Laboratory-scale experiments were conducted to evaluate the joint action toxicity and biochemical
effects of sublethal concentrations of Forcados light crude oil (FLCO) and three dispersants against Clarias
gariepinus over a period of 28 days. The derived 96hrLC50 values revealed that the dispersant, DS/TT/066
(0.03mL/L) was the most toxic, followed by dispersant, OC/TT/OSI (0.19mL/L), FLCO (5.06mL/L) and
crystal clear oil dispersant (CCOD = 12.06mL/L) the least toxic when acting singly. Joint action toxicity
evaluations of FLCO and dispersants showed that the interaction between FLCO : DS/TT/066 and FLCO :
OC/TT/OSI was synergistic (synergistic ratio (SR) > 1) with SR values of 10.5 and 3 respectively. However,
for the mixture of FLCO : CCOD, the interaction was antagonistic (SR < 1) with SR value of 0.97. The result
of the biochemical effects study revealed that malondialdehyde (MDA) levels decreased significantly (P<0.05) in the exposed fishes, reduced gluthathione (GSH) and gluthathione-s-transferase (GST) activities increased significantly (P<0.05) in fishes exposed to FLCO : CCOD mixture alone while there was no significant difference (p>0.05) in superoxide dismutase (SOD) and catalase (CAT) activities in all the exposed fishes compared to control animals. The observed increase in GSH and GST levels in conjunction with a decrease in MDA concentration in the liver of test animals exposed to binary mixtures of FLCO and CCOD reveals the ability of the animals to overcome the effects of lipid peroxidation in this group. Further studies on the mechanism of toxicity of these dispersants in field and laboratory assays are recommended.