Far North's Lake Ngakapua now free of oxygen weed

Sophia Clark and Andrew Macdonald (NRC), Jennifer Troup (Marine Environmental Field Services) and Lisa Forester (NRC) were delighted to emerge from Lake Ngakapua empty handed. Photo / Supplied

The Far North has recorded another extinction - and this time it's good news.

Privately-owned Lake Ngakapua, north of Kaitaia, is now officially free of the pest oxygen weed lagarosiphon, just the second time such an eradication has been achieved in Northland.

A comprehensive survey of the lake and reed bed by a team of snorkellers confirmed that the 9ha dune lake was free of the weed.

Northland Regional Council biodiversity manager Lisa Forester said Ngakapua was 5.2m deep, with a moderate to high ecological score, and was ranked as one of Northland's top 20 lakes.

Lagarosiphon was thought to have been introduced by eel fishermen or duck shooters, and was discovered during the NRC's annual ecological lakes survey in October 2014.

Keen to clear the lake, which had also been fenced via the council's Environment Fund, council staff applied 235 litres of herbicide over 1.3ha of the surface in 2015.

"The oxygen weed infestation was localised, and over a small area, so prompt action was important to prevent it from choking the entire lake, which lagarosiphon can do very quickly," Forester said.

The herbicide targeted the lagarosiphon while sparing native plants, was non-toxic to fish and aquatic invertebrates, and the water could be safely used again within a relatively short time frame, typically several days.

"Niwa staff involved with the Ngakapua project commented that it was one of very few examples in New Zealand where weed eradication in a lake had been achieved, with fast action and commitment by all sectors, and the owner," she added.

The only other time that oxygen weed had been successfully eradicated in Northland was in the much smaller (0.9ha) Lake Phoebe, near Pouto, where it had been dominating the lake bed.

Information on a variety of water and land pests can be found at www.nrc.govt.nz/pestcontrolhub