Counting tagged giant kōkopu

In the last week of March 2010, NIWA scientists in waders with fishing gear and night spotlights counted tagged whitebait in the Nukumea Stream in Ōrewa. However, one species of the tiny fish proved to be elusive.

In December 2009, the scientists had released 30 giant kōkopu with tiny transponder tags (PIT tags) into the Nukumea Stream. They were trying to test whether the native fish could be successfully stocked into a stream.

Each of the transponder tags has a unique ID so that, when the tagged fish pass by the antennae positioned in the stream, they can be identified.

The first follow-up survey was carried out in January 2010, with another the following month. “We knew that some of the fish had left the study area as they had been detected on our antennae, but we managed to locate 15 [out of 30] of the giant kōkopu, which was a pleasing result,” says NIWA freshwater fish scientist Dr Paul Franklin.

Follow-up survey failed to find any tagged kōkopu

“Unfortunately, the recent [March] follow-up survey failed to locate any of the giant kōkopu that were released into the Nukumea Stream in December and had been found during previous follow-up surveys in January and February.

"It was expected that many of the giant kōkopu would eventually leave the pilot study area but it was hoped that a few would remain. It is thought that extremely low water levels in the river, caused by the recent lack of rain, may have restricted the amount of habitat available for the fish and contributed to the giant kōkopu no longer being present,” says Dr Franklin.

The scientists will keep monitoring the area to see if any of the fish come back as the water levels rise again over winter.

Partial success

Follow-up surveys were conducted in 2010 and 2011 and three of the original 30 fish were recaptured. All of the fish displayed good growth rates and were healthy, suggesting that the giant kokopu successfully adapted to living in the stream following their release.

In subsequent surveys during 2012 - 2015 no PIT tagged giant kokopu were captured. Three fish with healed incision marks at the same location as the original tags have been trapped so it is thought that these were part of the original release group. The discovery of a one large giant kokopu in the stream indicates that a small remnant of natural population was still present.

Kōkopu species in decline

Adult giant kōkopu breed in freshwater. Their larvae drift out to sea and then return to rivers and streams as one of the species that make up ‘whitebait’. The species has been in decline, mainly because of loss of habitat.

Scientists think that the adult fish produce a chemical signal or pheromone to tell whitebait which streams to return to. However, it is too early for the scientists to tell whether this pheromone production has occurred in the Nukumea Stream.

The giant kōkopu used for the trial were hatched and reared in tanks at Mahurangi Technical Institute from eggs collected from the Waitetuna River.

The news was not all bad for the scientists on the survey. As well as the giant kōkopu, they were also studying another whitebait species called the banded kōkopu. A sample population had been captured in the stream, marked and returned in October 2009. Many of these fish were still resident in the study area. This means that they were not displaced or outcompeted by the giant kōkopu whilst they were in the study area. It also suggests that these fish may be more resilient to the low flow conditions within the stream than the giant kōkopu.