One of the first wild fish that a child is most likely to catch in a net in small streams and ponds is the three-spined stickleback, Gasterosteus aculeatus, a remarkably tough species that will eat any other unwary smaller passing creature.
At this time of year the breeding males – maximum length 8cm (3in) – turn from silvery fish into red-chested, green-sided, blue-eyed warriors. They build a nest in shallow oxygen-rich water in preparation for being an attentive and protective father.
Their vivid colours and enticing swimming display are used to lure a female to lay up to 200 eggs in the nest. At that point the female is chased away and the brightly coloured male takes on all comers, meanwhile fanning the eggs in the nest to ensure they get enough oxygen to develop.
After eight to 10 days the tiny hatchlings are herded together and protected, sometimes being taken into the mouth and back to the nest if they stray too far.
The species is common in Britain and is often the first fish to recolonise streams that have been polluted. Sticklebacks can also tolerate brackish and salt water. Their spines can be locked in position, making it difficult for predators to swallow them.
