Heavy metals are natural trace components of the aquatic environment, but their levels have increased due to domestic, industrial, mining and agricultural activities. Discharge of heavy metals into river or other aquatic environment can change both species diversity and ecosystems, due to their toxicity and bioaccumulation. These heavy metals are enters in the aquatic ecosystem as a result of direct input of atmospheric deposition, leaching of mineral and soil erosion due to rain water which causes the hazardous effects on aquatic biota especially fishes. These heavy metals when accumulated in the fish tissues, they damage and weaken the mechanisms concerned leading to physiological, pathological and biochemical changes.The Cadmium is non essential element for living organisms and its presence in fresh water in higher concentration are toxic to organisms, liver and ovary of the fish.The zebrafish were exposed to sublethal concentrations of Cd (20% and 80% of 96 h LC50 i.e. 1.05 and 4.18 mg/l) for 7, 14, 21 and 28 days period. The activity of antioxidant enzymes, catalase (CAT) and reduced glutathione (GSH) in zebrafish were decreased. There was increased lipid peroxidation (LPO) in liver and ovary. These observations clearly indicate the defensive nature and adaptive mechanism of cells against free radical induced toxicity.
