Question

Fish die in water bodies polluted by sewage due to-
A. Pathogens
B. Clogging of gills by slit
C. Reduction in oxygen
D. Foul smell

Answer 1

Hint:
Aquatic organisms need to be dissolved in oxygen to respire. DO is necessary for aquatic animals, other microorganisms, and underwater plants. Dissolved oxygen is also needed for the decomposition of organic matter. Dissolved Oxygen is defined as the amount of gaseous oxygen dissolved in the water.

Complete answer:
Most water bodies are polluted due to industrial waste, human activities.
Sewage is wastewater that creates environmental hazards as it transmits diseases that are contaminated by water.
The chemical sewage contains a raw material that is utilized by aquatic plants and algae for their development and thus they increase in number which blocks the penetration of sunlight. As they are increased in amount, the level of dissolved oxygen decreases which in turn creates a lack of oxygen availability and that affects the fishes and other aquatic organisms and may kill them as they uptake the oxygen from water.
Due to the waste present in the water, the microorganisms present in the sewage consume all the dissolved oxygen, thereby depriving the fishes of the dissolved oxygen and ultimately fishes die due to lack of availability of oxygen.

Hence, reduction in oxygen is the cause of the death of fishes in water bodies polluted by sewage. So, the correct answer is Option (C).

Additional information-
Microbial activity increases BOD. When there is high turbidity in the water body then it can lead to the clogging of the gills of fishes and then their death.
Eutrophication can also cause the death of fishes. If there is an increase in sulfur dioxide level in the water body, then it has the potential of killing fishes.

Note: The presence of an algae bloom can lead to oxygen depletion. The introduction of a large amount of decaying biological material in general to a body of water leads to a reduced level of dissolved oxygen as microorganisms use up available oxygen in the process of breaking down organic matter.
Waste coming from industries is harmful and it not only pollutes the water body but also leads to the death of aquatic organisms.