Question

Fish die in water bodies polluted by sewage due to:
A.pathogens
B.clogging of gills by silt
C.reduction in dissolved oxygen
D.foul smell

Answer 1

The contamination of water bodies is called water pollution. The main reason for water pollution is usually the result of human activities. Raw sewage and industrial waste discharge in water causes a decrease in oxygen level, altar of food chain, reduction of biodiversity and increase in the number of pathogens.

Complete step by step answer:
Waste disposal in water bodies is the main cause of water pollution. Waste like plastic products which do not decompose can block the respiratory system of marine animals causing their death. Release of chemicals into rivers by industries alter the pH of water which leads to negative effects like decrease of oxygen content in water.
Sewage release in water bodies increases their turbidity which blocks sunlight from reaching the bottom of the water bodies. This causes the plants at the bottom to die out due to lack of photosynthesis. This in turn leads to decrease in dissolved oxygen level in water. Just as we require oxygen dissolved in the atmosphere to survive, marine organisms also require dissolved oxygen to survive. The reduction of dissolved oxygen in water bodies is the main reason why fish die.

$\therefore$ The correct option is option C, i.e. reduction in levels of dissolved oxygen.

Note:
The disposal of plastic waste in water bodies is also one of the reasons why the marine population is endangered. The plastic clog the gills of fish and cause their death. Sometimes sewage disposal also results in an increase in algae in the water bodies which block out sunlight and reduces the dissolved oxygen level in water. This process is also called eutrophication.