Despite risks fish farms are booming in Africa:.

"It was horrible," says Allan Ochieng of the disaster that struck the fish farmers of Lake Victoria late last year. Thousands of fish were killed when Africa's largest lake experienced a natural and recurring phenomenon known as upwelling. It happens when deep waters mix with surface water, causing a sudden depletion of dissolved oxygen in the water, killing the fish. Some of the farmers believe that raised levels of algae, or pollution might have played a role. Mr Ochieng lost all his 120,000 tilapia, of which half were almost ready for harvest. Many...