This has been called a homing instinct. It may come from the rocks, the soil and the decaying parts of dead animals and plants. The journey upriver is difficult. They may travel up a river in great numbers, called a salmon run. At this time they are not eating but are living on foods stored in their bodies.
Males develop hooked snouts.
Humps may form in front of the dorsal fin. Their skin changes color, depending on the kind of salmon, changing from silver to shades of red, black or green. Once the salmon reach the upper, shallow parts of the stream, male and female salmon form pairs and get ready to spawn. The female digs a nest, or redd, with her tail. The male will often chase away or fight off other male salmon.