By Amber McEwan
As the temperatures gradually get warmer, we aren’t the only ones thinking about hopping into our local river. A small, yet spectacularly attractive little native fish called the redfin bully uses this time of year to make the move from the sea to fresh water.
The redfin bully is usually a freshwater fish but they undergo a migration when they are young. Adults lay eggs in streams and rivers and when they hatch, the tiny larvae are swept downstream and out to sea. They spend 3–6 months living in the sea (often travelling long distances with inshore currents!), then venture up into river mouths and begin to gradually make their way upstream. This is a kind of migration called diadromy and it is very common in New Zealand freshwater fish.
This species is one of our most brightly coloured, especially the males which have bright red stripes and electric blue borders on their large, frilly fins. Adults are commonly around 100 mm long. Redfin bully males are exemplary fathers. After eggs are laid, dad is left to guard the nest, a job he undertakes with great ferocity! He chases away all intruders, puffing himself up and flashing his stripes and he often goes without food for long periods so he can stay close and keep his offspring safe.
Redfin bullies are found in many streams and rivers throughout New Zealand but they do prefer clean, cold water. They are nocturnal, so the best time to see them is with a strong torch at night in a local stream. Look closely—although they are colourful, they blend in well with the riverbed and tend to ‘freeze’ as camouflage which means that they can be hard to spot! Remember to be gentle with your observations, these colourful little fish are found nowhere else in the world and have a more serious conservation status than the little spotted kiwi!
Amber McEwan is a freshwater ecologist based in the Wairarapa.