Abstract
The setting of safe limits for lead (Pb) into the lagoon and protection of fishes such as Oreochromis niloticus has been based on its toxicity ignoring the influence of salinity, an important parameter in the lagoon. The study therefore investigates the salinity tolerance and relative acute toxicity of lead nitrate [Pb(NO3)2] under fresh water and varying salinity against O. niloticus fingerlings. A total of 280 fingerlings were used as test animals and Pb(NO3)2 as test chemical. Concentrations were 1, 2, 4, 6, 8, 10, 30, 40, 50, 60, 80, 120, 160 and 180mg/l at salinity levels of 2, 12, 18, 22, 32, 35 parts per thousand (ppt) and fresh water. Dose response data were analyzed by Probit. Mortality assessment showed that the fingerlings could not survive at salinities of 22, 32 and 35 ppt within 24 h but survived well in 2 ppt. The 96 h LC50 was 130.094 mg/l at 12 ppt but below 12 ppt there was a steady increase in toxicity (3.255 and 6.243 mg/l) in fresh water and 2 ppt respectively. Similarly, the toxicity also increased with an increase in salinity from 12.1 to 18 ppt (113.191 mg/l). The significance of this study shows the need for inclusion of varying salinity in setting of safe limits to confer protection on the delicate biotic components of the rich lagoon biodiversity.
Key words: Lead nitrate, Oreochromis niloticus, salinity.
