For the vast majority of salmon, the trip upriver to spawn is a suicide mission. The fish expend nearly every ounce of energy they have fighting currents, leaping up waterfalls, and dodging predators. Their bodies change—they absorb parts of their skeleton and parts of their skull, using the calcium to fuel the trip. Males grow a hook on their lower lip. In most cases, after they’ve laid and fertilized their eggs, the salmon die.
Occasionally, salmon will survive and go on to spawn again—and again, and again.
LINK (via: Hakai Magazine)