All fish species reproduce sexually. This means that a new individual is created by the combination of an egg and a sperm. The unifying of egg and sperm is known as fertilization, and the juvenile fish develop within the egg. However, there are some variations among fish in the methods of reproduction.
The two organs responsible for producing eggs and sperm in fish are, respectively, ovaries and testes. Usually, the female has ovaries and the male testes, however a few species of fish are hermaphrodite, containing both sets of sexual organs in one individual.
Over 97 percent of fish species are classified as oviparous reproducers. This means that the development of the embryos inside the eggs occurs outside of the parent's body. This is the same reproductive method used by insects, amphibians, birds, reptiles and arachnids.
Oviparous Reproductive Method
Female oviparous fish release unfertilized eggs (either laying them on a rock or plant surface, or emitting them into the surface water where they join the zooplankton layer) which are then fertilized by the male by either rubbing his sexual organs on the eggs, releasing his sperm, or emitting his sperm into the water so that fertilization takes place in the zooplankton layer.
Some species of fish, notably rays and chimaeras, are classed as ovoviviparous reproducers, or, more colloquially, livebearers. This means that fertilization and development of the embryonic fish take place inside the female. The fish hatch inside the female's body, being born as live young. Unlike in mammals, the eggs do not receive extra nutrition from the mother's body, developing with energy solely from the egg's yolk.
Like ovoviviparous reproducers, viviparous fish develop their young within the female's body. However, the eggs hatch inside the mother and remain in the placenta where they receive oxygen and nutrients from the mother's biological system via an umbilical cord. When mature they are also born as live young.
The reproductive cycle of fish varies widely. Some fish, such as the clown fish, reproduce many times a year, while others reproduce annually. Some species, notably the Pacific salmon, reproduce only once in their lifetime, dying after releasing their eggs or sperm.