Threespine Stickleback, Gasterosteus aculeatus





The threespine stickleback is a relatively small, silvery fish averaging two inches in length, and having a laterally compressed body, which narrows at the caudal peduncle area.   These fish have three dorsal spines, two pelvic spines, and one anal spine at the posterior of the anal fin. The pelvic fins have one spine and one soft ray differentiating it from the blackspotted stickleback, which has two soft rays.  The spines can be locked in an erected position for defense against predators. Threespine sticklebacks have bony lateral plates, the number of which is largely dependent on whether the fish inhabits salt or fresh water, saltwater fish typically having more. 


Threespine sticklebacks inhabit fresh, salt, and brackish waters.  It is common in slow moving backwaters of rivers, lakes, ponds, sheltered bays, and harbors.  Threespine sticklebacks feed on small crustaceans, oligocheates, and insect larvae, fish larvae and eggs, even those of its species. 

Predators of the threespine stickleback include a variety of piscivorous birds and fish.  Threespine sticklebacks are generally a schooling fish except during the spawning season which is generally between May and July.   At this time, females continue to school, while the males claim and defend territories.  Once in a territory, the male becomes more aggressive and will try to drive off other males and any other intruders.  A distinct change in colouration between the sexes occurs; the females remain silvery with dark patterns throughout their body, and the males develop a red breast and throat, the irises of their eyes become blue, and the back often turns greenish.  The male threespine stickleback builds a nest in a shallow depression usually escavated near shelter, typically rocks or vegetation.  The nest is built using algae and aquatic plants.  The male then entices a female to enter the nest and deposit her eggs for him to fertilize.  The male then chases the female away and begins guarding and aerating the nest.