Chemical Pollutants and its Effects on Fish Physiology with Special Reference to Accumulation, Absorption and Excretion

Water pollution that alters the chemical composition, temperature, dissolved oxygen, or microbial composition of water and affects aquatic organisms including fishes has implications on human health, both directly and indirectly. The xenobiotics or contaminants come in contact with fishes through water or the natural food available in the medium. The chemical pollutants including heavy metals, once absorbed via gills or gut, undergo metabolism and finally absorbed through the epithelial cells, then usually bound to a protein and then transported by the blood to either storage point, such as, fat, or to the liver for transformation and/ or storage then excreted in the bile, or passed back into the blood for possible excretion by the kidney or gills. Harmful chemicals have different mode of actions and may tend to act on certain organs or physiological functions, usually more than one being affected. Fishes are exposed to both bioconcentration and biomagnification from eating lower on the food chain. Environmental hypoxia, i.e., lower air saturation level is one of the important thrust to both freshwater and marine fishes where oxygen concentration varies with depth, temperature, salinity, productivity, nutrient enrichment, meteorological conditions and plankton blooms. Environmental hypoxia or exposure to other chemical pollutants can induce a reduction in number of fish erythrocytes or inadequate amount of hemoglobin in the cells resulted in anemia. Cadmium causes anemia in a variety of fish species also. The most metals are absorbed by fishes in the ionic form except methylmercury. The outer surface of gills and intestinal tissues is a negative charge; therefore, it will attract metallic ions. The affinity of metals in the specific tissue is determined by the micro-environment of the tissue surface, viz., the mucosal surface of fishes is complex one, because it includes the epithelial membranes as well as mucous layer, encompassing a mixture of glycoproteins, mucopolysaccharides, assorted low molecular weight compounds and water. Uptake as well as accumulation of metals is directly correlated with pH of the luminal medium. The proximal portion of the intestine possesses more absorptive surface area where the lipophilic toxic metals are assimilated in much quantity. In the intestine metal accumulation is largely associated with plasma albumin and lipoproteins. The mechanism of metal uptake through the gut has been assumed to be simple diffusion and biotransformation of chemicals before excretion may be a process of passive diffusion.