How do fish reproduce?

Female fish lay eggs-they spawn. The male fish eject their semen on the eggs, and thus fertilize them. Many fish like the salmon return to their place of birth for laying eggs. The salmon eggs hatch in streams and from freshwater fish turn into saltwater fish in 1-3 years. Then, they remain in the sea up to 4 years. 

When they wish to reproduce, they return to their birthplace covering a distance of as much as 6ooo km. They have to swim against the current and overcome all the obstacles in their way. Once at the destination they spawn. It is a mystery how the salmons find their birth place. Scientists assume that they are guided by the magnetic field of the Earth.