Evaluation of acute toxicity of ammonia in Genetically Improved Farmed Tilapia

Tilapia is one of the most widely cultivated cichlid species in the world. GIFT (Genetically Improved Farmed Tilapia) is an important strain for the international market. The continuous or frequent exposure to concentrations of non-ionized ammonium (NH3) above certain values induce morphological changes in the gills, affects the growth of the fish, and could also be lethal due to the toxicity of the compound. Acute toxicity of Ammonia in GIFT Tilapia strains was studied in three experiments (average weights of fish: 19.11, 6.54 and 0.47 g) conducted in recirculating aquaculture systems with 20 L aquariums. The effects of Ammonia concentrations: 0.50, 0.89, 1.58, 2.81, 5.00 mg NH3/L and stocking densities: 5, 10, 20 fish/aquarium were tested through three Acute Toxicity Tests of 96 h. The greatest susceptibility to the concentrations of NH3 evaluated was recorded in the smallest size and highest stocking density. The high mortalities observed were attributed to the toxic effects of NH3 in the configuration of the gills, as confirmed by the histopathological analysis of the tissue. The Branchial changes were hyperplasia, dilation of blood vessels, Lamellar fusion, aneurysm, and hemorrhage producing hypoxia to the fish. The results obtained suggest that GIFT is a strain with high tolerance to the toxic effects of Ammonia and surpass the values previously reported in the literature. The maximum concentration of NH3 recommended for the safe culture of tilapia fingerlings is 0.48 mg NH3/L.