These changes are caused by bacteria, enzymes and chemical action. Spoilage By Bacteria Bacteria are the most important cause of seafood spoilage. Millions of bacteria are present in the surface slime, on the gills, and in the gut of living seafood species. When seafood species die, bacteria, or the enzymes they produce, invade the flesh through the gills, along blood vessels, and directly through the skin and belly cavity lining. If food poisoning bacteria are present, they can multiply and cause illness when the seafood is eaten.
Many different enzymes are present in living seafood species.
They help build tissue, contract and relax muscles, and digest food. When seafood species die, enzymes continue to work and start to digest or breakdown the flesh. This causes the flesh to soften and lowers the quality. Enzymes also produce more food for bacteria to feed on, increasing the rate of spoilage. This is especially important in fatty fish such as salmon and mackerel.