The primary means of fish reproduction are egg laying and live bearing. Egg-laying fish release eggs that are later fertilized. Live-bearing fish internally fertilize the female, who carries the offspring within until they develop and are birthed.
The release of fish eggs and their subsequent fertilization is called spawning. Most marine fish reproduce by releasing thousands of small eggs into the water for external fertilization.
Egg-laying fish have a variety of methods for releasing the eggs. Some fish scatter eggs, which are fertilized by the male fish as they fall. Other fish attach the eggs to a surface, where they are then fertilized as they are released. Some fish release eggs into a nest of bubbles blown by the male fish. A few fish are mouth-brooders, keeping the eggs in their mouth until they hatch.
Fish sexually mature at many different ages, depending on the species and some environmental factors. Some fish are born sexually mature while others do not mature until 15 years of age. Most bony fishes reach sexual maturity between one and five years. The gestational period of fish ranges from a few days to a few months. Some species of fish are hermaphroditic, either simultaneously having both egg- and sperm-producing organs or changing sex from male to female or vice versa.