The upper thermal tolerance of brook trout Salvelinus fontinalis was estimated using critical thermal maxima (CT max) experiments on fish acclimated to temperatures that span the species' thermal range (5-25 C). The CT max increased with acclimation temperature but plateaued in fish acclimated to 20, 23 and 25 C. Plasma lactate was highest, and the hepato-somatic index (I H) was lowest at 23 and 25 C, which suggests additional metabolic costs at those acclimation temperatures. The results suggest that there is a sub-lethal threshold between 20 and 23 C, beyond which the fish experience reduced physiological performance. K E Y W O R D S body condition, critical thermal maxima, fish, hepato-somatic index, lactate, osmolality Freshwater ectotherms have negligible heat retention capacity relative to the amount of metabolic heat they generate; thus, their body temperature is mainly determined by the environment (McDonnell & Chapman, 2015). Freshwater ectotherms are therefore especially vulnerable to climate change effects such as warming water temperatures and fluctuating thermal regimes (Woodward et al., 2010). The physiological function of an ectotherm is maintained over a range of temperatures; nonetheless, reduced performance may occur when

By understanding range-wide intraspecific variation in metabolic rate we can better understand how organisms have adapted to their environment. However, methods to quantify metabolic rate of fishes from remote areas or those that cannot be brought back to the laboratory because of imperilment status are lacking. Consequently, practical and reliable field-based methods are needed. To address this need, we developed a simple yet robust intermittent-flow respirometry system, adapted from a design commonly used in the laboratory that is readily suited for field use. Standard metabolic rate (SMR), maximum metabolic rate (MMR) and aerobic scope (AS) estimates were obtained from juvenile lake trout () and brook trout () using both field- and laboratory-based systems. Whole-fish SMR, MMR and AS estimates from the field and laboratory methods did not differ from one another (ANCOVA and LMM: all > 0.05) for either species and were comparable to estimates previously reported. Our field setup is a simpler system than the conventional laboratory-based system that requires less power and equipment to operate, yet still offers users the ability to: (1) acclimate fish to the respirometry chamber; (2) measure oxygen consumption during a shorter period (1 h), which yield metabolic rate estimates comparable to systems that take measurements over longer periods; and (3) take repeated oxygen consumption measurements with manual user-defined flush and measurement phase routines. Developing practical and reliable field respirometry methods, as demonstrated here, is important if we wish to improve our ability to predict how imperiled species will respond to changes in their environment. Such knowledge is critical for informing conservation strategies.

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