Often in water bodies subjected to sewage pollution, fishes die because of the:

Options

A. foul smell
B. Pathogens in the sewage
C. Clogging of the gills by solid particles
D. reduction in dissolved oxygen caused by microbial activity

Answer

To solve the question regarding the death of fishes in water bodies subjected to sewage pollution, we will analyze each option provided and determine the correct reason for the fish deaths.

### Step-by-Step Solution:

1. **Understanding Sewage Pollution**:
- Sewage pollution introduces various contaminants into water bodies, including organic matter, pathogens, and solid particles. It is essential to understand how these factors affect aquatic life.

2. **Analyzing the Options**:
- **Option A: Foul Smell**:
- While sewage may produce a foul smell, this does not directly affect the health of fish. Therefore, this option is incorrect.

- **Option B: Pathogens in the Sewage**:
- Pathogens can cause diseases, but they do not significantly impact the immediate survival of fish in the same way that oxygen levels do. Thus, this option is also incorrect.

- **Option C: Clogging of the Gills by Solid Particles**:
- Although solid particles can clog gills, fish have a protective structure called the operculum that helps manage water flow and can mitigate this issue. Hence, this option is incorrect.

- **Option D: Reduction in Dissolved Oxygen Caused by Microbial Activity**:
- Sewage introduces a high level of organic matter, which leads to increased microbial activity. This activity consumes dissolved oxygen in the water, leading to hypoxia (low oxygen levels). Fish require sufficient dissolved oxygen to survive, and a reduction in oxygen levels can lead to their death. Therefore, this option is correct.

3. **Conclusion**:
- The correct answer is **Option D: Reduction in dissolved oxygen caused by microbial activity**. This is the primary reason for fish deaths in water bodies affected by sewage pollution.