Which of the following is not a freshwater fish ?

Options

A. Salmon
B. Mrigal
C. Catla
D. Rohu

Explanation

To determine which of the following is not a freshwater fish, we will analyze each option provided:

1. **Salmon**: This fish is primarily found in marine environments, particularly in the North Atlantic and Pacific Oceans. It is known for its migratory behavior, often moving from saltwater to freshwater to spawn. Therefore, Salmon is classified as a marine fish.

2. **Mrigal**: This is a type of carp that is commonly found in freshwater bodies such as rivers and lakes. It is a popular fish in aquaculture and is considered a freshwater fish.

3. **Katla**: Katla is another species of carp that thrives in freshwater environments. It is widely cultivated in India and is also classified as a freshwater fish.

4. **Rohu**: This fish is also a freshwater species and is commonly found in rivers and ponds in South Asia. It is a popular choice for consumption and is considered a freshwater fish.

After analyzing all the options, we can conclude that:

- **Salmon** is the only fish listed that is not a freshwater fish. It is a marine fish.

### Final Answer:
**Salmon** is not a freshwater fish.