Scientists' crucial discovery in Hamilton Park

NIWA scientists have made an important discovery about the spawning habits of New Zealand's largest whitebait species in a central Hamilton stream.

The discovery solves a scientific mystery that will help restore the declining population of a Kiwi icon.

The giant kokopu is the largest of the five native New Zealand whitebait species (Galaxias argenteus) but the only one in which spawning has never previously been observed or eggs discovered.

NIWA fisheries technician Josh Smith and fisheries scientist Dr Paul Franklin have been working on a project to find out more about the spawning habits of the giant kokopu for just over a year.

They fitted electronic tags to 43 adult giant kokopu in the Bankwood Stream in Hamilton's Donny Park and placed antennae at the lower end of the stream to detect any fish leaving.

"When only one left, it gave us a good idea that they were spawning in the stream," Dr Franklin said.

A hand-held electronic tag reader was used to track the movement of fish in the stream during the day. At night, the scientists used spotlighting with a torch to find and then catch fish in a net so they could identify them and check their condition.

Last year, they found several ripe (ready to breed) males which made them "reasonably confident" they were spawning in the stream. Then it became a matter of narrowing down where and when the fish were actually spawning.

"In the first week of June this year, we finally found a couple of ripe females in the stream for the first time. We expected them to spawn on a bank when the water level was higher after rain, and with a storm due a few days later, we knew the time was right."

A few days later their endeavours paid off when Smith discovered several nests of eggs on the banks of the Bankwood Stream. A few of the eggs were brought back to the NIWA laboratory in Hamilton to examine more closely. As the eggs developed, they were able to confirm their discovery.

When the giant kokopu spawn, they lay their eggs in the grasses and vegetation on the bank where there is shade, high moisture content and humidity to provide the optimum conditions for hatching. But hatching will only take place when the stream rises again.

"This finally fills in the gap for all whitebait species. We know that spawning is a critical and susceptible part of the life cycle. They require a specific vegetation and water flow to lay and hatch eggs which is a risky strategy. But the more the environment is modified by humans the more risky that strategy becomes," Dr Franklin said.

Both scientists say it is important the right vegetation remains on the banks undisturbed.

"The giant kokopu are a special fish with significant biodiversity value. There is a good chance they are spawning in other streams around Hamilton and if we can provide and protect the right kinds of habitiat others may stay."

Dr Franklin says it is important to understand the habitats the fish use, and the different habitats they need throughout their life cycle.

"If one of these habitats is not available, the species will ultimately die out."

There is still a lot of work to be done on the spawning habits of the giant kokopu but the scientists say they are starting to gain an important understanding into a key phase.

Their findings will be presented at a freshwater and marine sciences conference at Waikato University next month.

Fact about the giant kokopu

  • Largest of the five whitebait species native to New Zealand
  • Can grow to over 40 cm long
  • Spawn on river banks about half a metre above the normal water level
  • Historically an important food source for Maori
  • May live for over 20 years
  • Prefer slow-flowing waters
  • Most common at low elevations and not very far inland
Josh Smith
The egg of a giant kokopu, part of a new discovery about their spawning habits. [Josh Smith]
Josh Smith
An adult giant kokopu, the largest of New Zealand’s five native whitebait species, which can grow to more than 40cm long. [Josh Smith]
Josh Smith
NIWA fisheries scientist Dr Paul Franklin looks at a nest of eggs laid by the giant kokopu on the edge of the Bankwood Stream in Hamilton. [Josh Smith]