Natural diel thermal cycles impact thermal tolerance (Wehrly et al., 2007), bioenergetics (Beauregard et al., 2013; Eldridge et al., 2015), and metabolic status (Oligny-Hébert et al., 2015) of fishes. However, largely due to practical issues of temperature control in the laboratory, comparatively few studies investigate how temperature cycling, compared to stable acclimation temperatures, affect fish biology (see Threader and Houston, 1983; Houston and Gingras-Bedard, 1994; Mesa et al., 2002; Narum et al., 2013; Tunnah et al., 2017 as examples). Given that water temperatures in traditionally productive Atlantic salmon habitat, such as the Miramichi River, are increasing beyond the presumed thermal limit for this species (Elliott and Elliott, 1995), we were interested in whether the nature of the thermal cycle differentially influenced physiology. Specific to current federal (DFO) regulations whereby river closures occur when water temperature is ≥20°C for two consecutive nights (DFO, 2012), we investigated the effects of warming temperature cycles with distinct thermal minimums on the physiological response (i.e. thermal tolerance) of juvenile salmon. Although federal guidelines are designed primarily with mature life stages in mind, juveniles were used as a readily accessible and abundant proxy for their adult counterpart. The link between adult and juvenile fish physiology has been defined in many fishes (see Rodnick et al., 2004; Pörtner et al., 2008; Fowler et al., 2009; Morita et al., 2010), and although a more thermally tolerant life stage, juveniles provide insight into the physiological capacity of salmonids to withstand extreme events.