Giant kokopu are New Zealand’s largest native freshwater fish other than eels, but few people have ever seen one.
They are also the largest member of the galaxiid family world-wide, and one of the species whose young forms part of the whitebait catch. Their numbers have declined markedly and they’re now listed as ‘vulnerable’ – the same status as brown kiwi.
And yet, though they have disappeared from many parts of the country, there are still goo numbers in a small stream in Gordonton.
“So they need our help,” says Niwa freshwater ecologist Paul Franklin. “Over summer we undertook fish surveys and put little electronic tags – much like the ones that go in to cats and dogs – in to the kokopu that we caught, so we can track them. We’ve subsequently been following their movements in a bid to try and identify if and where they might be spawning in the stream.”
The team has been working at the Gordonton site over the last year. It is part of a larger project looking at giant kokopu spawning, which includes sites in the Hakarimatas and a Hamilton stream.
“The spawning season is typically somewhere in the April to July period so we’ve been out several times in the last few weeks (first week of June) trying to track down potential spawning sites.”
The work is weather dependent: the surveys are carried out after high flows, following heavy rain.
“The fish lay their eggs in the bankside vegetation when river levels are high, and the eggs then develop out of water. They rely on more rainfall around 4 to 6 weeks later to re-inundate the spawning sites and allow the eggs to hatch.
“Our efforts have been focused on the reach of the stream between the State Highway 1B bridge and Peach Rd, but we have also looked upstream of Peach Rd.”
The potential spawning sites were near to aggregations of giant kokopu that were found using an instrument that detects their electronic tags.
One challenge is telling the difference between kokopu eggs and slug eggs. “Eggs were found in the vegetation, but both look very similar to the naked eye, so we have to follow their development to see if they turn in to fish before we can confirm!”
There is research ongoing which involves training dogs to search for eggs but that is some way off, he says. “We’re hopeful something like that will work out in the future, but for now we are reliant on our team getting down on their hands and knees to search in the grass!”
Paul says it has been known for some years that giant kokopu live in the Gordonton stream.
“It is sometimes surprising what lives in our streams around us, and it is important that people are aware that despite appearances, these heavily modified streams can support important fish communities.
“Giant kokopu like slow flowing streams with pools, as long as there’s some cover along the edges where they can hide. There are also plenty of eels present in the stream, and we’ve also caught inanga there.”
They are generally diadromous, meaning they migrate between the sea and freshwater as part of their life-cycle.
“However, in the Waikato catchment, it has been shown that often giant kokopu don’t migrate to the sea, and instead use some of the lower Waikato lakes to rear in. We believe that many of the fish that we’re finding at Gordonton probably reared in Lake Waahi.”
Response from landowners has been positive, he says. “Most show a keen interest in the fish that we find in their streams. Sometimes they are surprised what’s there though! There has obviously been a move by many landowners to fence off waterways, and even better, plant up their riparian zones in recent years, which benefits all streams.
“We’re also starting to see more awareness of the need to avoid creating barriers in streams, such as weirs or badly installed culverts, that can block the movements of fish between critical habitats that they need to survive.”
The project, funded by the MBIE Endeavour Fund, is set to run for another three years.
“We’ll be continuing searching for evidence of spawning at the site over the next month or so. Next summer we will do more fish surveys and try to tag more fish for us to track in the next spawning season when we will be back to search for eggs again.”
The team were out again in Gordonton last Friday and caught six giant kokopu in nets set overnight. However, none were showing any signs of spawning.