Global climate change is altering freshwater ecosystems and affecting fish populations and communities. Underpinning changes in fish distribution and assemblage‐level responses to climate change are individual‐level physiological constraints. In this review, we synthesize the mechanistic effects of climate change on neuroendocrine, cardiorespiratory, immune, osmoregulatory, and reproductive systems of freshwater and diadromous fishes. Observed climate change effects on physiological systems are varied and numerous, including exceedance of critical thermal tolerances, decreased cardiorespiratory performance, compromised immune function, and altered patterns of individual reproductive investment. However, effects vary widely among and within species because of species, population, and even sex‐specific differences in sensitivity and resilience and because of habitat‐specific variation in the magnitude of climate‐related environmental change. Research on the interactive effects of climate change with other environmental stressors across a broader range of fish diversity is needed to further our understanding of climate change effects on fish physiology.

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Using critical thermal maximum (CTMax) tests, we examined the relationship between upper temperature tolerances and fish size (fry–adult or subadult lengths) of rainbow trout Oncorhynchus mykiss (41–200‐mm TL), Apache trout O. gilae apache (40–220‐mm TL), largemouth bass Micropterus salmoides (72–266‐mm TL), Nile tilapia Oreochromis niloticus (35–206‐mm TL), channel catfish Ictalurus punctatus (62–264 mm‐TL), and Rio Grande cutthroat trout O. clarkii virginalis (36–181‐mm TL). Rainbow trout and Apache trout were acclimated at 18°C, Rio Grande cutthroat trout were acclimated at 14°C, and Nile tilapia, largemouth bass, and channel catfish were acclimated at 25°C, all for 14 d. Critical thermal maximum temperatures were estimated and data were analyzed using simple linear regression. There was no significant relationship (P > 0.05) between thermal tolerance and length for Nile tilapia (P = 0.33), channel catfish (P = 0.55), rainbow trout (P = 0.76), or largemouth bass (P = 0.93) for the length ranges we tested. There was a significant negative relationship between thermal tolerance and length for Rio Grande cutthroat trout (R2 = 0.412, P < 0.001) and Apache trout (R2 = 0.1374, P = 0.028); however, the difference was less than 1°C across all lengths of Apache trout tested and about 1.3°C across all lengths of Rio Grande cutthroat trout tested. Because there was either no or at most a slight relationship between upper thermal tolerance and size, management and research decisions based on upper thermal tolerance should be similar for the range of sizes within each species we tested. However, the different sizes we tested only encompassed life stages ranging from fry to adult/subadult, so thermal tolerance of eggs, alevins, and larger adults should also be considered before making management decisions affecting an entire species.

Natal origin and dispersal potential of the federally threatened Pecos bluntnose shiner (Notropis simus pecosensis) were successfully characterized using otolith microchemistry and swimming performance trials. Strontium isotope ratios ( 87 Sr: 86 Sr) of otoliths within the resident plains killifish (Fundulus zebrinus) were successfully used as a surrogate for strontium isotope ratios in water and revealed three isotopically distinct reaches throughout 297 km of the Pecos River, New Mexico, USA. Two different life history movement patterns were revealed in Pecos bluntnose shiner. Eggs and fry were either retained in upper river reaches or passively dispersed downriver followed by upriver movement during the first year of life, with some fish achieving a minimum movement of 56 km. Swimming ability of Pecos bluntnose shiner confirmed upper critical swimming speeds (U crit ) as high as 43.8 cm·s −1 and 20.6 body lengths·s −1 in 30 days posthatch fish. Strong swimming ability early in life supports our observations of upriver movement using otolith microchemistry and confirms movement patterns that were previously unknown for the species. Understanding patterns of dispersal of this and other small-bodied fishes using otolith microchemistry may help redirect conservation and management efforts for Great Plains fishes.Résumé : L'origine natale et le potentiel de dispersion du méné (Notropis simus pecosensis), une espèce menacée au niveau fédéral, ont été caractérisés avec succès par la microchimie des otolithes et des essais de performance natatoire. Les rapports d'isotopes de strontium ( 87 Sr: 86 Sr) des otolithes chez le fondule résident (Fundulus zebrinus) ont été utilisés comme substituts des rapports d'isotopes de strontium dans l'eau et ont révélé trois tronçons distincts sur le plan isotopique le long de 297 km de la rivière Pecos (Nouveau-Mexique, États-Unis). Deux motifs distincts de déplacements associés au cycle biologique ont été décelés chez méné. Les oeufs et les alevins étaient soit retenus dans des tronçons supérieurs de la rivière ou dispersés passivement vers l'aval pour ensuite se déplacer vers l'amont durant la première année de vie, le déplacement minimum de certains poissons atteignant 56 km. La capacité natatoire de méné a confirmé des vitesses de nage critiques supérieures (U crit ) pouvant atteindre 43,8 cm·s -1 et 20,6 longueurs du corps·s -1 30 jours après l'éclosion des poissons. Une bonne capacité natatoire tôt durant la vie appuie nos observations de déplacements vers l'amont obtenues de la microchimie des otolithes et confirme des motifs de déplacement jusqu'ici inconnus pour cette espèce. La compréhension des motifs de dispersion de cette espèce et d'autres poissons à petit corps à l'aide de la microchimie des otolithes pourrait aider à réorienter les efforts de conservation et de gestion des poissons des grandes plaines de l'Ouest. [Traduit par la Rédaction]