Abstract
In coho salmon Oncorhynchus kisutch , no significant differences in critical thermal maximum ( c. 26·9° C, C Tmax ) were observed among size‐matched wild‐type, domesticated, growth hormone ( GH )‐transgenic fish fed to satiation, and GH ‐transgenic fish on a ration‐restricted diet. Instead, GH ‐transgenic fish fed to satiation had significantly higher maximum heart rate and Arrhenius breakpoint temperature (mean ± s.e. = 17·3 ± 0·1° C, T AB ). These results provide insight into effects of modified growth rate on temperature tolerance in salmonids, and can be used to assess the potential ecological consequences of GH ‐transgenic fishes should they enter natural environments with temperatures near their thermal tolerance limits.