In Raron in Valais, the perchs of the lake are raised in basins. Close to the Lötschberg tunnel, the exploitation benefits from the spring water evacuated by the digging of the tunnel. When the tunnel was dug, water flowed naturally from the bowels of the earth. It should have been evacuated in the Rhone, but with its 19 ° C, it would have been wasteful. Fish farming therefore installed a one-kilometer pipe to recover it, which gives a constant flow. It is an ideal temperature since the poles are at their best in water at 21 ° C.