Fish are vertebrates that are cold-blooded. This means that their body is as hot or cold as the water around them. All fish live and breathe in water, and most baby fish hatch from eggs.


Female fish can lay hundreds of eggs. Inside every egg there’s a bag of food called the yolk. The unborn baby fish eats the yolk, so that it can grow bigger and stronger.

Fish often try to hide their eggs because many other animals like to eat fish eggs. Female seahorses lay their eggs into a special pouch on the male’s front, where they stay safe until they are ready to hatch.

Some species of shark lay their eggs in a hard egg case to keep the eggs safe until they hatch.

Young Fish

When young fish hatch, the yolk bag is still stuck to their body. The young fish use this as food at first.

Life is very dangerous for young fish. Most of them don’t have fins, so they can’t swim well. They often float into colder water or places where there’s no food. Also, many predators, like seals, birds, frogs, and other fish, hunt them. This is why fish lay so many eggs. A large female tuna fish can lay up to six million eggs at one time, but only about two of these will grow into adult tuna!

Growing Up

Most fish never see their parents because they swim away after they have laid their eggs. The young fish have to find food for themselves so that they can grow bigger. They eat small plants and insect eggs or larvae.

Most small fish grow into adults just a few months after hatching, but whale sharks do not grow into adults until they are 25 years old!

The whale shark is the biggest fish in the world. An adult whale shark can be about 12 meters long and it can weigh more than 21 metric tons.


Fish need to find mates so that they can breed. When the male three-spined stickleback is ready to breed, he builds a nest.

Part of his body changes to a red color. Females see his bright colors and they lay their eggs in his nest.

Many fish go back to the same place to breed every year. Salmon live in the ocean, but to breed, they swim back to the rivers where they hatched. Some salmon travel hundreds of kilometers up rivers, and sometimes they have to jump up waterfalls. They change color from silver to red when they are swimming back to their breeding places.


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