What causes salmon to die off after spawning?

Salmon change color to attract a spawning mate. Pacific salmon use all their energy for returning to their home stream, for making eggs, and digging the nest. Most of them stop eating when they return to freshwater and have no energy left for a return trip to the ocean after spawning.

Can you eat salmon that die after spawning?

Avoid keeping fish that have visible decay, as their flesh may contain a higher number of bacteria than a fresher fish. Keep the fish alive as long as possible. These salmon, especially if they are showing any visible signs of decay, are covered in bacteria, including their mouths.