Found in ponds, lakes, ditches, streams and rivers, the three-spined stickleback is a small torpedo-shaped fish which doesn’t always have three spines along its back. Sometimes there are only two and occasionally there are four. Body colour varies, though it generally has brown-to-green barring above and paler, silvery colouring below. 

Behaviour

Sticklebacks are carnivorous, feeding on tadpoles, fish and insect larvae. 

They are noted for their highly ritualized breeding behaviour. Breeding takes place during the spring, when the male develops a bright red throat and belly and green flanks and performs a striking ‘zig-zag’ courtship dance to attract a mate. He builds a nest out of vegetation, all glued together by threadlike, mucous secretions from his kidneys, known as spiggin. The male then coaxes up to three females into the arched nest to lay her eggs, which he then fertilizes. Each female may lay up to 200 eggs. The male defends the nest and attacks any intruder that dares to come close, while also fanning the eggs with his pectoral fins to supply them with oxygen. The eggs hatch four weeks later, but the male typically continues to defend the fry up to two weeks after hatching.

Their typical lifespan is 3-5 years.

Size

• Length: 4-7cm
• Average weight: 1g

Status

Common; one of the most common freshwater fish in UK canals and rivers.

Distribution

Sticklebacks are endemic to the temperate zone and can be found in both oceans and in fresh water. Freshwater taxa were trapped in Europe, Asia, and North America after the Ice Age 10,000-20,000 years ago, and have evolved features different from those of the marine species. The three-spined stickleback is the most commonly found freshwater species throughout the British Isles. 

When to see

All year round.

Facts

• There are three recognised species of stickleback in the UK: the three-spined stickleback (Gasterosteus aculeatus) and the nine-spined stickleback (Pungitius pungitius) which can be found in freshwater, saltwater or brackish waters. The latter is not widely distributed in Scottish freshwaters and is most commonly associated with slow moving waters in the south and western parts of the UK. The fifteen-spined stickleback (Spinachia spinachia) is purely marine and much larger (up to 22 cm). 

• The sticklebacks’ spines serve as defence against predators. The spines are raised and locked if a fish or bird tries to swallow it, usually resulting in the predator having to spit it out, unharmed.  

• Sticklebacks don’t have proper fish scales and are usually smooth along the body or sometimes covered with bony plates known as scutes. 

• Spinelesss sticklebacks only occur in northern Scotland and the Western Isles in river systems that are lacking in calcium. They are referred to as anomalous morphotypes, which means they display some degree of ecological or genetic divergence. The loss of spines is determined by a change in how genes which control development is expressed in a particular tissue, a change thought to be an evolutionary adaptation to reduced predation risk.