Ammonia 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 of Arapaima gigas to 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 juvenile A. gigas were 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.

Since temperature plays an important role in feed consumption, chemical reactions and metabolism, it was hypothesized that it is below ideal it could interfere in the masculinization of tilapia in biofloc system (BFT) and the ideal methyltestosterone (MT) concentration in the feed. The masculinization rates of Nile tilapia in a zero-water exchange BFT (and without clarification) at 25°C and 28 °C combined with concentrations of MT in the feed (0 (control), 10, 20, 30 and 40 mg ∙ Kg-1 of feed) were evaluated using 3 replicates per treatment (30 tanks of 50 liters, 2 larvae ∙ L-1). Larvae were fed five times a day for 28 days. The water quality and growth performance did not diverge between MT concentrations (p > 0.05). Larvae grew 2.7 times higher in 28°C than 25°C. The control treatments did not differ from each other for male proportion (mean = 66,75%) but differed from all hormonal treatments. These treatments presented masculinization rates above 98.7% and 96.9%, at temperatures of 25°C and 28°C, respectively, and did not differ from each other at the same MT concentrations. Therefore, it is feasible to use an even lower concentration (10 mg of MT ⋅ Kg-1 of feed) in a zero-water exchange BFT, regardless of these temperatures. At the lower temperature, the input of MT in the system was smaller due to smaller feed intake, however the fingerlings would take longer to reach commercial body weight. Furthermore, after 2 hours of the last hormone feeding, MT residues were not detected in any biofloc/water mixture samples.

How intensive the masculinization period of Nile tilapia can be in biofloc technology (BFT) based on higher stocking densities and a zero-water exchange protocol to mitigate the environmental impact has not been studied yet and needs investigation. Thus, our objective was to determine the optimal stocking density for reduction of hormonal effluent in BFT, better growth and masculinization rate of Nile tilapia larvae, and lower total variable cost. Five stocking densities (1.5, 3, 4.5, 6, and 7.5 larvae ∙ L-1) with four replicates were tested. Tilapias were fed with 60 mg of 17α-methyltestosterone (MT) ∙ Kg-1 of feed for 28 days, and with hormone-free feed until the common selling body weight of 1g. Water quality variables showed no significant differences between treatments, except for total settleable solids, pH, and final total organic carbon, that had a negative, positive and quadratic linear pattern with the increasing of stocking density, respectively. Growth performance variables such as final body weight, specific growth rate, survival and individual feed intake (and consequently hormone input) were superior at the lower stocking density. Masculinization rate (98.87%) did not differ among treatments, but total variable cost increased to produce 1g fingerlings in higher stocking densities. Hormone residue was not detected in BFT water of any treatment 12 hours after the last feeding so, in conclusion, even being the highest hormone input in the system, 1.5 larvae ∙ L-1 is the best level of stocking density studied for tilapia masculinization in a zero-water exchange BFT.