Fight against lagarosiphon going well

Niwa diver and aquatic ecologist Aleki Taumoepeau hovers over the hessian bottom lining in Parkins Bay, Lake Wanaka. Photo / NIWA

Land Information New Zealand claims it is winning the war on lagarosiphon weed in Lake Wanaka.

In an update this week, Linz biosecurity manager Dave Mole said recent monitoring showed large parts of Lake Wanaka were free of the invasive weed "thanks to co-ordinated efforts'' led by Linz.

More than 600 catch-bags of weed were pulled out by divers over the past year, more than 20,000sq m of the lake bed had been lined with hessian matting and at least 3ha of the lake sprayed with herbicide.

"Monitoring by the National Institute of Water and Atmospheric Research [Niwa] has confirmed the success of these efforts,'' Mr Mole said.

"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.

"We're now seeing native plant species thriving in areas previously overrun with lagarosiphon.''

Mr Mole said lagarosiphon was particularly effective at colonising lakes and killing off native plant species. Just 1cm or 2cm of the plant is able to infest an entire lake "if left to take hold''.

Native plants colonise hessian matting in Lake Wanaka after lagarosiphon has been smothered. Photo / Boffa Miskell

Growing at the rate of up to a metre a month, at its full height lagarosiphon can reach 5m, which means it can cause problems for boaties and swimmers.

"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.''

The weed remains in Glendhu Bay and Roys Bay.

Boffa Miskell project manager Marcus Girvan said eradication was focused on high use and boat launching areas.

"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.''

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 is planned over the coming months, and several areas are due for another inspection by Niwa within six months.