Juvenile fish - postlarval stage of fish development, during which immature fish change body proportions, fins, coloration. It is a fully formed fish of a given year of birth with well-developed scales, possessing all the characteristics of an adult fish. Its age is indicated by the sign 0.

Juvenile fish a few days, weeks, months old. It resembles adult fish in body shape, but differs from it in size and weight.

Refers to more developed calves in which the yolk sac is nearly gone and their swim bladder is functional to the point where the fish can move around and forage for food to feed themselves in a limited fashion. At this stage, fish usually filter feed on plankton because they are still too small and slow to come out of hiding without being eaten by predators.