The cold aquatic environment is also responsible for the notorious tendency of fish and shellfish to spoil faster than other meats. The cold has two different effects. First, it requires fish to rely on the highly unsaturated fatty acids that remain fluid at low temperatures: and these molecules are highly susceptible to being broken by oxygen into stale-smelling, cardboardy fragments. More importantly, cold water requires fish to have enzymes that work well in the cold, and the bacteria that live in and on the fish also thrive at low temperatures. The enzymes and bacteria typical of our warm-blooded meat animals normally work at 100°F/40°C, and are slowed to a crawl in a refrigerator at 40°F/5°C. But the same refrigerator feels perfectly balmy to deep-water fish enzymes and spoilage bacteria. And among fishes, cold-water species, especially fatty ones, spoil faster than tropical ones. Where refrigerated beef will keep and even improve for weeks, mackerel and herring remain in good condition on ice for only five days, cod and salmon for eight, trout for 15, carp and tilapia (a freshwater African native) for 20 days.