Oxygen weed gone from Lake Ngakapua

Sophia Clark (left) and Andrew Macdonald (NRC), Jennifer Troup (Bay of Islands-based Marine Environmental Field Services) and Lisa Forester (NRC) were delighted to emerge from Lake Ngakapua emptyhanded.

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

Privately-owned Lake Ngakapua, just north of Lake Ngatu, has been officially declared 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 there during the council'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 Endothall 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," Ms Forester said.

The herbicide effectively targeted the lagarosiphon while sparing non-target 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.

Regional council chairman Bill Shepherd said the only other time to his knowledge that this 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.

"Within a few months the lake bed there was fully occupied by native plants once more, and already at Ngakapua we're seeing evidence of a very healthy eel population," he said.

He added that Northland's dune lakes, especially those (unlike Ngakapua) that were easily accessible to the public, faced numerous threats, including nutrient runoff, invasive pest fish and aquatic weeds, as well as cumulative effects from public use.

Against that backdrop, the successful removal of oxygen weed from Ngakapua was an inspiration to, and a welcome victory for, those battling to protect Northland's lakes.

"While it represents just one step along a long journey, this oxygen weed eradication project is a practical demonstration that the aspirational vision of a pest-free Northland is achievable," Mr Shepherd said.

He said a few simple steps, including following the 'clean, check, dry' regime when moving between lakes and other waterways, could go a long way to protecting them from unwanted invaders.

"One of the most important things you can do to help to protect our precious waterways is ensure you always check, clean, then dry any equipment that comes into contact with the water, between every waterway, every time," he said.

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