The sudden mass killing of fishes is likely to be seen in a :

Options

A. mesotrophic lake
B. oligotrophic lake
C. salt lake
D. eutrophic lake

Answer

### Step-by-Step Solution:

1. **Understanding the Question**:
The question asks about the conditions that lead to the sudden mass killing of fishes in a specific type of lake.

2. **Identifying Lake Types**:
The options provided are:
- Mesotropic lake
- Oligotropic lake
- Salt lake
- Eutropic lake

3. **Analyzing Eutrophic Lakes**:
- Eutrophic lakes are characterized by high nutrient levels, particularly nitrogen and phosphorus. This leads to excessive growth of algae, known as algal blooms.

4. **Impact of Algal Blooms**:
- When algal blooms occur, they can block sunlight from reaching other aquatic plants, which affects photosynthesis.
- As the algae die and decompose, decomposing bacteria consume a significant amount of dissolved oxygen in the water.

5. **Depletion of Oxygen**:
- The activity of decomposing bacteria, along with the high biomass of algae, leads to a rapid depletion of dissolved oxygen in the water.
- This condition is referred to as hypoxia, where oxygen levels drop to a point that is insufficient to support aquatic life.

6. **Consequences for Fish**:
- Fish and other aquatic organisms rely on dissolved oxygen for survival. When oxygen levels fall drastically, fish cannot breathe, leading to mass mortality.
- The dead fish may float to the surface, indicating a sudden mass killing.

7. **Conclusion**:
- Based on the analysis, the sudden mass killing of fishes is most likely to occur in a **eutropic lake** due to the processes of nutrient accumulation, algal bloom formation, and subsequent oxygen depletion.

### Final Answer:
The sudden mass killing of fishes is likely to be seen in a **eutropic lake**.

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