ACUTE TOXICITY TESTS OF COPPER ON JUVENILE MOZAMBIQUE TILAPIA, OREOCHROMIS-MOSSAMBICUS (CICHLIDAE), AT DIFFERENT TEMPERATURES

In the Olifants River, copper is one of several metals which pose a th reat to the conservation status of the river. The LC(50) values attain ed during this study, can be used as an indication of the levels at wh ich copper becomes lethal to O. mossambicus and must only be seen as t he limits within which the concentration of copper can be regarded as lethal for this species in the Olifants River. Acute toxicity tests we re performed to determine the lethal copper concentrations for juvenil e Oreochromis mossambicus at representative mean summer (29 degrees C) and winter temperatures (19 degrees C) of the Olifants River, Kruger National Park, Northern Province, South Africa. Fish were exposed to v arious copper concentrations (0.0, 1.0, 1.7, 1.8, 2.0, 2.2, 2.3, 2.4, 2.5, 2.6, 2.7, 2.8, 3.0, 3.5, 4.0 mg/l) for 96 h in experimental flow- through systems at pH levels which ranged between 7.4 and 8.1, and mor talities were monitored. Dosage-survival curves of percentage survival versus actual copper concentrations were plotted after 24, 48, 72 and 96 h, and thereafter the LC(50) or median lethal concentration (the c oncentration of the test material that will kill or immobilize 50% of the test organisms over a predetermined length of time, usually 24 to 96 h) values were calculated for both summer and winter temperatures. Time survival curves of percentage survival versus time were also plot ted, after which the LT(50) or median survival time (the time required for half the fish to die at a specific toxicant concentration) values were calculated for the two temperatures. Toxicity curves were constr ucted using both the LC(50) and LT(50) values to determine the incipie nt LC(50) values (ILC(50) - the lethal concentration for 50% of the te st organism in a long-term exposure or lethal threshold concentration) . The LC(50) values (lethal copper concentration) of 2.61 mg/l and 2.7 8 mg/l as well as the ILC(50) values (lethal threshold concentration) of 2.95 mg/l and 3.32 mg/l for copper at 29 +/- 1 degrees C and 19 +/- 1 degrees C respectively, are considerably higher than the copper con centrations occurring in the water of the Olifants River, during summe r (0.055 +/- 0.016 mg/l) and winter (0.085 +/- 0.032 mg/l). Thus the f ish in the Olifants River are not exposed to acute lethal copper conce ntrations.