We used radiotelemetry to assess thermoregulatory behaviors for 14 populations (n = 3985) of adult summer steelhead (Oncorhynchus mykiss) as they passed through the Columbia River migration corridor. Steelhead use of small cool-water tributaries (''thermal refugia'') rapidly increased when the Columbia River reached a temperature threshold of about 19 8C. When main stem temperatures were warmest (i.e., >21 8C), more than 70% of the tagged fish used refugia sites and these fish had median refugia residence times of 3-4 weeks. Thermoregulatory responses were similar across populations, but there were large among-population differences in the incidence and duration of refugia use likely linked to population-specific migration timing patterns. In survival analyses using 1285 known-origin steelhead, fish that used thermal refugia were significantly less likely to survive to natal basins, were harvested at relatively high rates in refugia tributaries, and had greater unknown mortality in the main stem. These results highlight the trade-off between the presumed physiological benefits of thermal refugia use and a likely increase in harvest and other mortality risks that arise when preferred thermal habitats are severely constricted.Résumé : La radiotélémétrie nous a servi à évaluer le comportement thermorégulateur de 14 populations (n = 3 985) de truites arc-en-ciel anadromes (Oncorhynchus mykiss) adultes d'été lors de leur passage à travers le corridor de migration du fleuve Columbia. L'utilisation par les truites arc-en-ciel anadromes de petits tributaires d'eau froide (« refuges thermiques ») augmente rapidement quand la température du Columbia atteint un seuil d'environ 19 8C. Lorsque les températures du cours principal sont maximales (c'est-à-dire >21 8C), plus de 70 % des poissons marqués utilisent les sites de refuge et la durée médiane du séjour de ces poissons dans les refuges est de 3-4 semaines. Les réactions de thermorégula-tion sont les mêmes dans les diverses populations, mais il y a d'importantes différences entre les populations en ce qui concerne la fréquence et la durée de l'utilisation des refuges, vraisemblablement liées aux patrons temporels de migration spécifiques à chaque population. Des analyses de survie impliquant 1 285 truites arc-en-ciel anadromes d'origine connue montrent que les poissons qui utilisent les refuges thermiques sont significativement moins susceptibles de survivre jusqu'à leur retour à leur bassin versant de naissance, qu'ils sont récoltés à des taux relativement élevés dans les tributaires refuges et qu'ils ont une mortalité de cause inconnue plus forte dans le cours principal. Ces résultats mettent en évidence le compromis entre les bénéfices physiologiques présumés de l'utilisation des refuges thermiques et une vraisemblable augmentation de la récolte et des autres risques de mortalité qui apparaissent lorsque les habitats thermiques préférés sont grandement réduits.[Traduit par la Rédaction]

We used radiotelemetry to evaluate the temporary staging of adult migrating steelhead Oncorhynchus mykiss into nonnatal tributary rivers of the Columbia River and to determine the effects of staging behavior on migration rate. By monitoring the movement patterns of 2,900 individual steelhead over 3 years (1996, 1997, and 2000), we determined that an average of 61% of the steelhead destined for upstream areas temporarily staged in one or more tributaries in the lower Columbia River for durations from less than 1 h to 237 d. Median residence time varied significantly by tributary used and year and, based on canonical correlation analysis, was correlated with main‐stem Columbia River water temperature. Steelhead that temporarily staged in tributary rivers migrated through the lower Columbia River significantly more slowly than steelhead that did not use tributaries. Use of coolwater tributaries as thermal refugia during warm summertime conditions significantly influences the migratory behavior of Columbia River adult steelhead. Our results highlight the need to preserve the water quality parameters of existing cooler‐water Columbia River tributaries and to rehabilitate watersheds that historically maintained cooler‐water tributaries as sources of thermal refugia for adult summer steelhead returning to the basin.

Despite the success of catch-and-release regulations, exposing fish to air during release has emerged as a growing concern over the past two decades. We evaluated the effect of air exposure during midsummer on survival of Yellowstone Cutthroat Trout Oncorhynchus clarkii bouvieri, Bull Trout Salvelinus confluentus, and Rainbow Trout O. mykiss exposed to catch-and-release angling. Fish were sampled by angling on Palisades Creek (August 2016), Sawmill Creek, and the Main Fork of the Little Lost River, Idaho (July−August 2017). After capture, fish were kept underwater while they were measured and individually tagged. Anglers, in groups of two to four, caught study fish and gave them an air exposure treatment of 0, 30, or 60 s. Single-pass backpack electrofishing was then used to recapture tagged fish and estimate relative survival. In total, 328 Yellowstone Cutthroat Trout were sampled (0 s: n = 110; 30 s: n = 110; 60 s: n = 108), 278 Bull Trout (0 s: n = 92; 30 s: n = 94; 60 s: n = 92), and 322 Rainbow Trout (0 s: n = 103; 30 s: n = 106; 60 s: n = 113). The majority of fish were caught using artificial flies (≥92%) and were hooked in the corner of the mouth, lower jaw, or upper jaw (≥78%) in all three species. No difference in survival was observed among air exposure treatments for all three species. Results from the present study along with those from prior field studies of air exposure times during angling suggest that mortality from exposing fish to air for ≤60 s is not likely a population-level concern in catch-and-release fisheries for these species.Catch-and-release angling regulations are one regulatory tool available for fisheries managers (Isermann and Paukert 2010) and are implemented for a variety of reasons such as to prevent the consumption of contaminated fish (Carline et al. 1991), protect species that are easily overexploited (Sullivan 2003), improve the quality of the fishery (Perry et al. 1995; Schneider and Lockwood 2002), and for social reasons (Schill and Scarpella 1997). When