Fish is a highly nutritious and perishable food commodity, and its quality is greatly influenced by the biochemical and microbiological changes that occur after death. These changes, known as postmortem changes, play a critical role in determining the freshness, texture, flavour, and storage life of fish and fishery products. After death, the cessation of blood circulation and oxygen supply leads to a shift from aerobic to anaerobic metabolism in muscle tissues. This results in the depletion of energy reserves such as ATP, breakdown of glycogen into lactic acid, and subsequent alterations in muscle proteins. The process typically progresses through several stages including pre-rigor, rigor mortis, and post-rigor, followed by autolysis and microbial spoilage. These stages cause significant changes in muscle structure, biochemical composition, and sensory characteristics of fish. Factors such as temperature, handling stress, species, and storage conditions strongly influence the rate of these changes. Understanding postmortem changes is essential for the fish processing industry because proper handling, chilling, and preservation techniques can effectively slow down deterioration and maintain product quality.