Seafood Spoilage Bacteria are the major cause of seafood spoilage. Millions of bacteria are on the surface, on the gills, and in the gut of living fish and shellfish. After harvest, bacteria invade the flesh of fish and shellfish through the gills, along blood vessels, and directly through the skin and belly cavity lining. These bacteria grow and multiply in the flesh. If food-poisoning bacteria are present, they can multiply and cause illness.
Enzymes also cause 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 or breakdown the flesh. This causes the flesh to soften and lowers the quality. Enzymes also produce more food for bacteria which increases the rate of spoilage. This commonly occurs in fatty fish such as salmon and mackerel.