Threespine  Stickleback      Gasterosteus aculeatus



This small fish is a common resident of estuarine and fresh waters throughout North American including both the Atlantic and Pacific Coasts and as far east as Korea. It can reach four inches in length by adulthood.  The individual in the photograph is about three inches long and was using the floats at the Boston Harbor Marina as a refuge along with hundreds of its siblings.


The name Stickleback comes from the presence of several very sharp spines on the back of the fish that serve as protection from predators. Three dorsal spines are present in this species and are raised when threatened.  The last spine, which is located just in front of the first ray, is very small.  The marine form is heavily armored as you can see from the vertical plates on it’s side.  The freshwater ones have fewer plates.


Threespine Stickleback feed on small zooplankton such as baby crab and barnacles, amphipods, and young fish. In turn they are eaten by a host of predators including larger fish and fish-eating birds.


They usually breed in fresh water from March through October. The male builds a nest of plant material and debris with an opening at either end. The nest is glued together using a material manufactured in the fish's kidneys. The male with his dark green and orange-red body colors lures the female into the nest using an elaborate courtship ritual. After spawning the eggs are fertilized and then guarded by the male. The male will also fan the eggs with its fins and clean them with its mouth. The eggs hatch in a week and the young are protected by the male for several days until they begin foraging on their own.


Threespine Stickleback breeding behavior has been the subject of numerous scientific studies. In fact this animal was one of the first species to be studied in the early years of the development of the scientific discipline about animal behavior called ethology.