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

Hint:
Sewage contains the biodegradable pollutants which when dumped into water, degraded by water bacteria with the help of dissolved oxygen. Sewage or domestic/municipal wastewater is wastewater that is produced by a community of people.

Complete step by step answer:
Sewage is what enters the sewage system from household bathrooms, toilets, kitchen, or laundries etc. It comprises more than $99\% $ of water and about $1\% $ of biodegradable pollutants. It also contains toxic constituents. Sewage usually travels from a building into a sewer. The sewer takes the sewage to the onsite sewage where its sewage is treated to remove harmful constituents from it and then discharged. But reality is that more than $90\% $ sewage is directly dumped into water bodies without even treating it which leads to water pollution. Sewage contains pathogens such as bacteria, viruses, parasites etc. which affect the life of both humans and aquatic organisms. All categories of swage contain pathogenic organisms. So the sewage should be properly treated before dumping the sewage.
Fish die in underwater bodies polluted by sewage due to reduction in dissolved oxygen. This is because the water bodies use dissolved oxygen and cause death of a fish.
Hence option (c) is the correct answer.

Note: The total amount of oxygen consumed by microorganisms in decomposing the waste present in a certain volume of a sample of water is called BOD or Biochemical Oxygen Demand of the water. As the BOD increases, dissolved oxygen in water decreases.