Some fish, like guppies, give birth to live young, but most fish lay eggs.
In spring and summer, when the water is warm enough for eggs to hatch, the male and female fish send signals to each other with their fins or their body. At this signal, the female lays her eggs in the water or in a nest at the same time that the male releases his sperm. Some sperm reach the eggs and fertilize them. This entire process is called spawning.
These fertilized eggs have a see-through membrane for a shell. Inside the egg is a yolk, which will feed the growing fish, and cells, which will become the fish itself. As the cells divide and surround the yolk, they gradually start to look like a fish. The fish feeds on the yolk, to which it is attached by blood vessels.
When it is time for the fish to hatch, usually about one month, the shell softens and tiny fish swim out into the water. But the newly hatched fish, called larvae, continue to feed from the yolk, which remains attached to them. When that supply of yolk is used up, the fish begin to hunt for food in the water.
In some fish, such as the sea horse, the female lays her eggs in a pouch on the underside of the male, who carries them until they hatch; while in others, such as the catfish, the eggs are carried and hatched in the male’s mouth!