Kohada or gizzard shad is a fish that's especially prized for being used in the preparation of sushi. The fish is not called kohada until it reaches 4 inches, usually in April and May. If it's smaller, it's called shinko, and if it's fully grown, it's called konoshiro or nakazumi.
Kohada is traditionally filleted, deboned, washed, salted, rinsed with vinegar, then marinated in vinegar for a long time. This fish is acidic, sweet, and full of umami, similar to fresh sardines (iwashi) or less strong mackerel (saba).