Buyers of fresh tuna, whether at the sushi bar or the supermarket, often look for cherry-red flesh to tell them that the fish is top-quality. But it has become increasingly likely that the fish is bright red because it has been sprayed with carbon monoxide.
Carbon monoxide, a gas that is also a component of wood smoke, prevents the flesh from discoloring. It can even turn chocolate tuna red, according to some who have seen the process.
People in the seafood industry estimate that 25 million pounds of treated tuna, about 30 percent of total tuna imports, were brought into the United States last year, mostly from processors in Southeast Asia. Retailers in the United States buy it already treated.
https://www.nytimes.com/2004/10/06/dining/06TUNA.html?ex=1098138633&ei=1&en=9920ea462067f5bc