A significant heat event occurred in the Little Southwest Miramichi River (LSWM) in early July 2010 (Fig. 1A). Maximum daytime water temperatures during the July 6–8 event ranged from 28.0°C to 30.7°C (mean: 29.3 ± 0.5°C); nighttime temperatures ranged from 20.7°C to 24.5°C (mean: 22.8 ± 0.9°C). Such temperatures are known to alter metabolism in juvenile Atlantic salmon (Breau et al., 2011). To address our objectives, we chose to model this particular heat event for our experimental treatments. We established an acclimation regime based on the mean diel Tmax and Tmin of the 2 weeks prior to the event (16–21°C). To minimize thermal stress mortality, Tmax was set at 27°C for all treatments, just below the upper incipient lethal limit (7-day survival: 27.8°C; Elliott, 1991). Three temperature treatments were established within the limits of ΔT measured in the LSWM River in 2010 (Fig. 1B). We established minimum (i.e. nighttime) water temperatures as: (i) above the current legislated diel minimum temperature that determines river closure (≥20°C; DFO, 2012), 22–27°C; (ii) at the threshold, 20–27°C; and (iii) within the optimum range for growth and survival, 18–27°C. In order to subject parr to 27°C without exceeding the maximum natural diel ΔT (9°C) measured in the LSWM, an intermediate (‘ramp’) day was placed between the 16–21°C-acclimation regime and the temperature treatments. Temperatures on this day reached a maximum of 23°C and minimum of 18°C (Fig. 1B). Figure 1:(A) Thermal profile of the Little Southwest Miramichi (LSWM) River throughout early July 2010. A known thermal event occurred July 6–8. (B) Thermal profile of the LSWM River overlaid with experimental treatments. Details provided in text of Methods.