Tactical weed control paying off

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Recent monitoring has shown large parts of Lake Wanaka remain free of the invasive weed lagarosiphon, after more than 600 catch bags of the weed were pulled out by divers over the past 12 months.

Almost $380,000 is being spent on clearing aquatic weeds in the lake as part of the Lake Wanaka aquatic weed management programme, which is funded by Land Information New Zealand, the Otago Regional Council, Queenstown Lakes District Council and a donation from a philanthropist.

Under the programme, about 20,000sq m of hessian matting has been laid on the lake bed to starve the weed of sunlight, and about 3ha of the lake has been sprayed with herbicide.

The weed is monitored by the National Institute of Water and Atmospheric Research.

Linz biosecurity manager Dave Mole said using several techniques was working well. Native plant species were now thriving in areas that were previously overrun with the weed, he said.

“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 last week’s Niwa inspection found good progress is being made in controlling the weed.”

While about two thirds of Lake Wanaka had been kept free of lagarosiphon, it did remain in the southern bays of the lake, particularly in Glendhu Bay and the Roys Bay area, he said.

The weed can grow at up to a metre a month, quickly colonise lakes and can kill native plant species.

Project manager Marcus Girvan, of Boffa Miskell, said recreational lake use was increasing each year and everyone had a part to play preventing the weed’s spread.

“Our strategy is to focus our efforts in the 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 from Lake Wanaka is planned over the coming months, with several areas due for another inspection from Niwa within six months.