This study assessed the production carrying capacity for Nile tilapia (Oreochromis niloticus) in net cages in reservoirs of the Brazilian semi-arid region during a supra-seasonal drought. It used the phosphorus mass balance models to estimate nutrient assimilation, with phosphorus concentration limits in the water column were set at 30 mu g center dot L-1, according to Brazilian legislation, and 60 mu g center dot L-1 for eutrophied environments. In the Santa Cruz reservoir, the phosphorus assimilation capacity was 5,848 kg center dot year-1 at 60 mu g center dot L-1, while in Umari it was 572.7 kg center dot year-1 at 30 mu g center dot L-1 and 4,620 kg center dot year-1 at 60 mu g center dot L-1. The carrying capacities were 680 tonnes in Santa Cruz at 60 mu g center dot L-1, and in Umari, 63 tonnes at 30 mu g center dot L-1 and 508 tonnes at 60 mu g center dot L-1. Despite higher phosphorus loading in Umari, Santa Cruz showed greater capacity at 60 mu g center dot L-1 due to its larger water volume. The study highlights the importance of reservoir morphometry and hydrological parameters for aquaculture sustainability during extreme droughts, and uses mass balance models to propose practices that balance production and environmental conservation.