Among the list of pollutants, the heavy metals group is attracting the focus of Aquaculturist, Environmentalist and Fisherfolks alike, since they not only affect aquatic organisms but also have the potential to ultimately affect human beings. The deleterious effects of heavy metals such as lead (Pb) on aquatic ecosystems necessitate continuous monitoring of its accumulation in key species since it affords an indication of its impact on organism’s health. In natural waters, the total Pb concentrations range between 0.05 and 10.0 mg/l (Galvin 1996). While the standard recommended a limit for water for consumption range between 0.01 and 0.05 mg/L Pb. In the current study, we exposed juvenile tilapias (Oreochromis niloticus) to waterborne Pb in five concentrations 0 (control), 0.01, 0.05, 0.25 and 1.25 mg/L Pb, over six weeks period. The present study focused on the bioaccumulation of low to moderate concentrations of Pb while simultaneously assessing the effects of waterborne Pb on O. niloticus feed intake, growth performances, blood plasma Lysozyme, Immunoglobulin M (IgM), Complement 3 (C3) and Cortisol levels. Results from the present study showed that among the Pb exposed groups a trend emerged, which indicated a high bioaccumulation rate in fish exposed to the lower (0.01, 0.05 and 0.25 mg/L) Pb concentrations. Fish exposed to 0.01 mg/l Pb showed highest accumulation rates, after 2 weeks of exposure, they were able to accumulate muscle Pb level that was equal to the concentration of Pb to which they were exposed. Meanwhile, data showed that fish exposed to 0.05 and 0.25 mg/L waterborne Pb recorded mean muscle Pb levels equal to or above the concentration of Pb to which they were exposed in the 4th week of exposure time. Furthermore, the mean muscle Pb recorded in fish after the 6th week of exposure to 0.01 mg/L waterborne Pb was 6 times above the level of the water concentration, while fish exposed to 0.05 mg/L, 0.25 mg/L and 1.25 mg/L waterborne Pb recorded mean muscle Pb after the 6th week that was 5, 3 and 2 times above the concentration of Pb in their respective water environment. Compared with the control group no significant difference in C3 activities were observed in fish exposed to 0.01 mg/L, 0.05 mg/L, and 0.25 mg/L Pb; however, as the dose of Pb increased to 1.25 mg/L significant decrease in C3 activity was observed after 4 weeks of exposure when compared with the control group. Additionally, no significant difference in IgM activities was observed in the 0.01 mg/L, 0.05 mg/L and 0.25 mg/L Pb treated groups of fish when compared with the control respectively. However, as the dose of Pb increased to 1.25 mg/L significant decrease in IgM activity was observed after 2 weeks of exposure compared with the control group. Similarly, as the dose of Pb was increased to 1.25 mg/L significant decrease in lysozyme activity was observed after 2 weeks of exposure compared to the control group. While Pb dose 0.25 mg/L after 4 weeks of exposure showed a significant decrease in lysozyme activity compared with the control. On the other hand, results on cortisol showed no significant sustained variations in activity when the respective Pb treated groups were compared with the control. The present study also showed that 1.25 mg/L waterborne Pb significantly depressed O. niloticus feed intake, weight gain, and absolute growth rate. While the mortality records showed that 75% of deaths occurred during the first two weeks of Pb exposure and the highest mortality was recorded in the group that was exposed to 1.25 mg/L Pb. The present study concluded that at low waterborne Pb concentrations fish bioaccumulate Pb faster. Also, tilapias may have become tolerant to the low Pb exposure levels over time by producing metalloproteinase and or their organs may have coped by reaching a state of homeostasis; however, further research will be needed to verify these hypotheses.
