CREATURE FEATURE: The Redfin Bully

By Stella McQueen - Own work, CC BY-SA 4.0

One of the surprising results from our eDNA test at Gill Ave Stream, Kelvin Strand, is a small endemic fish, the redfin bully. 

Meet the redfin bully

The redfin bully (Gobiomorphus huttoni) is only around 8–12 centimetres long, but it is one of Aotearoa’s most distinctive freshwater fish. Look closely at an adult male redfin bully and you can see why it has earned a reputation as New Zealand’s most colourful native freshwater fish. Its fins develop vivid red and orange colouring, while both males and females have distinctive diagonal stripes across their cheeks.

These fish are remarkably well adapted to life in fast-flowing streams. They are strong climbers, using their large pectoral fins to grip the substrate and have been recorded above waterfalls more than five metres high.

The redfin bully needs both freshwater and the sea to complete its life cycle. Adults live and spawn in freshwater. The eggs are laid in a ‘nest’ created by the male, this could be under  rocks or in a hollow log, this helps protect the eggs from the flow of the water and hides them from predators. The male guards the nest until the eggs hatch. Once hatched, the tiny larvae are carried downstream towards the sea. Later, juvenile redfin bullies make their way back into freshwater, where they grow and mature.

Redfin bullies have quite particular habitat requirements. They are generally associated with small, stony or bouldery streams with moderate to fast flows. Too much sediment can smother the spaces between rocks and interfere with the habitat and food sources these fish depend on. NIWA notes that redfin bullies are more sensitive to stream siltation than many other native fish because of their dependence on this type of rocky habitat.

A surprising discovery in Gill Ave Stream

We tested the Gill Ave Stream because there are proposed plans to divert flow from it. Auckland Council was under the belief that the salinity levels in the stream would not support fresh water species; the DNA now proves otherwise. 

The Gill Ave Stream. flowing through the Kelvin Strand area, might not look like the sort of place you would expect to find rare native fish. It is an urban stream, surrounded by roads, houses and development. Yet eDNA testing has revealed that the stream is home to all sorts of native species including redfin bully, a species with a threat level of at risk: declining.

Water flow is important to redfin bully, their preferred habitat includes flowing, oxygenated water over rocky stream beds and the flow of the stream is what carries juveniles out to sea for an important part of their life-cycle. This makes proposed changes to the flow of Gill Ave Stream particularly concerning, changing the amount, timing or pattern of water moving through the stream could have consequences on the redfin bully's ability to feed and breed in the stream. Change the flow, and you potentially change the habitat.