Reproduction All fish reproduce sexually. In sexual reproduction, a sperm unites with an egg in a process called fertilisation. The fertilised egg develops into a new individual. In almost all fish species, males produce sperm and females produce eggs. In a few species, the same individual produces both sperm and eggs. The eggs of most fish are fertilised outside the female's body. A female releases her eggs into the water at the same time that a male releases his sperm. Some sperm come in contact with some of the eggs, and fertilisation takes place. This process is called external fertilisation. The entire process during which eggs and sperm are released into the water and the eggs are fertilised is called spawning. Almost all bony fish reproduce in this way. Sharks, rays, chimaeras, and a few bony fish, such as guppies and mosquito fish, reproduce in a different manner. The eggs of these fish are fertilised inside the female, a process called internal fertilisation. For internal fertilisation to occur, males and females must mate. The males have special organs for transferring sperm into the females. After fertilisation, the females of some species release their eggs into the water before they hatch. Other females hatch the eggs inside their bodies and so give birth to living young. Fish that bear living young include many sharks and rays, guppies, and some halfbeaks and scorpionfish.

Preparation for spawning Most fish have a spawning season each year, during which they may spawn several times. But some tropical species breed throughout the year. The majority of fish spawn in spring or early summer, when the water is warm and the days are long. But certain cold-water fish, such as brook trout and Atlantic cod, spawn in fall or winter. Most fish return to particular spawning grounds year after year. Many freshwater fish have to travel only a short distance to their spawning grounds. They may simply move from the deeper parts of a river or lake to shallow waters near shore. But other fish may migrate tremendous distances to spawn. At their spawning grounds, the males and females of some species swim off in pairs to spawn. Among other species, the males and females spawn in groups. Many males and females tell each other apart by differences in appearance. The females of some species are larger than the males. Among other species, the males develop unusually bright colours during the spawning season. During the rest of the year, they look much like the females of their species. In some species, the males and females look so different that for many years scientists thought they belonged to different species. Among other fish, the sexes look so much alike that they can be told apart only by differences in their behaviour. For example, many males adopt a special type of courting behaviour to attract females. A courting male may swim round and round a female or perform a lively "dance" to attract her attention. Among some species, including cod, Siamese fighting fish, and certain gobies and sticklebacks, a male claims a territory for spawning and fights off any male intruders. Many fish, especially those that live in fresh water, build nests for their eggs. A male freshwater bass, for example, uses its tail fin to scoop out a nest on the bottom of a lake or stream.

Spawning and care of the eggs After the preparations have been made the males and females touch in a certain way or make certain signals with their fins or body. Depending on the species, a female may lay a few eggs or many eggs - even millions - during the spawning season. Most fish eggs measure 1/8 inch (3 millimetres) in diameter or less. Some fish, such as cod and herring, abandon their eggs after spawning. A female cod may lay as many as 9 million eggs during a spawning season. Cod eggs, like those of many other ocean fish, float near the surface and scatter as soon as they are laid. Predators eat many of the eggs. Others drift into waters too cold for hatching. Only a few cod eggs out of millions develop into Adult fish. A female herring lays about 50,000 eggs in a season. But herring eggs, like those of certain other fish, sink to the bottom and have an adhesive covering that helps them stick there. As a result, herring eggs are less likely to be eaten by predators or to drift into waters unfavourable for hatching. A number of fish protect their eggs. They include many freshwater nest builders, such as bass, salmon, certain sticklebacks, and trout. The females of these species lay far fewer eggs than do the females of the cod and herring groups. Like herring eggs, the eggs of many of the freshwater nest builders sink to the bottom and have an adhesive covering. But they have an even better chance of surviving than herring eggs because they receive some protection. The amount and kind of protection given by fish to their eggs vary greatly. Salmon and trout cover their fertilised eggs with gravel but abandon them soon after. Male freshwater bass guard the eggs fiercely until they hatch. Among ocean fish, female seahorses and pipefish lay their eggs in a pouch on the underside of the male. The eggs hatch inside the male's pouch. Some fish, including certain ocean catfish and cardinal fish, carry their eggs in their mouth during the hatching period. In some species, the male carries the eggs. In other species, the female carries them.

Hatching and care of the young The eggs of most fish species hatch in less than two months. Eggs laid in warm water hatch faster than those laid in cold water. The eggs of some tropical fish hatch in less than 24 hours. On the other hand, the eggs of certain cold-water fish require four or five months to hatch. The males of a few species guard their young for a short time after they hatch. These fish include freshwater bass, bowfins, brown bullheads, Siamese fighting fish, and some sticklebacks. But most other fish provide no protection for their offspring.