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Intrusion . . . Work to control the spread of oxygen weed lagarosiphon has been carried out at Lake Aviemore. PHOTO: DANIEL BIRCHFIELD

Herbicide control work to remove an invasive weed at Lake Aviemore has been carried out by Land Information New Zealand.

A survey conducted in April identified some patches of lagarosiphon between Benmore Dam and Parsons Rock, according to information provided by Linz.

One of those patches spanned 280 sq m in size.

A Linz spokesperson said the discovery of such a large amount of of lagarosiphon was a “serious concern”, and while no plant fragments had been discovered downstream of Parsons Rock, it was “highly likely” some would have spread to the main area of the lake.

Lagarosiphon, commonly known as South African oxygen weed, was originally used in fish bowls, aquariums and as a pond plant.

It was reported as a naturalised species in New Zealand in 1950 and made illegal to sell or distribute in 1982.

Since April, scattered plants and small patches have been removed by hand, while the largest patches were covered by a biodegradable hessian mat to shade them out.

The hessian mats were used as opposed to dredging, which increased the risk of plant material breaking off and seeding other areas in the lake.

Herbicide control work, using Diquat in gel form, was carried out via boat on May 19 and by helicopter on May 22.

“Much of the spraying was targeting Elodea canadensis, another oxygen weed that is widespread within New Zealand,” the spokesperson said.

“Although elodea doesn’t cause the same issues as lagarosiphon, it looks similar and can act to hide lagarosiphon. By controlling this species, we will greatly improve our detection rates.”

Further dive-based surveillance will now be carried out to detect and remove lagarosiphon plants.

Lagarosiphon was first found in Lake Aviemore in 2014 by Niwa, when it carried out an aquatic weed survey.

Since then, a number of isolated plants have been removed as part of routine surveillance activities.

Linz advised lake users to be vigilant and ensure they were not transporting weed fragments from one lake or river to another.

“One of the pathways for the spread of the weed is from Lake Benmore’s Ahuriri arm and neck. If boat trailers or equipment are snagged with lagarosiphon fragments and are then launched in Lake Aviemore, a whole new infestation can establish.

“If left uncontrolled, lagarosiphon could become surface reaching within two years, becoming a hazard for swimmers and restricting boat access.”

Spread of the weed has been attributed mainly to recreational boat traffic.