Question

insert one dash where needed.
salmon hatch in freshwater and migrate to the ocean though they eventually return to freshwater to spawn and die.

Explanation:

The dash is used to set off a contrasting or clarifying clause that interrupts the main thought. Here, we use an em dash (represented by the given dash) to separate the unexpected contrasting information about the salmon returning to freshwater from the initial statement about their migration to the ocean.

Answer:

Salmon hatch in freshwater and migrate to the ocean — though they eventually return to freshwater to spawn and die.