Behavioral thermoregulation and associated mortality trade-offs in migrating adult steelhead (Oncorhynchus mykiss): variability among sympatric populations

Abstract

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]

Climate change effects on stream and river temperatures across the northwest U.S. from 1980–2009 and implications for salmonid fishes

“…The trends in river and stream temperatures we document, in combination with increasing evidence of thermal constraints on some populations (Cooke et al 2004;Goneia et al 2006;Sutton et al 2007;Keefer et al 2007;Doremus and Tarlock 2008;Keefer et al 2009), suggest these predictions are being realized. Although most species have persisted through greater climatic perturbations in past millennia, modern climate change is happening especially rapidly, at the end of an already warm period, and is being imposed on populations that are often already depressed and fragmented from a century of intense human development (McIntosh et al 2000;Hessburg and Agee 2003).…”

“…Several anadromous salmon species that return to spawn each year in this region do so during the summer period when average temperatures and seasonal extremes (i.e., MaxWATs) are warming most rapidly. In some instances, these migrations are now periodically disrupted during especially warm periods as the fish pause to congregate near coldwater sources (Goneia et al 2006;Sutton et al 2007;Keefer et al 2009) and fish that migrate during the year's warmest temperatures often return less successfully to their spawning areas (Cooke et al 2004;Keefer et al 2008). Thermal "events" wherein hundreds or thousands of adult salmon die simultaneously because thermal tolerances are exceeded have been documented in recent years (Lynch andRisley 2003, Doremus andTarlock 2008;Keefer et al 2010) and important recreational fisheries are now sometimes suspended during warm periods to minimize additional fish stress (Brick et al 2008).…”

A collective navigation hypothesis for homeward migration in anadromous salmonids

“…At confluences of similar sized tributaries, salmon presumably could sample the odour cues of both branches by travelling a short distance up both or by simply traversing the river channel at the confluence perpendicular to the direction of flow. Indeed, short term sampling of non natal streams appears to be common in sal mon migrating in large rivers (Keefer et al 2009). …”

Thermal exposure of adult Chinook salmon in the Willamette River basin

“…Short-duration exposure to potentially stressful temperature and cumulative exposure elicit different behavioral and physiological outcomes, and either may have sublethal or delayed effects that reduce migrant survival or reproductive success (e.g., Lee et al, 2003;Clark et al, 2008;Crossin et al, 2008;Pankhurst and King, 2010). The third objective was to compare Chinook salmon temperatures with Willamette River temperatures to assess whether fish exploited cool-water refuges when temperatures along the migration route were warm (i.e., 'behavioral thermoregulation': Keefer et al, 2009;Mathes et al, 2010). We evaluated four hypotheses: (1) that the warmest exposure would occur in the main stem Willamette River; (2) that cumulative exposure would vary among individuals as a function of migration timing and duration; (3) that cumulative exposure would differ among sub-basin populations as a function of tributary thermal regimes and pre-collection holding time; and (4) that salmon would use cool-water refuges when Willamette River temperatures reached 18-20°C, a range associated with behavioral thermoregulation by other Chinook salmon populations.…”