Bacteria from the environment and the gut can grow and multiply in fish tissue. This activity diminishes fish quality and eventually causes spoilage. Bacteria also grow rapidly at warm temperatures. Finally, the highly unsaturated fat in fish is also affected by oxygen in the air. Oxygen reacts with this fat to produce the odors and flavors associated with rancidity.
Fat oxidation can be a serious problem if the fish is to be frozen or stored for very long.
This is one reason why fattier fish like bluefish do not remain in good condition during frozen storage as long as leaner fish like flounder. Many fish are wasted because of mishandling. The quality of the fish you bring home ultimately depends on how far the normal spoilage processes have progressed. Although you cannot stop these processes, by handling your catch properly you can control the rate at which they occur. The factor that is easiest and most important to control is temperature.