Giant kokopu hotspot discovered on Waiheke Island

An “incredibly exciting” find of a new population of native giant kokopu has been made on Waiheke Island.

A freshwater ecologist for Auckland Council’s biodiversity team, Matthew Bloxham, had a hunch a stream at the eastern end of Waiheke had the qualities giant kokopu need to call a place home.

Teams had already searched 25 streams on the Auckland mainland where sightings of giant kokopu had been recorded. Two surveys in 2014 and 2018 turned up no signs of the threatened native fish.

Then 25 giant kokopu were found in the Waiheke stream a few months ago. A further six of the fish, which can grow up to 45cm long, were found in a different part of the stream a few weeks later.

“That’s really big news for us,” says Mr Bloxham.

“Nationally they are listed as a ‘vulnerable’ species, but we have to try even harder to sustain them in Auckland, given we’re at the upper limit of their distribution.”

A stream at Whakanewha Regional Park on Waiheke has lost all its giant kokopu, but another island stream at Awaawaroa still has a population of 10 to 20 of the native fish.

Extremely small numbers of giant kokopu have been found in a handful of other new sites in Auckland recently, including two at a stream in Whatipu.

The council biodiversity team is now focussing on Waiheke as the main giant kokopu stronghold in the Auckland region, says Mr Bloxham.

“The mainland populations are generally all failing – they’re functionally extinct. So we’re putting most of our efforts into giant kokopu recovery on Waiheke, because we think it holds the greatest promise.”

That means tackling the pressures that have driven many populations of the fish to extinction – development, habitat loss, sedimentation, predators, and a lack of logs to hide under.

The team plans to reintroduce giant kokopu at Whakanewha and to potentially boost numbers in the other Waiheke streams once the habitats are improved.

Giant kokopu are being bred in captivity by Manaki Whitebait in Warkworth and fish from the hatchery will be released on Waiheke over the next few years. In July, shortjaw kokopu were bred in captivity for the first time and these will also be released into Watercare dams in the Waitakere and Hunua ranges over the next few years.

The council team is working with Waiheke Forest & Bird members to improve pest management around the Waiheke streams where giant kokopu have been found.

“The eggs are like caviar to many animals – mice, rats, hedgehogs, everything eats them."

Giant kokopu gather to spawn in stream-side vegetation during floods, then must wait for another flood to wash the eggs into the stream. The larvae are carried downstream to the sea, where they are “nailed by all the things out in the briny”. Only a small number make it back into the same stream to replenish adult populations.

Sediment and flooding are other serious pressures on native fish.

Tree planting along stream edges is being carried out to act as a buffer.

Conservation New Zealand, local Forest & Bird branch members and the island community have been helping plant trees around Awaawaroa stream.

Mr Bloxham will give a talk about shortjaw and giant kokopu at Waiheke Sustainability Centre in Oneroa on August 9 at 6pm.

Planting days to support the giant kokopu project will be held at Awaawaroa on August 11 and 25 at 10am.

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