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EFFECTS OF HEAVY METALS TOXICITY ON FRESH WATER FISH

Aquatic environment gets contaminated with a variety of pollutants generated from diverse sources (industries, agriculture and domestic). Amongst the pollutants, pesticides, heavy metals and detergents are the main cause of concerns for the aquatic environment because of their toxicity, persistency and tendency to accumulate in organisms. Heavy metals are a group of 19 elements, which have many similar physical and chemical properties and are remarkable varying from the remaining 97 known elements. Metals in the fishes are transported by the bloodstream, which brings it into contact with various organs and tissues. Fish can regulate metal concentration to a certain extent; where after bioaccumulation will occur. Therefore the ability of each tissue to either regulate or accumulate metals can be directly related to the total amount accumulated in that specific tissue. Furthermore, physiological differences and the position of each tissue in the fish can also influence the bioaccumulation of a particular metal. Among the 19 heavy metals Lead, Cadmium and mercury do not have any biological significance or beneficial use and are known   to be extremely toxic.  Other toxic metals are Chromium, Copper, Manganese, Nickel, tin and Zinc. Once dispersed in the biosphere, these metals cannot be permanent.
Toxicity of Heavy Metals
Mercury: Banerjee (1985) studied the effects of heavy metals poisoning on peripheral Haemogram in Heteropneustes fossilis; due to exposure to Mercury sulphate. Palanichamy and Bhaskaran (1995) studied physiological responses of the fish, Channa straitus. It was observed that feeding, growth, protein conversion efficiency, oxygen consumption, protein, glycogen and SDH activity of heavy metals treated Channa striatus decrease with increasing concentrations of Mercury, whereas the protease activity increased. Mercuric chloride affects the internal cells of the kidneys of the fishes.
Cadmium: Vijayram et al. (1989) studied the Cadmium induced changes in the biochemistry of an air breathing fish, Anabas testudineus. On short term exposure (3 days) a decrease was observed in liver and muscles glycogen content and liver weight too was recorded. During long term exposure (54 days) a reverse was observed. Cadmium inhabits the acid phosphatase activity when present in the liver.
.lead: Palalnichamy and Baskaran (1995) studied physiological responses of the fish, Channa stritatus. It was observed that feeding, growth, protein conversion efficiency, oxygen consumption, protein, glycogen and SDH activity was decreased with increasing concentration of Lead, whereas the protease activity increased. Rao and Manjula Sree Patnaik (1999) studied the Lead accumulation in the catfish, Mystus vittatus (Bloch). The concentration pattern on the different organs for Lead revealed that metal in muscle was low while on other organs the levels varied. The concentration of heavy metal from the natural habitat was also analyzed and there was some evidence that accumulation in different organs influenced by the variations in pH and it was more in acidic pH.
Manganese: Nussey et.  al. (2000) studied the accumulation of Manganese in the tissues of the moggel, Labeo umbratus (Cyprinidae) from Witbank Dam, Mpumalanga. Manganese and Nickel behave similar to the Chromium when present in the tissues of fishes. Higher amount of these elements are found to be accumulated in gills and liver.  Bioaccumulation of Manganese decrease with increase in length of the fish.
Nickel: Nussey et. Al. (2000) studied the bioaccumulation of Nickel in the tissues of the moggel, Labeo umbratus, from Witbank dam, Mapumalanga. Bioaccumulation of nickel decreased with increase Length of the fish.

Iron: Aurn et. Al. (2000) observed the reduction in biochemical parameters (protein, carbohydrate, and lipid) of fish, Oreochromis mossambicus when exposed to Iron for (15 days). The detoxification mechanism in Orechromis mossambicus was scrutinized by monitoring the induction of xenobiotic conjugation enzyme (Glutathione-s-trnasferase) at every 5 days interval.
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