The common bully (Gobiomorphus cotidianus), or toitoi (Māori), is a fish endemic to New Zealand, and is present throughout the country.

It is thought that these populations of fish inhabited lakes or relatively high in river systems which were eventually blocked off from the sea due to natural processes.

In these land-locked environments the common bully is an important prey for larger freshwater fish such as eels and trout.

[5] In the summer months Gobiomorphus cotidianus can be found moving more into the littoral zones of lakes and streams as the females prepare to reproduce.

The females attach clusters of a several hundred to a few thousand eggs onto the underside of hard surfaces on the lake bottom where they are fertilized and guarded by the males.