Kawerau River cordons up to prevent 'invasive' weed spread

The Kawarau bridge, at the outlet to Lake Wakatipu. Photo / File

Government scientists are developing a plan to keep the invasive lake weed lagarosiphon out of Lake Wakatipu.

Three sections of the upper Kawarau River have been cordoned off with buoys to restrict lagarosiphon - also known as South African oxygen weed - from spreading into the lake's Frankton Arm.

Land Information New Zealand confirms scientists at National Institute of Water and Atmospheric Research have been commissioned to develop a management plan for Lake Wakatipu.

Linz biosecurity manager Dave Mole said lagarosiphon had infested surrounding rivers and lakes but Lake Wakatipu was clear.

"If we let the weed creep into the lake it will establish quickly, having an economic impact and threatening recreational activities such as fishing, swimming, water-skiing and jet-boating.''

The weed can choke waterways strangle native plants and pose a hazard to lake users.
In May of last year, Linz contractors sprayed a herbicide on the weed in the upper Kawarau. Further herbicide control work is planned for the river this month, if conditions allow.

The Lake Wakatipu Aquatic Weed Management Group has been established to oversee control work. Its members include Linz, the Otago Regional Council and Queenstown Lakes District Council, Ministry of Primary Industries, the Department of Conservation and commercial jet-boat operators KJet and Thunder Jet.

Ngai Tahu has been invited to join and the group will meet again next month.
An initial weed bed cordon extends 200m down the first major bend of the Kawarau River.

Queenstown Lakes harbourmaster Marty Black said commercial companies and jet boat drivers were required to keep clear of all weed beds until further notice.

"River cordons have been installed to make it obvious where the out-of-bounds area is.
"Drivers will also notice signage advising jet-boat drivers to switch off their engines before they head under the Kawarau Falls Bridge and into the lake.

Switching off the engine will help weed drop from the jet unit and limit the weed spreading.''

The council had closed the boat ramp below the bridge, accessed via State Highway 6.
Mr Black said human activities were the major factor in the spread of lagarosiphon, through the accidental transfer between water bodies on contaminated boating or fishing gear.