Marion Valentine of Palmerston North asks :-

Are cock-a-bullies seasonal?

David Rowe, a zoologist with the National Institute of Water and Atmospheric Research, responded.

First of all what are cock-a-bullies? They are small fish. Most are no larger than your thumb and they live on the bottom of rivers, lakes, and estuaries where they sit still, hoping that their camouflage colouring will help them blend into the background. When they are disturbed, they quickly dart away to another hiding place.

Strictly speaking, the word cockabully should be used only for marine fish in the family Trypterygiidae. However, cockabully was probably used by early settlers in New Zealand to also include similar looking freshwater fish which we now call bullies (short for cockabully). Some people think that the term cockabully came from the Maori word kokopu (a small bottom-dwelling fish of rivers and creeks) but this is not correct.

There are six species of freshwater bully that are widespread in New Zealand and they are all seasonal because they reproduce during summer months. The females stick hundreds to thousands of eggs to the underside of a clean rock, one at a time: a task that takes many hours! The male fish then guards the eggs, fanning them to remove silt and scaring away any animal that threatens to eat them. When the eggs hatch, the larvae are about 3 mm long and almost invisible.

The larvae of four species of bully are carried straight out to sea where they live in the plankton until they become juveniles. The juveniles then return to the rivers and streams in a mass, seasonal migration of thousands to hundreds of thousands of fish. No one yet knows where these fish live at sea, or how they find the rivers and streams to return to! As adults, bullies are a common food for large eels and trout in both rivers and lakes, so are an important link in the food chain underpinning valuable freshwater fisheries.