Fish farming is a dirty business. Uneaten food pellets and feces contaminate the seabed, and disease is a constant threat in the static, overcrowded cages. Chile's salmon farming industry-the world's second largest-has been devastated in the past two years by outbreaks of lethal viruses and parasites. Conventional farm cages can be moved by tugboats, but this is cumbersome and inefficient. MIT researcher Cliff Goudey has a better idea. In ocean waters off Puerto Rico, he has tested mobile cages equipped with electric motors and propellers. Goudey's design will keep the cages away from sensitive inshore waters, while the increased flow of dissolved oxygen will make for healthier fish. Whether it will make them taste any better is an open question.