Lakes infested with smothering weed

An invasive aquatic weed now has a grip on two South Westland lakes and West Coast Regional Council staff are warning it could spread to more.

The freshwater pest plant Lagarosiphon, or oxygen weed, is now well established in Lake Ianthe and Lake Paringa, more than seven years after it was first found there.

The weed is spread by boats, trailers and fishing nets and its stems are up to 5m long.

It not only smothers native aquatic plants, but restricts water flows and keeps fish from their spawning grounds.

This week’s council meeting heard the weed is also present in the Kapitia reservoir near Kumara.

Council biosecurity co-ordinator Taylor Blyth reported the Lagarosiphon infestations at Lake Paringa and Lake Ianthe were now widespread.

"The South Westland Freshwater Management Unit is citing it as a major issue in the area — Department of Conservation has undertaken some control in these lakes, however there has been no response to the Kapitia incursion."

The council funds the surveillance of some West Coast lakes to the tune of $10,000 a year plus staff time, with co-funding of $15,000 from Doc and $5000 from Trustpower.

However, it has no budget for eradication.

The weed has made stealthy gains on the Coast since 2004 when a Niwa study found it was common in ornamental ponds but natural waterbodies were largely free of aquatic pest plants.

Niwa recommended an annual surveillance programme — but the suggestion was not taken up.

In 2010, Lagarosiphon was discovered by chance, well established in Lake Paringa.

Again, Niwa recommended annual surveys.

However, no surveillance began until 2015 — two years after the second Lagarosiphon infestation was discovered, again by chance, at Lake Ianthe.

The council then contracted Niwa to survey eight lakes under an Envirolink grant, and found no new incursions.

In 2019, the council and Doc co-funded a specialist dive team to survey eight lakes, and the pest plant was detected in the Kapitia Reservoir.

Late last year, the council and Doc re-evaluated the problem and proposed surveillance for 15 lakes.

This year’s budget allowed for nine lakes to be surveyed, and scientists are still evaluating the DNA plant results collected in February from lakes Brunner, Kaniere, Mapourika, Moeraki, Ianthe, Wahapo, Pratt, Matheson and Paringa.

Council operations director Randal Beal said Doc had done some weed control in the affected lakes, but not in the Kapitia Reservoir.

"We don’t currently have money for eradication work, but the South Westland freshwater group is making the issue a priority, and if it comes through in their recommendations to council, we will have to find a budget for it," Mr Beal said.

Councillors noted that boaties could help stop the spread of the pest by entering lakes up rivers downstream where possible, so that any weed on their boats or trailers would be sent seawards to be killed by salt water.

- By Lois Williams, Local Democracy Reporter

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