Fish go through various life stages between fertilization and adulthood. The life of fish start as spawned eggs which hatch into immotile larvae. These larval hatchlings are not yet capable of feeding themselves and carry a yolk sac which provides stored nutrition. Before the yolk sac completely disappears, the young fish must mature enough to be able to forage independently. When they have developed to the point where they are capable of feeding by themselves, the fish are called fry. When, in addition, they have developed scales and working fins, the transition to a juvenile fish is complete and it is called a fingerling, so called as they are typically about the size of human fingers. The juvenile stage lasts until the fish is fully grown, sexually mature and interacting with other adult fish.
Growth stages Ichthyoplankton (planktonic or drifting fish) are the eggs and larvae of fish. They are usually found in the sunlit zone of the water column, less than 200 metres deep, sometimes called the epipelagic or photic zone. Ichthyoplankton are planktonic, meaning they cannot swim effectively under their own power, but must drift with ocean currents. Fish eggs cannot swim at all, and are unambiguously planktonic. Early stage larvae swim poorly, but later stage larvae swim better and cease to be planktonic as they grow into juveniles. Fish larvae are part of the zooplankton that eat smaller plankton, while fish eggs carry their own food supply. Both eggs and larvae are themselves eaten by larger animals. According to Kendall et al. 1984 there are three main developmental stages of fish:
Egg stage: From spawning to hatching. This stage is named so, instead of being called an embryonic stage, because there are aspects, such as those to do with the egg envelope, that are not just embryonic aspects. Larval stage: From the eggs hatching till to when fin rays are present and the growth of protective scales has started (squamation). A key event is when the notochord associated with the tail fin on the ventral side of the spinal cord develops and becomes flexible. A transitional stage known as the sac larval stage, lasts from hatching to the complete resorption of the yolk sac. Juvenile stage: Starts when the morphological transformation or metamorphosis from larva to juvenile is complete, that is, when the larva develops the features of a functional fish. These features are that all the fin rays are present and that scale growth is under way. The stage completes when the juvenile becomes adult, that is, when it becomes sexually mature or starts interacting with other adults. This article is about the larval and juvenile stage.
Hatchling – refers to a recently hatched fish larva that is still too immature to achieve motility, and therefore not yet capable of active feeding. A hatchling still possesses a yolk sac upon which it depends for nutrition, and is thus also known as a sac fry. Fry – refers to a more developed hatchling whose yolk sac has almost disappeared, and its swim bladder is functional to the point where the fish can move around and perform limited foraging to nourish itself. At this stage, the fish usually filter-feeds on planktons as it is still too small and slow to venture away from covers without being consumed by predators. Fingerling – refers to a fish that has reached the stage where the fins can be extended and protective scales have covered the body. At this stage, the fish is typically about the size of a human finger, hence the name. Once reaching this stage, the fish can be considered a juvenile, and is usually active enough to move around a large area. The feeding diet also changes from planktons to invertebrates and algae, occasionally other fish. The fish also starts to morphologically resemble adult fish gradually (though still smaller), although it is not yet achieved sexual maturity.
Juvenile salmon
Fry and fingerling are generic terms that can be applied to the juveniles of most fish species, but some groups of fishes have juvenile development stages particular to the group. This section details the stages and the particular names used for juvenile salmon.
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