摘要：During the present investigation, the 96-hr LC50 and lethal concentrations of copper and cobalt for Tilapia nilotica were determined under controlled laboratory conditions at constant pH (7.25), total hardness (255 mgL-1) and temperature (30 °C). During acute toxicity studies, the physico-chemical parameters of water viz. temperature, pH, dissolved oxygen, electrical conductivity, carbon dioxide, total ammonia, calcium, sodium, magnesium, potassium and total hardness were monitored at 12-hr intervals for each test. Fish were exposed to different concentrations of copper and cobalt, separately, starting from zero with an increment of 0.05 and 0.5 mgL-1. After 96-hr exposure of various concentrations of each metal, the fish mortality data were recorded with three replicates for each concentration. The 96-hr LC50 and lethal concentrations for each metal was computed by using Probit analyses method at 95% confidence interval. The 96-hr LC50 and lethal concentrations of copper for Tilapia nilotica were computed as 25.00±0.65 and 47.56±1.18 mg L-1, respectively. However, the tolerance limits of fish for cobalt, in terms of 96-hr LC50 and lethal concentrations were calculated as 96.14±0.58 and 178.46±2.04, respectively. The tolerance limits of fish for both copper and cobalt varied significantly in terms of 96-hr LC50 and lethal concentrations. However, fish were significantly more tolerant to cobalt than that of copper. With the increase in metallic ion concentration of the test media (water), the level of ammonia and carbon dioxide increased, while that of dissolved oxygen decreased constantly. Total ammonia of the test media showed significantly direct relationship with carbon dioxide while the same remained significantly negative with dissolved oxygen indicating decrease in oxygen consumption by the fish under metallic ion stress, at different concentrations of copper and cobalt that enhanced the ammonia excretion by the fish.
