As hundreds of oilrigs in the Gulf of Mexico approach the end of their productive lives, oil companies are faced with the problem of what to do with them.
Now engineers at the Massachusetts Institute of Technology (MIT), Boston, have come up with an ingenious solution - turn the rigs into fish farms and cash in on the rising demand for quality farmed fish such as snapper and red drum.
MIT's engineers have developed a large, tent-shaped fish pen that uses the oil platform as its central support. The top of the net is attached to the rig, while the wider, bottom portion is anchored to the sea floor, up to 100ft below.
"It's a very efficient way of supporting a large fish pen," says Cliff Goudey, head of MIT's Sea Grant Programme. "Normally, pens have a floating support but these don't perform well in waves or strong currents. That's why you find most fish farms in sheltered sites near the shore."
But prime sites for fish farms are becoming scarce because tourists want unspoilt views and other fishermen want to use inshore waters to catch lobsters and shellfish.
Environmentalists are not too keen on traditional fish farms either. The poor circulation of water around the pens causes pollution because waste and uneaten food are not swept away. Disease and parasites are also a problem in such intensive farming and pesticides and antibiotics often add to the problem.
But taking fish pens offshore may solve the problems of the farmers and oilmen. The shape of the pen keeps the fish in the deeper, calmer water but the current is strong enough for water to circulate freely and disperse any pollution. The capacious pen could hold a bigger harvest and Goudey believes larger fish such as tuna could be farmed.
At present only a small-scale prototype pen exists, but tests in a large wave tank show that the pen would remain stable in the gulf's often stormy waters.
First, the engineers must convince the oilmen and fish farmers that offshore farms can turn in a profit. American law requires oil companies to remove or convert disused rigs. Goudey says the cost of setting up a fish farm compares favourably with the millions of dollars needed for dismantling.
