Simple SummaryThe world has been changing, and these changes have affected fish in the wild and in captivity. The Intergovernmental Panel on Climate Change (IPCC), which is an international body, predicts climate scenarios for the future that are based mainly on rising temperatures and carbon dioxide levels. Many species are sensitive to scenarios that are considered drastic, which suggests future losses in the rearing of aquatic organisms. In our study, we evaluate how these drastic climate scenarios and exposure to a sublethal concentration of ammonia, a product resulting from the consumption of dietary protein that can be toxic, affect the pirarucuArapaima gigas, a carnivorous Amazonian fish species that can reach up to 12 kg in one year of farming, but needs to consume a large amount of protein in its feed to do so. Our results show that the pirarucu is sensitive to ammonia and climate change, and that this can impair the animal’s growth and some vital body functions, implying economic losses for those who produce it and higher prices for those who consume it.AbstractAmmonia is toxic to fish, and when associated with global warming, it can cause losses in aquaculture. In this study, we investigated the physiological and zootechnical responses ofArapaima gigasto the current scenarios and to RCP8.5, a scenario predicted by the IPCC for the year 2100 which is associated with high concentrations of environmental ammonia (HEA). Forty-eight chipped juvenileA. gigaswere distributed in two experimental rooms (current scenario and RCP8.5) in aquariums with and without the addition of ammonia (0.0 mM and 2.44 mM) for a period of 30 days. The HEA, the RCP8.5 scenario, and the association of these factors affects the zootechnical performance, the ionic regulation pattern, and the levels of ammonia, glucose, triglycerides, sodium, and potassium in pirarucu plasma. The branchial activity of H+-ATPase was reduced and AChE activity increased, indicating that the species uses available biological resources to prevent ammonia intoxication. Thus, measures such as monitoring water quality in regard to production, densities, and the feed supplied need to be more rigorous and frequent in daily management in order to avoid the accumulation of ammonia in water, which, in itself, proved harmful and more stressful to the animals subjected to a climate change scenario.