“Space invader” fast growing lakeweed (lagarosiphon major) is throughout Lake Dunstan and stakeholders are taking a ‘sustained control approach’ aiming to stop the spread of the pest weed. 

The Central App asked Toitū Te Whenua (Land Information New Zealand, LINZ) the Government agency responsible for management of the lake beds and rivers, how management of lakeweed has been going.

Lagarosiphon control in Otago’s lakes and rivers is an “enormous challenge” according to a biosecurity leader. PHOTO: NIWA 

LINZ leader biosecurity Tracey Burton said ahead of summer, divers were hand-cutting weeds on the lakebed of Lake Dunstan. 

“A second round of weed control is scheduled for February to April of this year. 

“Again, we will have divers hand-cutting the weed in February and March, and will follow this up by boat-based and aerial herbicide treatment in April.

“The current strategy for Lake Dunstan is to prevent the spread of weed to lakes outside of Lake Dunstan and target high use areas such as boat ramps and swimming areas.” 

When asked if the weed was any worse this year an Otago Regional Council (ORC) spokesperson said it seemed to be on a par with other years.

Tracey agreed and said feedback from contractors was that in areas of control work and priority boat ramps and recreational sites, the amount of lakeweed is very similar to last year.

“Lake Dunstan is particularly challenging to control lakeweed as it is downstream of other waterways (for example Lake Wānaka) . . . infested with lagarosiphon. This, and the large amount of weed already present . . . means complete eradication is not achievable.

“Control of lagarosiphon in Otago’s lakes and rivers is an enormous challenge. Lagarosiphon is present in parts of Lake Wānaka, in the upper Clutha River/Mata-Au and in the Kawerau River.”

Weed management is paid for by LINZ, Contact Energy and ORC. The annual budget varies depending on identified priorities and what type of control work is being undertaken. 

During the 2022/23 financial year the three agencies spent $190,000 managing lagarosiphon in Lake Dunstan. For the current financial year (2023/24) there is $173,000 for this work.  

LINZ and the Lake Wakatipu Aquatic Weed Management Group have recently introduced prevention measures at the Frankton marina to help ensure the Lake Waakatipu remains weed-free and avoids an infestation similar to Lake Dunstan.

“Unlike Lake Dunstan, Lake Wakatipu has no known upstream sources of lagarosiphon. This means, any lagarosiphon present would have been spread there by people moving between waterways.”

Water-users who check, clean and dry wet equipment between water bodies play a major role in reducing the spread of lagarosiphon.

Tracey said lakeweed forms thick, dense weed beds that can grow up to five metres tall. 

“They can smother native aquatic plants, impede water flows, and impact on recreational activities like swimming, water sports and boating.

 “It grows incredibly quickly and can invade new waterways very easily – a fragment the size of your fingernail is all it needs to spread.”

LINZ partners with other key stakeholders in the district to manage lakeweed.

Control work is in line with the annual control programme set out by the Lake Dunstan Aquatic Weed Management Group (LDAWMG). 

LDAWMG has representatives from Otago Regional Council, Contact Energy, Central Otago District Council, Kāi Tahu (Hokonui Runanga), Cromwell and Districts Community Trust, Lake Dunstan Charitable Trust, Otago Fish and Game Council, and Clutha Fisheries Trust and LINZ. 

Read more: ORC: Lake Dunstan boatie behaviour generally good

Information on Check, Clean, Dry campaign here: Check, Clean, Dry: preventing didymo and other pests | NZ Government (mpi.govt.nz)