Shallow water ecosystems such as small streams and lakes often exhibit abrupt thermal fluctuations, and fishes inhabiting these systems must cope with these challenging thermal regimes daily. Presently, both the rate of change of thermal thresholds including the Critical Thermal Maximum (CT_max) within hourly or daily timescales and the thermal performance of mitochondrial energy transduction after acclimation remains unexplored. The objective of this study was to evaluate the short-term response in CT_max of the green sunfish (Lepomis cyanellus) while acclimating to a 30°C thermal regime, along with assessing mitochondrial energy transduction efficiency after acclimation. Individuals were held at 20°C for 60 days, then subjected to a 10°C increase in acclimation temperature. CT_max were obtained after 24, 72, 96, 120 and 192h of acclimation. CT_max increased after 96 h of acclimation, from 35.6°C at 72h to 36.6°C at 96h, however the differences were not statistically significant (p = 0.059). Oxygen consumption associated to proton conductance across the inner membrane and phosphorylation rates were significantly higher in 20°C acclimated specimens, and the reaction norm of both rates were positively correlated to assay temperature. Interestingly, the mitochondrial energy transduction efficiency, calculated as the flux control efficiency ratio, decreased after warm acclimation. The present study shows a decreased respiratory capacity of L. cyanellus mitochondria exposed to summer temperatures. The latter effect likely impinges challenges to energetically expensive processes including somatic growth, locomotion, and reproductive behavior.