Fishy Tales

Almost half of the fish we eat comes from farms rather than the wild ocean. With yields from fishing in decline, that proportion is set to increase But environmental issues have dogged fish farming down the years - pollution, disease. The BBC reports

But where are all the extra fish farms to go?

"The forecast is that by the year 2050 there will be a shortage of land for food production," says Patrick Sorgeloos from Ghent University in Belgium, an aquaculture adviser to the UN Food and Agriculture Organization . "And on top of that, freshwater will be a really precious resource that we will not be able to waste or use to expand freshwater aquaculture. So the FAO and other organisations are very much convinced that the future for aquaculture is much more in the marine environment - and with 70% of the globe covered by sea, we have a great opportunity to get more food produced there."

"We can't just rely on pillage and plunder from wild stocks anymore - we have to produce it ourselves, and we have to produce it in the open ocean, in deeper water," argues Neil Sims co-founder Kona Blue company, operating in Hawaii which believes it has a solution; stop eating bluefin tuna , and start eating Kona Kampachi, a species that is not currently staple choice , but that are palatable and ideal for farming. The company says Kona Kampachi is not only a good replacement , but the fish can be farmed sustainably. Key to that sustainability, it says, is to farm in open ocean, with the strong currents able to remove the fish's faeces and any other pollutants.

Also possible is vast special designed barges, full of salmon smolts which are the start of the grow-out period for salmon in the sea, as young fish of 60-80g - could be put on board such a large vessel, and then the vessel heads up the west coast of America to California. It would be a slow journey. By the time it gets there, the fish are at market size.

At the other end of the scale are urban fish farms - water in the heart of the city, close to the consumer. Special skyscrapers. Space and resources, not least water, are at a premium - hence the development of techniques such as aquaponics, a variant of the water-frugal plant-growing technique of hydroponics.

"In aquaponics, what you do is place the growing bed of the vegetables or flowers or whatever you're growing in the flow of the water coming out of the fish tanks," explains Martin Schreibman from Brooklyn College in New York. " It's the fish wastes, the breakdown products of protein metabolism, that provide the nitrates and phosphates for plants. So I now have a system with styrofoam floats, there are holes in the styrofoam that contain the plants - we've got pak choi and lettuce at the moment - and the water is plumbed back into the fish tank, so you've got a closed cycle."

Dickson Despommier, the Columbia University professor who developed the concept of vertical farming a decade ago explains "We can produce shrimp, we can produce molluscs; you can raise trout, striped bass, catfish...If vertical farming proves successful... then if you were to take a picture today of the skyline of New York or London or Shanghai and then compare that to 50 years later, you'd see a remarkable change. You'd see absolutely gorgeous, tall transparent buildings, apartment houses with three or four stories of transparent vertical farming on top... and you've taken food out of the hands of a few mass producers and placed it into the hands of all the people."