What are Juvenile fish?

In this post, we will understand fish reproduction and the different stages of fish development until reaching the juvenile stage and finally adulthood.

What are Juvenile fish?

Juvenile fish are called fingerlings. In fish, the embryo turns into a larva when born. The stage of development after the larvae is called fry. When scales and fins are developed they become fingerlings, also known as juvenile fish. This period is very delicate and risky for the survival of the fish.

Reproduction of fish

Most fish species have oviparous reproduction, the females do not protect the eggs and fertilization is external. In fish, there are males and females, and sexual dimorphism is very common.

Females usually lay or spawn their eggs in calm waters or even in nests. Then, the males also deposit or spawn their sperm in the water, enabling the eggs to be fertilized.

After the eggs hatch, the larvae remain with the sac. Then, they become fries, and finally fingerlings, that look somewhat similar to the adult fish. After this stage, fish do not usually pay attention to their young. Some species of fish can even feed on these fries (of the same species or others).

From the point of view of reproduction, because of the variation in the way the offspring is born, fish are classified as:

– Oviparous: embryos develop outside the mother’s body, inside the egg that contains the necessary nutrients. More than 90% of fish belong to this category.

– Viviparous: embryos develop inside the mother’s body, receiving the necessary nutrients directly from her.

– Ovoviviparous: there is a combination of the two forms, that is, the embryos develop inside an egg inside the mother’s body. At birth, the young hatch from the egg, and then from the mother’s body.

What are fingerlings?

Fingerlings are fish that have recently fully developed scale and have functioning fins. This means they are not classified as fries anymore, they are juvenile fish. At this point, the fish leave the fry stage.

What is the juvenile stage?

To be considered juvenile the fish must, morphologically, be similar to an adult. That is, its external structure must resemble that of a fish in adulthood, except for its size.

For the fish to reach this stage, it is necessary to go through the larval and fry stages. The transition period between the fry stage to the juvenile stage is around 30 to 60 days, depending on the species.

Why do some people confuse fry and juvenile?

Confusion occurs because many producers confuse the phases. Generally, the misunderstanding happens between the initial development of the fish and they are known as fries, which is when the fins appear.

Embryonic development of fish

To address the embryonic development of fish, we first need to know some basic concepts of embryology, such as the types of eggs and the phases that make up early embryonic development.

We can find different types of eggs, according to the way the nutritive material present in the egg of animals that contains protein, lectin and cholesterol is distributed and its quantity. To begin with, let’s call the result of the union of an egg and a sperm as an egg, and as a yolk, the set of nutrients that is inside the egg and will serve as food for the future embryo.

Types of eggs according to the organization of the yolk inside:

1.   Isolated eggs

The yolk is found evenly distributed throughout the interior of the egg. Typically found in sponges, cnidarians, echinoderms, and mammals.

2.   Telolocyte eggs

The yolk is displaced towards an area of ​​the egg, being opposite the place where the embryo will develop. Most animals develop from this type of egg, such as molluscs, fish, amphibians, reptiles, birds, etc.

3.   Centrilocyte eggs

 The yolk is surrounded by cytoplasm and this, in turn, surrounds the nucleus that will give rise to the embryo. Occurs in arthropods.

Types of eggs according to the amount of yolk

1.   Oligoletic eggs

They are small and have a little yolk.

2.   Mesolecites eggs

They are medium-sized with a moderate amount of yolk.

3.   Macrolecite eggs

These are large eggs, with a large amount of yolk.

General stages of embryonic development

1.   Segmentation

In this phase, a series of cell divisions occur that increase the number of cells needed for the second phase. It ends up in a state called a blastula.

In fish, segmentation is meroblastic, as the division does not completely cross the egg, as it is impeded by the yolk, and is limited to the area where the embryo is located. The first divisions are vertical and horizontal to the embryo and are very fast and synchronized. They give rise to a bunch of cells installed over the calf, constituting the discoidal blastula.

2.   Gastrulation

There is a reorganization of the blastula cells, giving rise to the blastoderms (primitive germ layers) which are the ectoderm, the endoderm and, in some animals, the mesoderm.

During the gastrulation phase, a rearrangement of the discoidal blastula cells occurs by morphogenetic movements, that is, the information contained in the nuclei of the different cells already formed is transcribed in a way that forces the cells to obtain a new spatial configuration. In the case of fish, this reorganization is called involution. Likewise, this phase is characterized by a decrease in the rate of cell division and little or no cell growth.

3.   Differentiation and organogenesis

Tissues and organs will form from the germinal layers, forming the structure of the new individual.

The endoderm invaginates forming a cavity called the archenteron. The entrance to this cavity will be called the blastopore and will result in the fish anus. From this point, we can distinguish the cephalic vesicle (brain formation) and, on both sides, the optic vesicles (future eyes). After the cephalic vesicle, the neural tube forms and, on both sides, the somites, structures that will eventually form the bones of the spine and ribs, muscles and other organs.

Development and temperature

Temperature can affect embryonic development. It is related to the incubation period and the development success (the same occurs in other animal species). There is usually an optimal temperature range for embryo development.

Eggs within this range will have a greater chance of developing and hatching. Likewise, eggs incubated for long periods at extreme temperatures (outside the optimal range of the species) will have a lower probability of hatching and, if they hatch, hatchlings may suffer from serious anomalies.


In this post, we understood fish reproduction and the different stages of fish development until reaching the juvenile stage and finally adulthood.

If you have any thoughts or doubts, feel free to drop us in a comment below!

Frequently Asked Questions (FAQs): Juvenile fish

How is fish development?

After fertilization of the egg by a sperm, the zygote is formed. In many species of bony fish, development is indirect, with larvae or fry, followed by fingerlings or juveniles, and lastly the adults.

What kind of reproduction do fish have?

The vast majority of fish reproduce through eggs. Fertilization is external and thousands of eggs are deposited in the water.

What is the fish reproductive system?

In general, fish have ovaries or testes paired with gonads. Some fish species have additional organs, such as the genital papilla.

When does the fish become adults?

The adult stage is only reached with the arrival of sexual maturity and the reproductive peak, some fish take months, others this reproductive peak can take 3 to 5 years.

How to know fish age?

Similar to the concentric rings of a tree trunk, in each scale of a fish, it is possible to identify the years of life, counting the concentric rings.

Which fish lived the longest?

A Greenland shark (Somniosus microcephalus) was the oldest vertebrate animal in the world. It reached, when alive, nearly 400 years of age, according to the journal Science.


Kimmel, C. B., Ballard, W. W., Kimmel, S. R., Ullmann, B., And Schilling, T. F. (1995). Stages of Embryonic Development of the Zebrafish. Developmental Dynamics 203:255-310.

Gilbert, S.F. (2010). Developmental Biology, Ninth Edition.