The answer is that after mating they produce a tiny mobile larvae totally different to the adult  this might drift for miles on ocean current before settling into the fixed adult form  Down Norwick power station in North Wales, generates the electricity by pump storage, in off peak hours thousands of gallons of water are pumped from a lower lake to an upper lake, when the demand for electricity is high bowels are opened and water falls back through turbine to the lower lake again, this generates the power  To ensure that the lower lake would never flood the  was diverted through mile long tunnel in the mountain side, no one knew for certain how the salmon, the trout and even rarer  that used to migrate up the old river would cope with the tunnels, pitch darkness and slow flowing water.