Tilde's Talks - Giant Kōkopu

It’s been a wet few weeks, and while our water tanks are grateful, there’s another upside to all this rain—one particular fish has been biding it’s time, waiting for floods.

That fish is the giant kōkopu or Galaxias argenteus, Galaxias referring to the striking spots along their sides which look like stars in a galaxy. The giant kōkopu is an endemic (found only in Aotearoa) freshwater fish which is classed as vulnerable and declining. As its name suggests, this fish is quite large and has been known to grow up to 58 cm and weigh in at 2.7 kg. It is the largest freshwater native fish in the country.

The giant kōkopu can live for several decades. It spends its time lurking under overhanging banks or vegetation, waiting for a tasty koura (freshwater crayfish) or a fallen insect to float by, which it strikes at speed. Giant kōkopu were first discovered on Waiheke by a local pig hunter (and my dad, which is my biggest claim to fame in conservation circles!). This is the only known breeding population in the Auckland region.

So why does the giant kōkopu like a flood? Because their spawning behaviour is crazy! As the summer droughts end and the autumn and winter floods begin, the giant kōkopu waits until the water rises. Once the streams begin to flood into the surrounding vegetation, eggs are laid on the reeds and branches, which are usually high and dry. Once the water recedes, the eggs are left high above the water's surface. The eggs will remain out of the water until the next flood, at which point, the movement of the water triggers hatching, and they will be swept out to sea.

The unusual breeding behaviour probably evolved to protect the eggs from other hungry fish, but, as is always the case, the behaviours which once helped have been more of a hindrance since the arrival of introduced predators. It turns out that introduced pests such as rats, mice and Indian mynas have a liking for caviar, and the eggs are easy targets. This, along with habitat loss, is causing the decline in this species.

The larvae will spend about four months in the sea before swimming back up the streams. This unusual lifecycle makes them amphidromous (word of the day!), a fish that migrates between saltwater and fresh. Juvenile giant kokopu are one of the migratory fish that make up whitebait. Many juveniles are caught on their way back up the streams and will never breed, causing further population declines.

Significant work is being done to protect this vulnerable fish on the island and safeguard Tāmaki Makaurau’s only breeding population by removing rats and mice from the area. We can all help to protect this special fish by planting out stream banks and doing pest control. Furthermore, we can limit whitebait consumption to allow these special creatures to recover.