The principle of aquaponics is to combine, in a closed circuit, aquaculture - here a trout farm - and hydroponics, a method of production in which the roots of crops are immersed directly in water. Near Bordeaux lies one of the largest aquaponics farms in Europe, which "uses ten times less water than open-ground cultivation and five times less energy" than a conventional farm with the same yield. IMAGES AND SOUNDBITES