Winning the battle against lagarosiphon

The health of Lake Wanaka is continuing to improve and its recreational opportunities are increasing, thanks to a programme for controlling the aquatic weed lagarosiphon.

A recent lake inspection showed progress containing the weed was well ahead of schedule.

Organised by the Lagarosiphon Management Committee, which was formed in 2004, the inspection included representatives from the Guardians of Lake Wanaka, Otago Regional Council, Department of Conservation, Queenstown Lakes District Council, Land Information New Zealand (Linz) and Niwa, which provides scientific advice and input.

The committee developed a 10-year management plan - now in its ninth year - to control the weed's spread and protect the lake's native aquatic biodiversity, using a combination of suction dredging, hand weeding and the herbicide diquat.

ORC environmental monitoring and operations director Jeff Donaldson said the lagarosiphon containment line had now been moved further south to extend from the southern tip of the peninsula to the southern end of Bishops Bay, which demonstrated the good results of the control programme to date.

''There's now improved access to parts of the lake, for example at Paddock Bay and Bishops Bay, which were previously restricted by dense weed beds. There's also been some regrowth of native aquatic plants.

''It means the committee can direct weed control efforts on to areas ... closer to Wanaka township.''

However, Guardians of Lake Wanaka's Peter Hook said continuing control operations in Lake Wanaka was crucial.

''If lagarosiphon's left untreated or proper precautions aren't taken when using the lake, it will tarnish the lake's natural heritage values, limit access for future generations and may result in spread to other New Zealand lakes such as Lake Wakatipu, which is free of lagarosiphon.''

Also known as South African oxygen weed, lagarosiphon can establish quickly in waterways if left untreated, choking the waterway and smothering native aquatic plant communities. It also poses safety problems for lake users by affecting underwater visibility, creating a hazard for swimmers, water skiing and fishermen.

The public can help stop the spread by visually checking boats or other craft for weed fragments after lake use and hosing down and drying all equipment such as water skis, biscuits or fishing gear.