After spawning, iteroparous steelhead Oncorhynchus mykiss from the Columbia River basin must navigate several hydroelectric dams on their way to the Pacific Ocean. We used radiotelemetry to investigate migration rates, downstream passage routes, and success of adult steelhead kelts migrating past lower Snake River and Columbia River dams during the springs of 2001 and 2002. Seaward-migrating kelts were collected, radio-tagged, and volitionally released from the juvenile bypass facilities at Lower Granite Dam (LGR) on the Snake River and at McNary Dam (McN) and John Day Dam (JDD) on the Columbia River. Migration success rates from LGR to the study area exit (8 km east of Portland, Oregon) were poorer during the low-flow nonspill conditions of 2001 (4.1%) than in the more typical flow year of 2002 (15.6%). Kelts tagged and released at Columbia River dams had substantially higher migration success than those released on the Snake River; 59.6% and 62.3% of the kelts released at McN and 63.6% and 80.0% of those released at JDD were contacted at the study area exit during 2001 and 2002, respectively. Kelt dam passage was predominately via spillways and surface flow routes, and during periods of spill 90.0% or more kelts typically passed via nonturbine routes. Only 47.2% of kelts were guided out of turbine intakes by screen systems during nonspill periods. Turbine passage, the primary alternative route during nonspill periods, may be a substantial source of kelt mortality. The poor migration success rate of Snake River kelts in both 2001 and 2002 suggests that additional management (i.e., kelt reconditioning, transportation, or both) may be warranted to boost iteroparity rates in this population.

We used ultrasound imaging and passive integrated transponder (PIT)-tagging programs to assess maturation status and iteroparity patterns in summer-run steelhead (anadromous rainbow trout, Oncorhynchus mykiss) of the interior Columbia River Basin (Pacific Northwest, USA). Postspawn kelts examined in downstream fish bypass systems at Columbia River and Snake River dams were disproportionately female (>80%) and majorities were of wild origin, unlike prespawn steelhead at these sites. Annual repeat migration estimates varied from 2.9% to 9.0% for kelts tagged at lower Columbia River dams (n = 2542) and from 0.5% to 1.2% for Snake River kelts (n = 3762). Among-site differences reflected greater outmigration distance and additional dam passage hazards for Snake River kelts. There was also strong evidence for condition-dependent mortality, with returns an order of magnitude higher for good-versus poor-condition kelts. Disproportionately more females and wild fish also returned, providing potentially valuable genetic and demographic benefits for the Columbia River's threatened steelhead populations. Results overall provide baseline data for evaluating kelt mortality mitigation efforts and basic life history information for steelhead conservation planning.Résumé : L'imagerie ultrasonique et les programmes de marquage transpondeurs intégrés passifs (PIT) nous servent à évaluer le statut de maturation et les patrons d'itéroparité chez les truites arc-en-ciel anadromes (Oncorhynchus mykiss) à montaison estivale du bassin intérieur du Columbia (nord-ouest pacifique, É .-U.). Les charognards d'après la fraie examinés dans les systèmes de passes migratoires de dérivation vers l'aval aux barrages des rivières Columbia et Snake sont de façon disproportionnée de sexe féminin (>80 %) et la plupart des individus sont d'origine sauvage, contrairement aux truites arc-en-ciel anadromes à ces mêmes sites avant la fraie. Les estimations de l'importance des migrations annuelles ré-pétées varient de 2,9-9,0 % chez les charognards marqués aux barrages du Columbia inférieur (n = 2542) et de 0,5-1,2 % chez les charognards de la Snake (n = 3762). Les différences entre les sites reflètent la plus grande distance de migration vers la mer et les risques additionnels du passage des barrages pour les charognards de la Snake. Il y a aussi de fortes indications de l'existence d'une mortalité reliée à la condition; en effet, les retours sont dix fois plus abondants chez les charognards en bonne condition que chez ceux en mauvaise condition. De façon disproportionnée, il y a aussi plus de retours de femelles et de poissons sauvages, ce qui apporte potentiellement des bénéfices génétiques et démographiques précieux aux populations menacées de truites arc-en-ciel anadromes du Columbia. Globalement, nos résultats fournissent les données de base nécessaires pour évaluer les efforts de mitigation de la mortalité des charognards, ainsi que des informations démographiques fondamentales pour planifier la conservation de la truite arc-en-ciel anadrome.[Traduit...