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Brackish water ecosystems characterized by fluctuating physicochemical parameters are more susceptible to the toxic effects of heavy metals acting singly or jointly. This study investigated the effect of salinity variations on the joint action toxicity of copper sulphate (CuSO4) and lead nitrate Pb(NO3)2 against fingerlings of Oreochromis niloticus. Fingerlings were exposed to binary mixtures of CuSO4 and Pb(NO3)2 (ratios 1:1 and 1:4) at varying salinities (0‰, 2‰, 12‰ and 18‰) in laboratory bioassays. The binary mixtures of the heavy metals were least toxic to the fish at 12‰ with 96 h LC50 values of 115.558 mg l-1 and 198.274 mg l-1 compared to 8.465 mg l-1 and 16.884 mg l-1 for 0‰, 46.084 mg l-1 and 69.843 mg l-1 for 2‰ and 13.196mg l-1 and 100.567 mg l-1 for 18‰ at ratios 1:1 and 1:4 respectively. Analysis using the Synergistic Ratio Model (SR) showed that both heavy metals were less toxic to the fish species when acting jointly irrespective of ratio than when acting singly at 12‰. Therefore, the need to consider the fluctuating salinity and joint interaction of heavy metals in setting ecologically safe limits for the discharge of effluents containing heavy metals into the aquatic ecosystems is important. Key words: Salinity, Heavy Metals, Toxicity, Oreochromis niloticus

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