Any of about 12 species of fishes of the family Gasterosteidae (order Gasterosteiformes), found in temperate regions of the Northern Hemisphere. Sticklebacks are small, elongated fishes reaching a maximum length of about 15 cm (6 inches). Some species live in freshwater, some in the sea, and some in both. The members of the family are characterized by a row of spines on the back, in front of a soft-rayed dorsal fin. They also have a sharp spine in each of the pelvic fins; a slender tail base (caudal peduncle); and a squared tail. The scaleless skin is generally protected by a variable number of hard armour plates on the sides.

Images are approximately actual size for typical adults. Marine forms tend to be larger than freshwater forms, and are more heavily armored.

9-spine stickleback, Pungitius pungitius

3-spine stickleback, Gasterosteus aculeatus

Sticklebacks are noted for their reproductive behaviour. Breeding usually takes place in spring, at which time the male becomes suffused with color (marbled gray-green with black underside and white pelvic spines in the ninespine, and blue-green body with red throat in the threespine). After building a nest of plant materials glued together with a threadlike, mucous secretion from his kidneys, he coaxes and drives a female into the nest to lay her eggs and follows her to fertilize them. More than one female may be chosen. When the nest is full, the male becomes its guard and caretaker, aerating the eggs until they hatch and aggressively defending eggs and young from intruders.

Several stickleback species are familiar and abundant fishes. The three-spined stickleback (Gasterosteus aculeatus) is found almost everywhere in the Northern Hemisphere, in freshwater and salt water. A small fish, 5 to 10 centimetres long, it has three dorsal spines. The nine-spined stickleback (Pungitius pungitius), also small but with more dorsal spines, is another widely distributed form. Other species include the brook stickleback (Culaea inconstans) of North American freshwaters; the four-spined stickleback (Apeltes quadracus), also North American but primarily marine; and the sea, or 15-spined, stickleback (Spinachia spinachia), a slim, many-spined fish of European coasts.

Sticklebacks can be collected with minnow traps or a seine, as shown by the two field biologists in this picture: