Time lost all meaning as Charlie continued the swim upstream. He had fought the salmon instincts as hard as he could, never able to do more than veer off from the main group a little. He finally gave up, allowing the salmon instincts to take over. It was so much easier to accept his fate when the spawning was all important, the only thing in his thoughts. Those few times when Charlie surfaced within the fish brain were terrifying and miserable, the blinding need cancelling out even the joy he might have had with the amazing swimming experience.
The salmon struggled against rapids, even leaping up small waterfalls. Many fell by the wayside, too exhausted to continue and doomed to die unfulfilled. Others were snatched up by predators and birds of prey. Charlie had minor wounds on his back where a huge bird had grabbed him with sharp talons and tried to carry him off. And still they swam, not resting, not eating, totally focused on reaching the spawning ground. Charlie felt his own body weakening, but also became aware of pressure in his gut. The sperm he needed to fertilize his mate's eggs. It would be released in a cloud over the roe, starting a new generation of salmon that would swim to to the ocean and return in a few years to repeat the cycle.
The water grew colder, and the river narrower. It was breaking up into tributaries now, and Charlie followed instincts towards the birthplace that was not really his. More swimming. He knew the end was near. Not only because his salmon brain was sensing the end of the journey, but awareness of his own failing body. The actual spawning was so anticlimactic it was almost funny. The salmon paired off, male and female, with no real emotion. The instincts that had driven them here now guided the former human to swim by the female, watching as she dug shallow depressions in the sand and then laid her eggs. Hundreds. Thousands. Then he would move over the eggs and release his sperm in s white cloud that drifted down.
It was done. Charlie was empty, totally spent both physically and mentally. Enough of his human mind remained that he still felt fear and sorrow. But he could feel his body failing, his heart slowing. Like the hundreds of salmon around him, his scales were falling off, his fins disintegrating, all of them rotting apart even while they still lived. His heart slowed again, and vision dimmed. All that was left to do was to die.