| Summary: | As climate change advances, coastal marine ecosystems are predicted to experience increasingly frequent and intense heatwaves. At the same time, already variable CO₂ levels in coastal habitats will be exacerbated by ocean acidification. High temperature and elevated CO₂ levels can be stressful to marine organisms, especially during critical early life stages. Here, we used a fully cross-factored experiment to test the effects of simulated heatwave conditions (+ 4 °C) and elevated CO₂ (1000 µatm) on the aerobic physiology and swimming performance of juvenile Australasian snapper, Chrysophrys auratus, an ecologically and economically important mesopredatory fish. Both elevated temperature and elevated CO₂ increased resting metabolic rate of juvenile snapper, by 21-22% and 9-10%, respectively. By contrast, maximum metabolic rate was increased by elevated temperature (16-17%) and decreased by elevated CO₂ (14-15%). The differential effects of elevated temperature and elevated CO₂ on maximum metabolic rate resulted in aerobic scope being reduced only in the elevated CO₂ treatment. Critical swimming speed also increased with elevated temperature and decreased with elevated CO₂, matching the results for maximum metabolic rate. Periods of elevated CO₂ already occur in the coastal habitats occupied by juvenile snapper, and these events will be exacerbated by ongoing ocean acidification. Our results show that elevated CO₂ has a greater effect on metabolic rates and swimming performance than heatwave conditions for juvenile snapper, and could reduce their overall performance and potentially have negative consequences for population recruitment.
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