FISH PRESERVATION
Since the ancient times, people had to invent ways to preserve their fish from spoilage. This was important, since there was not a way to quickly transport the fresh fish to the people who wanted to eat it, not to mention the absence of refrigerators. Of course, people who lived near the coast had the advantage of opportunities of buying decently fresh fish, but for other people something else had to be invented. So people came up with drying, smoking, salting, and even fermenting the fish. All these methods had in common that they were strongly anti-bacterial - dry fish doesn't allow bacteria to grow due to the lack of life-necessary water; smoking and salting kills bacteria and prevents them from growing. Fermentation actually makes use of bacteria but just for the flavor purposes.
Dried fish is probably as old as the humanity is. It is a simple method of putting the fish under sun rays or in the wind that will do the job of "gently" evaporating the water from the fish. Below 15% of water content, the microbes have a hard time to grow on a fish. Another advantage is that many flavor molecules, particularly amino acids, become more concentrated when there is less water, so the taste becomes more intensive. However, drying is not recommended for all fish, just the fish with lower fat content. Fat is easily spoiled and becomes rancid when exposed to the sun rays (as happens with butter that is treated in a similar way). Even if dried fish is an ancient method of preservation, it is still used, and certainly worth trying. You can purchase not only dried fish but also shellfish, and you can use them after soaking them in water, or as additional flavor ingredients in your dishes.
Salting fish is a more convenient and safer method to use in hotter climates, where natural sun is too intense to allow for drying the fish (it would spoil too quickly). Salt-curing can be done in a number of ways, depending on the purpose. Short-time salting can preserve the fish for a number of days, which has been done in old times just for transportation purposes. However, fish can also be put into very salt water solutions, where the salt content is so high that almost no more salt can be dissolved. Curing the fish in such soluion will make it stand a longer test of time, several months or more. A popular dish is salt cod, which is made by curing the fish meat in a lot of salt, something that actually enhances the flavor of the fish due to a controlled bacterial activity that breaks down bigger fish proteins into the tasteful amino acids.
Smoking the fish is done over an open fire, but with certain flammable products, usually sawdust. Cold smoking is done at about 30 C/85 F, a temperature in which the sawdust gives much more smoke than an intact piece of wood. This low temperature keeps the fish soft while some of the inner water is lost and the fish meat thus becomes more "shrunk" and meaty. The fish can also be smoked in hotter conditions, around 70 C/160 F, where the fish meat is actually cooked but only for a brief period. Smoked fish keeps for a month or two in cold compartments.
One more method of preservation, perhaps the one that people are (unfortunately) most acquainted with, is the canned fish. Advantage of putting a fish in a can is of course its long preservation time. However, this is also the fish that has been heated twice (one for cooking, two for sterilizing), and some additional flavors may have been added to improve the flavor. Actually, some canned fish is not that bad, but it's still far away from fresh fish, or from the intricate flavors of smoked or salt-cured fish. Of course, the choice is always yours!