Category: hikarimono (shiny, silver fish).
Season: available year round, best in the fall and winter months. Shinko (young kohada) is available in the spring and early summer.
Ah, kohada, the ultimate edo-style neta, and quite possibly my personal favorite. When young and only a couple of inches in length, this fish is called shinko by Japanese fishmongers. Shinko is at its best in the springtime or in early summer. The fish is not called kohada until it reaches about 4 inches, and this is often when it is served as a nigiri topping. Eventually, it can reach nearly 10 inches in length, and is then called konoshiro, which is its actual name. But by then, well, it's just not as tasty, and rarely served as sushi.
Many (myself included) consider kohada to be the best way to judge the skills of a sushi chef. The preparation requires that the fish be filleted, deboned, washed with water, salted, rinsed with vinegar (usually saved from the previous day, for additional strength and flavor!), and finally marinated in vinegar for an extended period of time. Many factors such as fish size, fat content, and even weather conditions are used to determine how long to salt and marinate the fish, and a good chef will know this mostly from previous experience, but also to a certain extent from instinct.
The final product can be amazing: full of umami from the omega 3 fats, acidic from the vinegar, but also deliciously sweet. Kohada pairs wonderfully with aka-su flavored shari, and is just a joy to eat when prepared well.