Fish reproduce from eggs. In herring, mackerel, and many other species, spawning (the act of reproduction) is a mass enterprise. Gathered in huge schools, the females expel eggs and the males discharge milt (sperm) into the water. In some species the eggs float; in others they sink. In all species thus spawned, vast numbers of eggs and young are eaten by fish and birds or are otherwise destroyed. However, each female produces thousands, even millions, of eggs at a spawning.

Many fish lay their eggs in nests. The nests may be depressions in the beds of streams and lakes, such as those made by salmon and bass. The stickleback's nest is made of plant material. Fighting fish use floating bubbles as nests.

The eggs and young of nesting fish are commonly guarded by one of the parents, usually the male. The males of certain ocean catfishes carry the eggs, and later the young, in their mouths for as long as six weeks, fasting until the young are large enough to fend for themselves. Male sea horses and pipefish carry the eggs in abdominal pouches similar to those of kangaroos.

In some species, especially shark species, the females give birth to living young from eggs that are fertilized and hatched within their bodies.

Unlike most other vertebrates, fish grow as long as they live and eat, though they grow more slowly as they age. The rate of growth varies greatly, being most rapid where food is most abundant. Growth is commonly quicker in warm regions than in cold.

Fish have a wider range of size than any other group of vertebrates. The smallest fish known, the pygmy goby of the Philippines, averages less than one-half of an inch (13 mm) at maturity. The largest is the whale shark, which is estimated to reach a length of 60 feet (18 m) or more. (The largest of all sea-dwelling animals, the blue whale, is not a fish but a mammal.)

The normal life span of fish varies greatly. Some die after one year. Certain large fish such as sturgeon, lake trout, and Atlantic halibut may live for decades.