The dark areas (which are sometimes individual muscles) often have a higher fat content than the predominant lighter-colored flesh and therefore have more flavor. On the negative side, the flavor of the dark-hued flesh can be too pronounced and oily for some tastes. Moreover, since this flesh is high in myoglobin, which turns brown when heated or overexposed to oxygen, it can detract from the lily-white appearance of the rest of the fish. The high proportion of fat normally associated with these dark areas also shortens storage life. On these grounds, the consumer sometimes chooses to remove the dark areas.
** Asian Recipes