Differential toxicity and influence of salinity on acute toxicity of copper sulphate and lead nitrate against <i>Oreochromis niloticus</i>

Abstract

This study investigated the salinity-tolerance of Oreochromis niloticus and the influence of salinity changes on the acute toxicities of copper sulphate (CuSO₄) and lead nitrate (Pb[NO₃]₂) against the fish species. On the basis of daily mortality assessment, O. niloticus was found to be unable to survive in media with salinity above 22‰ within a 24-hour period, but the species survived well at lower salinities. The 96-hour LC₅₀ values of both CuSO₄ and Pb(NO₃)₂ against the species was lower indicating a higher toxicity at salinities lower (0 and 2‰) or higher (18‰) than 12‰ which had the highest 96-hour LC₅₀ value and exerted lowest toxic action. CuSO₄ was however more toxic to the species than Pb(NO₃)₂ based on the recorded 96-hour LC₅₀ values. The significance of this study in setting ecologically safe limits that would be relevant to the varying salinity conditions that characterize the lagoon ecosystems in the tropics for the discharge of heavy metals into lagoons was discussed. The advantages of breeding O. niloticus under typical brackish water conditions instead of freshwater were also emphasized in the text.

Keywords: Oreochromis niloticus; acute toxicity; salinity; copper sulphate, lead nitrate.