Spawning is the laying of the eggs of fish. The eggs are called spawn and are laid in very large masses. After the eggs are fertilized by the male fish, they hatch. There are many ways that fish spawn.
Many fish leave their eggs unprotected after they spawn. The cod and flounder lay millions of eggs. The kinds that stay to guard their eggs after spawning lay fewer eggs. In some species, the eggs are fertilized when the male sprays sperm and liquid, called milt, over them; comparatively few such externally – fertilized eggs receive sperm cells and become fertile. In other species, the eggs are fertilized by the male inside the female’s body opening before lying.
Some fish, such as the sunfish and trout deposit their eggs in a depression that they make in a stream bottom. The male guards the eggs or covers them with sand for protection. Some males build a nest of algae and guard the eggs until they hatch. Female sea horses lay their eggs in a pouch in the male’s tail, where they are carried until hatching. The female saltwater catfish carries its egg s in its mouth for protection.
Fresh water fish usually spawn in the water where they live. Other fish migrate hundreds and even thousands of miles to reach their spawning region. This migration of schools of fish is called a run. Fish that go up the river to spawn are called anadromous. Fish that go down the river, out to sea are catadromous.
Some fish do not spawn. Their eggs hatch inside them and their young are born alive. They are ovoviviparous. Most sharks have young this way. Fish that spawn, laying eggs which hatch outside their bodies are called oviparous.