The return of the elusive giant kōkopu

NIWA Josh Smith and NIWA Dr Paul Franklin and NIWA Catriona Paterson.
Credit photographer David Tate.

NIWA media release 20 August 2010
The return of the elusive giant kōkopu

‘Whitebait’ tagged as part of a unique experiment have turned up. Earlier this year the giant kōkopu released into the Nukumea Stream in Orewa had disappeared, but when scientists returned in June the fish were back!

Scientists returned to the Nukumea Stream in Orewa in June, to investigate the trial release of giant kōkopu and found that they were back!

“We’ve found some of the giant kōkopu again, and it seems that our hypothesis that they'd previously moved to different habitats because of the extremely low flows in the river was correct,” says NIWA Freshwater Fish Scientist, Dr Paul Franklin.

Giant Kōkopu: Galaxias argenteus

In December 2009, NIWA implanted thirty giant kōkopu with tiny transponder tags
(PIT tags) and released them in the Nukumea Stream. Each tag has a unique ID so that when the tagged fish pass by the antennae positioned in the stream, they can be identified. By tagging the fish the scientists are able to monitor how well they survive and where they choose to live.

The scientists expected that many of the giant kōkopu would eventually leave the pilot study reach. When none of the released fish could be located in March 2010 it was thought that the extremely low water levels in the river, caused by summer drought, may have restricted the amount of habitat available for the fish.

However, in June 2010 when river levels were higher, the scientists located four of the released giant kōkopu in or close to the original release site using a handheld tag reader. A further survey a month later found six of the fish.

As river levels rose following rainfall in the autumn, a further five giant kōkopu were also recorded passing an antenna located further downstream. NIWA will continue to monitor the stream for returned fish as water levels increase.

“It’s great news to see some of our fish coming back to where we originally released them. The information we are collecting on their movements and what habitat they are using will be invaluable for the conservation of giant kōkopu in the future,” says Dr Paul Franklin.

Adult giant kōkopu breed in fresh water. Their larvae drift out to sea, and then return to rivers and streams as one of the species that make up ‘whitebait’. A major cause of their decline has been loss of habitat. The NIWA study is the first-ever controlled trial to see whether the native giant kōkopu can be successfully stocked in a stream. Scientists think that the adult fish produce a chemical signal or ‘pheromone’ to tell whitebait (juvenile kōkopu) which streams to return to.

Background
This work contributes to the restoration of native New Zealand species.

Banded kōkopu

Scientists have also been studying banded kōkopu, a different species of whitebait. In February, many of the banded kōkopu that had been captured in the stream and marked during a previous baseline survey were still resident in the pilot study reach. This means that they were not displaced or out-competed by the giant kōkopu whilst they were in the reach, and it also suggests that these fish may be more resilient to the low flow conditions within the stream.

The fish are monitored by NIWA and a team led by Dave Slaven, Ecology Team Leader Northern Gateway Alliance (NGA). The fish used for the trial were hatched and reared in tanks at Mahurangi Technical Institute from eggs originally collected from the Waitetuna River.

The NZ Transport Agency and its NGA partners undertook this project as part of the mitigation programme associated with the recent construction of the NZTA’s Northern Gateway Toll Road on SH1.

If the trial is successful, the NZTA and its NGA partners plan to release 500-1000 juvenile giant kōkopu into the stream at a future date to augment the declining population.

ENDS

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