Salmon are born in fresh water, then migrate to the ocean before returning to fresh water to reproduce.
It's a myth that salmon return to the exact spot where they were born to spawn.
There are differences between the Atlantic salmon and the Pacific salmon. While there is only one species of Atlantic salmon, there are six species of Pacific.
Salmon stocks are under threat and the Wye & Usk Foundation is trying to improve the water quality of the Wye and Usk to encourage salmon and other species back to their natural habitat.
Salmon stocks in the North Atlantic are at their lowest levels since records began in the early 1960s.
Try one of the walks from Derek's latest walking series on BBC One Wales.
Find out about the wildlife you can find on your doorstep.