Genetic engineers have designed fish for better flavour and colour as well as for faster growth, improved disease resistance, and other traits. Two farmed species -- silver carp and grass carp -- already rank among the ten most-consumed fish worldwide. A specially bred tilapia, known as the "aquatic chicken," takes six months to grow to a harvestable size of about 700 grams. A biotech firm in Waltham, Massachusetts, has applied for government approval to commercialize salmon genetically engineered to grow four to six times as fast as their naturally occurring cousins.