The initial quality of the tuna when hooked is not under the control of the fisherman. Many natural factors influence initial quality. One of the most important is spawning. After spawning, their fat content is very low and the water content of the muscle is high, rendering the quality of the fish inferior. Shelf life is relatively lower, even under ideal storage conditions, in tuna and other fishes having red muscle tissue.
Pigmented muscle is rich in iron and copper, two minerals that promote oxidative rancidity.
Removal of blood with its iron containing pigments that foster oxidation will extend the keeping time of fresh tuna. Therefore, if tunas are landed while still alive, they should be bled. After bleeding, they should be submerged in an ice-seater brine to bring down the core temperature of the fish. As with all seafood products, conscientious handling and proper icing of tuna are prerequisites for a high quality product. To avoid bleaching of the flesh, loins, steaks or fillets produced from a whole tuna should be wrapped in plastic so that there is no direct contact with ice or water.