The three-spined stickleback is native to Ireland and is one of our smallest fish species. Three-spined sticklebacks have three or four spines along their backs and a pair of pelvic fins with prominent bony spines on their bellies. They have no scales but are sometimes armoured with hard, bony plates along their sides. Its high salinity tolerance allows the three-spined stickleback to inhabit a very diverse range of aquatic habitats where backwaters provide a refuge, including slow-flowing streams and rivers, ponds, lakes, canals, lagoons, estuaries and sheltered coastal waters. Coastal populations migrate back to fresh waters or brackish waters to breed.
The breeding behaviour of three-spined sticklebacks is quite interesting. At breeding time during the summer, the male develops a vivid red belly and bright blue eyes, and he becomes territorial and builds a tunnel-like nest from small pieces of aquatic plants on the riverbed. He uses a courtship dance to attract the female, which then swims through the nest to lay her eggs. The male follows to fertilise the eggs but then chases away the female and watches over the developing eggs by himself, fanning water over them to aerate them and guarding them from predators. A nest may contain eggs from several females, and the male may build more than one nest in a breeding season. Once the young hatch, the male protects them at the nest until they disperse.