FISH farming has a bad reputation. Its supporters argue that it promises to meet the growing shortfall as the world's wild fisheries become more and more exhausted. But its critics have the louder voice. They argue that farmed fish is fatty, dyed, polluting and stuffed with antibiotics. Moreover, they say that it is unsustainable. When a carnivorous fish—such as the salmon—is reared on a farm, it too must be fed with fish. And these fish must be caught in the wild, thus putting even more pressure on marine life, not less.