‘Top 20’ Northland lake declared free of pest oxygen weed

A privately-owned dune lake ranked among the region’s top 20 lakes 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 snorkelers and divers recently confirmed lagarosiphon has been eradicated from Ngakapua, a nine-hectare lake on the Aupouri Peninsula.

Lisa Forester, the Northland Regional Council’s (NRC) Biodiversity Manager, says Ngakapua is 5.2m deep with a ‘moderate-high’ ecological score and is ranked as one of Northland’s top 20 lakes.

She says lagarosiphon is thought to have been introduced to the lake by either eel fishermen or duck shooters and was discovered there during the council’s annual ecological lakes survey in October 2014.

Keen to rid weed from the lake – which has also been fenced via the council’s Environment Fund – 235 litres of Endothall, an approved herbicide targeting lagarosiphon was spread from a boat over 1.32 hectares of the lake’s surface by council staff 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 says 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 timeframe; typically several days. (The NRC also hopes to treat another 10 Northland lakes for several invasive exotic aquatic weed species over the next two years.)

“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.”

Regional council chairman Bill Shepherd says the only other time to his knowledge this oxygen weed had been successfully eradicated in Northland was from 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.”

Chairman Shepherd says Northland’s dune lakes – especially those (unlike Ngakapua) that are easily accessible to the public – face many threats including nutrient runoff, invasive pest fish and water weeds, as well as cumulative effects from public use.

Against that backdrop, the successful removal of oxygen weed from Ngakapua is 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.”

Chairman Shepherd says a few simple steps – including following the ‘clean, check, dry’ regime when moving between lakes and other waterways – can 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.”

Meanwhile, Ms Forester says information on a variety of water and land-based pests is available online via www.nrc.govt.nz/pestcontrolhub

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