Difference between marine, sea run, and freshwater stickleback fish populations

The marine populations live only in salt water. They do not survive in freshwater.

Similarly, the fresh water populations live in fresh water bodies alone. They live in rivers and lakes, but do not go into salt water bodies.

Finally, the sea run populations live their lives in fresh water, but they also go into salt water in order to reproduce. After breeding, they come back to their fresh water habitat.