In the final stage of the salmon’s life cycle, the adults re-enter their home river and swim back to the stream or lakeshore in which they grew as fry. Salmon from inland rivers may travel many hundreds or thousands of kilometres, swimming from 30 to 50 km a day against the current. They follow the scent of the water from their home stream, past rapids and other obstacles, such as dams, rock slides and log jams, before reaching their destination. Fishers and predators, such as bears, otters, racoons and eagles, catch many salmon on their trip upstream. When they enter fresh water, the salmon stop eating and live only on stored body fat. Their kidneys, gills and skin change to regulate the water and salt balance in their cells. To save energy, they lose the slime coating that helps protect them; their skin becomes thick and leathery, and they absorb their scales. The salmon’s appearance changes dramatically, with males and females developing distinct differences. Both males and females lose their silvery colour and take on deep red, green, purple, brown and grey colours. Their teeth become long and they develop a hooked jaw, which is particularly noticeable in the males. The body shape can change, with some species developing a pronounced hump on their back. Eggs ripen in the ovaries of the females, while sperm in the males changes into liquid milt. When they reach their home stream or lake, the female uses her fins and tail to find a spot with the right gravel size and water conditions. With strong sweeps of her tail, she rearranges the stones in the gravel bed to form a redd, the nest-like depression in the stream- or lakebed where she will lay her eggs. Males fight among themselves to get close to a female. When a female chooses a male, they nudge and bump each other in an underwater courtship dance. The female deposits some of her eggs in the redd, and the male deposits his milt to fertilize them. Some species deposit up to 6,000 eggs, but the average is about 2,500. The female covers the eggs with gravel to protect them, and often moves on to build a second or third redd, which is fertilized by other males. Both males and females die within a few days of spawning. Their bodies, battered and injured by the difficult trip upstream, decompose. Valuable nutrients from the carcasses form a rich food source for other fish and wildlife by fertilizing the stream or lake. Salmon carcasses that are carried onto riverbanks fertilize the forest and bushes. If most of the adult salmon are caught, the water will have few nutrients for the next generations of salmon and for the rest of the ecosystem.