35. Trout farm sites require a source of clean flowing water because the fish are usually held at high stocking densities. The traditional farm consists of a set of purpose built narrow ponds, 1m or less in depth, lying parallel to one another with a common inlet channel for water and a common outflow channel which is often also used for fish production. Water requirements are in tens of millions of litres per day and it is important that there is adequate water flow. Sophisticated farms employ concrete tanks, called raceways, for which the water intake and outflow discharges are of pipework. Some farms use circular tanks in which spiral water movement towards the central drain helps carry away detritus. Some farmed trout are kept in cages, similar to those used for salmon, in fresh or salt water locations. FAWC believes that with all designs, the aim should be good fish welfare rather than just fish survival and growth.