The toxicities of three refined petroleum products and their binary mixtures based on predetermined and equitoxic ratios of 1:1, 1:6 for petrol-kerosene; 1:1, 1:2 for petrol-diesel and 1:1, 1:3 for diesel–kerosene respectively were evaluated against the fingerlings of Oreochromis niloticus, in laboratory bioassays. The interactions between binary mixtures showed significant departures from the action of the individual constituent compound when acting singly. On the basis of synergistic ratios (SRs) and concentration-addition models (RTU) used for the joint action evaluations, the interactions between the constituent toxicants in the various test proportions of the mixtures were largely in conformity with the model of synergism (SR>1 and RTU > 1) except for petrol-kerosene (1:1) which was additive (SR=1). In most of the test combinations petrol was found to consistently increase the toxic effect of kerosene and diesel. The joint action toxicity studies provides a means of identifying the integrated impact of chemical mixtures on different levels of biological function and could make a valuable and viable addition to routine management protocols for protecting fragile aquatic ecosystems.
Key words: Joint action, binary mixtures, refined petroleum products, synergism.
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