This could also be caused in the handling of the food item or when it is placed on bacteria infested surface. In seafood, millions of bacteria are present in the surface slime, the gills and in the gut of living seafood species. When the seafood dies the bacteria or enzymes they produce invade the flesh through the gills, along blood vessels, and directly through the skin and belly cavity lining. When bacteria is present in seafood, it grows and multiplies causing discoloration and unpleasant odors. This is the most common food-borne illness found in seafood.
Enzymes also cause food spoilage.
Enzymes in living seafood help build tissue, contract and relax muscles and digest food. After harvest, enzymes continue to work and start to digest and break down the flesh. This causes the flesh to soften and lowers the quality. Enzymes also produce more food for bacteria, which also increases the rate of spoilage. Oxygen in the air attacks the oils in seafood and causes rancidity, bad odors and bad flavors.