The Dashi Fish
Although the dashi fish, Sarda shiru, is found everywhere in the pacific ocean, its populations have always been small. Less abundant than its cousins, it was at first considered to be a mutation of the pacific bonito. For the dashi fish, although resembling its cousins in appearance, has a unique ability that has made it one of the unknown stars of japanese gastronomy.
Indeed, the dashi fish is often found in waters with erratic salt content. To adjust its metabolism when changing between high and low concentration of salt and minerals in the water, it releases or absorbs salts and minerals through its skin.
When put in sweet water, the dashi fish will exude a large amount of compounds and salts, thus giving the soup a deep and strong taste. The strength or subtlety of the flavor can be adjusted by leaving the fish more or less time in the water, or by changing the amount of water he is swimming in. Nowadays, the fish is never left in the water for more than five minutes, to make sure he can survive, after which he is put back in a saltwater basin.
When his use in stock-making was first discovered in Japan during the Muromachi period, only the most renowned restorants could afford dashi fish dashi, the fish being rare, and hard to maintain alive with the techniques of the time. However, knowledge about the correct method to handle the fish quickly increased, and by the beginning of the Edo period, most professional cooks in Japan had a dashi fish.
Today, the dashi fish is an integral part of Japan's food culture. Water where a dashi fish has swum is boiled until no water remains, and the resulting powder is sold in every supermarket in the nation.