What do you call these little fish used for making dashi (soup stock)?
If you are in the western part of Japan, you call the small dried fish "iriko" (炒り子); in the east, "niboshi" (煮干し).
Other names for the little fishies include mushi, jako, dashijako, and iriboshi . . .
The type of fish used in making niboshi vary, including anchovies, sardines, round herring, Pacific sand lance among others, but niboshi made with anchovies are the most common.
The translation of the term "niboshi" can cause some confusion in terms of whether it is made with sardines or anchovies, because in Japanese, both sardine and anchovy are referred to as "iwashi"(イワシ).
Niboshi (煮干し) literally means "boiled and (then) dried" and does not include the specific name of the fish used. Because of this, niboshi is often translated as "dried sardine" or "dried anchovy".
In Japan, niboshi dashi is one of the more common forms of dashi. It is especially popular as the base stock when making miso soup. Niboshi dashi is made by soaking niboshi in water. If left overnight or brought nearly to a boil, the flavor of niboshi permeates the water to make the stock.
Due to immigration mainly from western Japan to Hawaii in the 1800s, niboshi is known as iriko there.