Tuna: A fish about which there are several health concerns. One concern involves carbon monoxide that prevents the flesh of the fish from discoloring and keeps it red. The FDA permits this practice in the US but Japan, Canada and the European Union have banned the practice because of fears that it could be used to mask spoiled fish. A second concern involves the presence of mercury in tuna. The US government has warned pregnant women, nursing mothers, young children and women of childbearing age to limit their consumption of tuna because of concerns about mercury poisoning.