Juvenile salmonid (Oncorhynchus spp.) use of constructed and natural side channels in Pacific Northwest rivers

Off-channel habitats, critical components in the life histories of Pacific salmonids (Oncorhynchus spp.), have become increasingly rare in human-modified floodplains. The construction of groundwater-fed side channels is one approach that has been used in the Pacific Northwest to recreate off-channel habitats. We evaluated the effectiveness of this technique by comparing 11 constructed side channels with paired reference sites (naturally occurring channels fed by mixed groundwater and surface water) in western Washington. While total salmonid densities were not significantly different between channel types, coho salmon (Oncorhynchus kisutch) densities were higher in constructed channels and trout densities were higher in reference channels during the winter. Constructed channels were deeper than reference channels and warmer in the winter and cooler in the summer but had lower physical habitat diversity, wood density, and canopy coverage. We did not detect significant differences in water chemistry or invertebrate parameters between channel types. Summer coho density was inversely correlated with minimum daily temperature and with total nitrogen and total phosphorous concentrations. Relative to other stream habitats, both constructed and reference channels sup- ported high densities of juvenile coho salmon during the summer and winter. Resume : Les habitats hors du chenal, des composantes essentielles du cycle biologique des salmonides du Pacifique (Oncorhynchus spp.), deviennent de plus en plus rares dans les plaines de debordement modifiees par l'activite hu- maine. La construction de chenaux lateraux alimentes par la nappe phreatique est l'une des methodes utilisees dans la region pacifique du nord-ouest pour recreer les habitats hors du chenal. Nous evaluons l'efficacite de cette technique en comparant 11 chenaux lateraux artificiels a des sites apparies (des chenaux d'origine naturelle alimentes par un me- lange d'eau phreatique et d'eau de surface) dans l'ouest du Washington. Bien que les densites totales de salmonides ne soient pas significativement differentes dans les deux types de chenaux, les densites de saumons coho (Oncorhynchus kisutch) sont plus elevees dans les chenaux artificiels et les densites de truites plus grandes dans les chenaux temoins en hiver. Les chenaux artificiels sont plus profonds que les chenaux temoins, ils sont plus chauds en hiver et plus frais en ete, mais ils possedent une diversite d'habitat physique, une densite de bois et une couverture d'arbres plus faibles. Nous n'avons pas trouve de differences significatives de qualite de l'eau, ni de caracteristiques de la faune invertebree dans les deux types de chenaux. Les densites de saumons coho en ete sont en correlation inverse avec la temperature journaliere minimale et avec les concentrations d'azote et de phosphore totaux. Par comparaison a d'autres habitats de cours d'eau, tant les chenaux artificiels que les chenaux temoins contiennent de fortes densites de jeunes saumons coho durant l'ete et l'hiver.

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