The effect of two sublethal concentrations (15, 30 mg/l) of Cr6+ on Nile tilapia, Oreochromis niloticus has been studied using some hematological and biochemical parameters aiming to evaluate their possible potential use as biomarkers of stress and/or defense. Hemoglobin content decreased throughout the experiment period, compared to the control. Moreover, the plasma glucose content and uric acid increased in both Cr6+ concentrations. Plasma urea decreased during the first 24 hr of exposure, then it increased, thereafter decreased and finally fish recovered. Also, the brain AChE of the lower concentration showed significant fluctuations starting with inhibition of the enzyme then activation and final tendency to recover, while the higher concentration showed a significant increase throughout the whole experimental
period with no sign of recovery. Plasma AChE has been inhibited and no sign of recovery was detected. Gill Na+-K+-ATPase specific activity remained unchanged in fish exposed to both Cr6+ concentrations during the whole experimental time. Liver metallothionein (MT) content increased in both fish treated with Cr6+ concentrations.
The present results indicated that blood Hb, plasma glucose followed by brain AChE are more sensitive indicators of general stress. However, MT, to a lesser extent, may be considered a valuable biomarker of defense. Finally, plasma AChE may also be considered as a biomarker of defense.