9 April 2018

Recent monitoring shows large parts of Lake Wanaka remain free of the invasive weed lagarosiphon thanks to coordinated efforts led by Land Information New Zealand (LINZ).

The Lake Wanaka Aquatic Weed Management Programme has seen more than 600 catch-bags of weed pulled by divers over twelve months, more than 20,000 square metres of the lake bed lined with hessian matting and at least three hectares of the lake sprayed with herbicide.

Monitoring by the National Institute of Water and Atmospheric Research (NIWA) has confirmed the success of these efforts.

“Through working together with regional councils and local community groups we’re not only stopping the spread of this pest species but removing it from the lake as well,” says LINZ Biosecurity Manager Dave Mole.

“Through the use of various techniques we’re now seeing native plant species thriving in areas previously overrun with lagarosiphon.”

The weed species is particularly effective at colonizing lakes and killing off native plant species. Just one to two centimetres of plant material can be all it takes to infest an entire lake if left to take hold.

Growing at the rate of up to a metre a month, at its full height lagarosiphon can reach five metres. This means that closer to the shore, the weeds can cause problems for boat users and swimmers alike.

“Lake Wanaka is one of our most popular water spots. With the recent good weather over summer and increased use of the lake it’s particularly good news that NIWA’s inspection found good progress is being made in controlling the weed.”

While about two thirds of Lake Wanaka has been kept free of lagarosiphon, it does remain in the southern bays of the lake, particularly in Glendhu Bay and the Roy’s Bay area, meaning that agencies and the community need to remain vigilant.

“With recreational lake usage increasing each year, our strategy is to focus our efforts in the high use and boat launching areas”, says Marcus Girvan, of Boffa Miskell, who is the Project Manager for LINZ’s Biosecurity efforts.

“By reducing the chance of people coming into contact with lagarosiphon it’s a win-win; lake users’ experience is enhanced and we reduce the risk of weeds being spread around the lake and across the region.

“However, it’s vital that we all play our part to stop this pest from spreading, not only across Lake Wanaka but in other waterways as well.”

Anyone using a boat, kayak or other craft is advised to look for weeds and “Check, Clean and Dry” before and after every outing.

Further work to remove more lagarosiphon from Lake Wanaka is planned over the coming months, with several areas due for another inspection from NIWA within six months.

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