Upper thermal tolerances and vulnerability to global warming in a Brazilian Caatinga fish Astyanax bimaculatus (Linnaeus, 1758) population

Global warming is predicted to increase both average temperatures and the frequency and duration of heat waves. Tropical ectotherms, particularly those living in warm environments are more prone to receive heat impacts. Then, it is crucial to identify those populations already exposed to high temperatures that may beat risk of decimation by ongoing warming. We assessed heating risk in a population of the fish species Astyanax bimaculatus, inhabiting hot pools in intermittent streams in the Brazilian Caatinga ecoregion. We experimentally examined the upper thermal tolerance (CTmax), and provided estimates of warming tolerance (WT), as the difference between CTmax and micro-environmental peak temperatures (Tmax). Two CTmax estimates ranged between 37.6 and 40.6°C, whereas pond Tmax ranged between 34.6 and 41.3°C. This determines very narrow, even negative WTs, for these extreme heated A. bimaculatus populations, which ultimately may determine their local extinction in the coming decades.