As people become richer and move to cities, global demand for meat and fish keeps growing, and by 2050 there will be another two billion of us to feed. But with wild fish stocks declining and ever more pressure on natural systems from livestock farming, we need to find new ways of meeting this appetite.

Russell Beard finds out about a highly-efficient closed-loop system known as 'aquaponics', where fish and vegetables grow symbiotically. This means that produce and protein can be cultivated sustainably and profitably in cities. Founded by Will Allen, an ex-professional basketballer-turned-urban agriculturalist, Growing Power is an urban farm with a difference in Milwaukee, Wisconsin.

At their 0.8 hectare inner-city plot you will not only find vegetables and salad greens, but fish tanks holding 10,000 fish each. Waste from the fish is pumped into plant beds where bacteria turn it into fertiliser for vegetables, which in turn clean the water for the fish. This means these units do not pollute waterways or need chemical fertilisers, and they use up to 90 per cent less water than conventional farming.

Growing Power's headquarters is said to be the most productive farm of its size in the US mid-west, and it trains hundreds of volunteers from around the world each year.

Inspired by this approach, a group of entrepreneurs have now set up their own commercial aquaponics farm in a nearby industrial wasteland. Sweet Water Organics sell their tilapia, perch and salad greens to local restaurants and shops, and are planning to expand the operation to produce six times more food than they currently do, using half as much energy.

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