It is accepted now, even by environmentalists, that properly conducted aquaculture could feed the world without polluting it. A favoured method of the environmentalists is the hard-walled pen system, which isolates the fish from the surrounding water in 40 foot deep tanks which catch their waste on the bottom. Although salmon farming has been a highly profitable industry for a number of decades and shows strong promise for the long term, profits are being squeezed today making it more difficult for operators to adopt more expensive eco-friendly methods.