2023 Annual Lake Dunstan Aquatic Weed Management Meeting

On 9th May 2023, we attended the annual Lake Dunstan Aquatic Weed Management meeting to review the 2022/23 works and assist in the planning for 2023/24. Toitū Te Whenua LINZ, Boffa Miskell, NIWA, ORC, Otago Fish & Game, Clutha Fisheries Trust, and ourselves were in attendance.

Background

• Since 1988, Lagarosipon (or lakeweed) has been 'unmanageable in the Clutha'
• By 1996, Lake Dunstan was 'saturated'
• Lake Dunstan management is 'sustained control' as it is not possible to effectively eradicate due to reinvasion from upstream (Clutha/Wanaka & Kawarau/Queenstown)
• Efforts in Wanaka are focusing on eradication techniques
• Queenstown is on prevention and monitoring as currently little is found in Whakatipu
• Lake Dunstan - 12 High Amenity areas (boat ramps) and 14 High Recreational areas where monitoring and works occurs.

Overview of methods

• Hessian not possible in Lake Dunstan
  • For eradiation not sustained control
  • Costs approx. $60,000 per ha plus maintenance costs
• Hand cutting is no longer popular option for Lake Dunstan
  • Due to high cost
  • But an appropriate backup if spraying is not possible.
  • 1024hr this financial year
• Harvesters
  • Slow & expensive
  • Need a lot of shoreline space to dry out weeds before transporting.
• Diquat spraying remains the best option.            
  • Diquat herbicide can achieve control at scale and within a short timeframe.
  • Risk of potential cancellation due to sediment in the water from recent rain
  • More cost-effective when can go ahead.

2022/2023 review

• Boat-based diquat spraying at all High amenity areas
  • ​Staggered due to unfit weed conditions for spraying
• Hand-cutting at all High Amenity areas (boat ramps) and High Recreational areas, except Jacksons.
• Advertisement of works shared to key groups (to share with members/on Facebook), Otago Daily Times and Cromwell Bulletin to inform when works planned.
• Signage increased significantly around work area to increase public awareness.
  • E.g. 46 signs in place on 9th May when Bendigo Boat Ramp and Approaches, Cromwell Swimming Beach, Lowburn Boat Harbour, McNulty's Inlet, Northburn Ski Lane, and Pisa Moorings sprayed.
• LINZ, NIWA and Boffa Miskell have seen an increase in public complaints about the use of herbicides and also lack of 'visible' impact on weeds.

Looking ahead

• In previous years spraying has not occurred at Lowburn harbour/boat ramp due to the Kiwi Water park operating at planned times of works
  • There is now a clause in their permit that will ensure spraying can occur. Correct precautions to prevent people entering the water during 24hr standdown time will be put in place.
• NIWA investigating another herbicide, Endothall, but no plans to trail in Lake Dunstan in near future
• NIWA, LINZ and Boffa Miskell to work with stakeholders to increase public awareness of the need for diquat spraying and the safety around its use.
• Check, Clean, Dry programme by ORC is going through a revamp to update information, resources and public awareness of how to prevent the spread of weeds to other water bodies.
• Boffa Miskell to liaise with ORC to increase monitoring of sites and ensure events on waterbodies noted and included in works scheduling.
• Ten Year Management Plan for Lagarosiphon at Lake Dunstan: 2016 to 2025 up for review soon
  • Will connect with stakeholders to update the plan and get community inclusion.

Learn more about Lagarosiphon