefish, and likely other nonanadromous fish, require distant habitats and also suggests that management and conservation strategies to keep connectivity of large river networks are imperative.Résumé : La connectivité des réseaux hydrographiques et les déplacements entre habitats peuvent revêtir une importance capitale pour le cycle biologique de nombreuses espèces de poissons, la compréhension des motifs de déplacement constituant donc un aspect central de la gestion des populations et des communautés, ainsi que des paysages utilisés par ces dernières. L'intégration de données de radio-étiquettes passives intégrées recueillies sur 4 ans et de données d'isotopes de strontium d'otolites permet de démontrer que 25 % des ménominis de montagnes (Prosopium williamsoni) échantillonnés se déplaçaient entre la rivière Methow et le fleuve Columbia (État de Washington, États-Unis). Des avalaisons saisonnières de la rivière Methow vers le fleuve Columbia pour y passer l'hiver se produisaient en automne, et des montaisons, au printemps. Nous avons observé que la migration était répandue durant la première année de vie, les individus migrateurs étant plus grands que les autres. Ces deux groupes présentaient toutefois des taux de croissance semblables. La température de l'eau était positivement corrélée à la proportion d'individus migrateurs et négativement corrélée au moment de la migration, aucune de ces variables n'étant toutefois corrélée au débit. Les grands déplacements spatiotemporels observés portent à croire que le ménomini de montagnes et, vraisemblablement, d'autres poissons non anadromes ont besoin d'habitats éloignés les uns des autres et qu'il est donc impératif d'adopter des stratégies de gestion et de conservation visant à préserver la connectivité des grands réseaux hydrographiques. [Traduit par la Rédaction]