Tolerance level of different life stages of Nile tilapia Oreochromis niloticus (Linnaeus, 1758) to low pH and acidified waters

Abstract Background Acidification or acid rain in freshwater ecosystem is increasing especially in developing countries due to modernization, urbanization and population increase. An effect of acid deposition on water bodies is the lowering of the pH, which affects fish species negatively. This study evaluated tolerance levels of different life stages of Oreochromis niloticus to different acidic pH as it affects their survival, morphology and behaviour. Concentrated sulphuric acid was used for the acidification. Four hundred fifty O. niloticus of different life stages of fingerlings, juveniles and adults were stocked in the different acidic pHs in triplicates for 14 days. Results Fingerling mortalities were 100%, 96%, 70% and 42% in pH 3, 4, 5 and 6. One hundred percent, 44%, 22% and 12% juvenile mortalities were noted in pH 3, 4, 5, and 6, respectively. One hundred percent, 34%, 18% and 14% adult mortalities were recorded in pH 3, 4, 5 and 6, respectively. Tolerance of O. niloticus to low pH was Adult > juvenile > fingerlings. Morphological and behavioural changes observed include erratic swimming, gasping, slimy mucous secretions, skin erosion, bleeding of fins, impairment in feeding and lethargy. Mortality was attributed to acid stress, acidosis of the blood, erosion of the epidermal layers, sodium ion influx inhibition and ionoregulatory system failure. Conclusion Mortalities, behavioural responses and morphological abnormalities recorded in O. niloticus were due to the toxic effect of the acid on the body physiology. Adult O. niloticus can be described as acid-tolerant species (acidophilic). For the culture of the species, the pH of the water should be regularly checked for acidity, and the fish should be reared at optimal pH similar or better than the one in which the fish were brought (i.e. from a fish farm) in order to prevent metabolic stress and for the survival of the species in culture