After an animal dies, some of its digestive enzymes erode the alimentary canal walls and invade the flesh. The flesh of an ungutted fish spoils faster under this assault than that of a land animal, partially because a fish's digestive enzymes are generally more efficacious. Remember, those biological catalysts that labor in the fish's digestive tract must chemically break down swallowed whole fish, bones and all. Compounding the disintegrating effect of the fish's potent digestive enzymes is the fact that fish flesh is easier to digest than terrestrial animal meat.
Glycogen also plays a role in storage. Fish (and other animal) store this carbohydrate in their muscle tissue as an energy source. At the animal's death, this supply is converted to lactic acid, an effective preservative. Unfortunately, the amount of lactic acid in the flesh of a dead fish is usually scant because a fish burns up most of its glycogen store struggling to escape from the fisherman's net or hook.
Another problem is that the potentially pathogenic bacteria that lie in wait on the outside of the fish or in its digestive tract are, unlike those of land animals, psychrophilic. That term means that these microorganisms can thrive in relatively low temperatures. At one degree above the freezing point, the bacteria that typically attack fish can thrive, whereas the action of those that normally assault beef is retarded. Even at temperatures below 32F, the bacterial in fish are more active than those in beef.
Freezing the fish at an exceedingly low temperature (minus 100F, for instance) is not a flawless solution because, though the bacterial growth may be impeded at such hypo-Siberian thermometer readings, freezing ruins the fish's texture - even more than it does the cell structure of red meat. Even dried fish does not store as well as dried red meat. Using any preservative method, fish is more perishable than meat from warm-blooded vertebrates.
Unsaturated fatty acids are more apt to oxidize than saturated ones. Since the ratio of unsaturated to saturated fatty acids is higher for fish than it is for mammals, it follows that fish will oxidize, and therefore become rancid, more quickly than beef, pork, or lamb.
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