Enzymes that normally regulate a fish's metabolism can work unchecked after it dies. Digestive enzymes may begin to digest the fish itself, causing belly burn or softening of the flesh around the gut. This is especially likely if a fish is caught while feeding, since its digestive enzymes will already be active. Other enzymes in fish muscle can also begin to affect the flavor and texture of the fillet. These enzymes work rapidly at warm temperatures.
Fish are also subject to bacterial degradation after death.
Natural barriers that protect fish while they are alive break down when they die. 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.