The term pollution is broadly refers to any undesirable change in the natural qualityof environment brought about by physical, chemical, or biological factors. Environmentalpollution is unfavourable alteration of our surroundings due to direct or indirect activities of man.The high rate of increase human population, rapid expansion in the industrial and urbanactivities and modernization of agriculture has resulted in generation of high volume of wastematerial causing gradual deterioration of valuable resources of biological productivity.
The water pollution has been defined as 'any man made alternation of physical,chemical or biological quality of the water which results in unacceptable depreciation of theutility of the environmental value of water'. The matter of unacceptability is to be decidedaccording to expectations and requirements at any time, bearing in mind the expectations andrequirements in environmental pollution change as knowledge, experience and perceptionadvance.
EFFECTS ON FISH AND FISHERIES.
Pollution due to domestic sewage increases the organic load and pollution due to agricultural waste and soil erosion containing nutrients fertilize the water and increase the rate of productivity of the aquaticecosystem. This results in higher growth of phytoplankton. Water becomes turbid due to excessive growth of phytoplankton and soil eroded particles. Excessive amount of nutrients change the algal community from one of great diversity of species to one of a few; the species which are eliminated are commonly those which form the food of the herbivorous animals which in turn feed the fisheries resources of the area. Aquatic lives face severe oxygen shortage.
ACCELERATED AGING OF LAKES AND PONDS .
Sewage pollution even at in small quantities may change the character of an aquatic environment over a period of years. Thus, with the gradual process of aging, deep, clearoligotropic lakes may be sedimented; becoming mesotrophic, then becomes eutrophic andeventually turning into bog.