Councils target invasive aquatic weed

A chemical treatment that’s harmless to fish and plant life will be used to eradicate an invasive weed choking lakes in The Groynes Reserve.

The weed, lagrosiphon major, which forms dense mats of vegetation, has affected lakes at the popular recreation reserve near Northwood since 2014. 

There have been concerns this year that the annual Take a Kid Fishing day which is held at the The Groynes in October might have to be postponed because of the weed infestation.

A chemical treatment will be used to control an invasive weed at The Groynes. Photo: Marcus Girvan, Boffa Miskell

The Christchurch City Council commissioned a report in June from environmental consultants Boffa Miskell which recommended using the chemical Endothall as the preferred option to fully eradicate the aquatic weed in the long term.

It found the herbicide had no known negative impacts on native fish and plants.

Environment Canterbury has publicly notified the plan to carry out aquatic spraying between Tuesday, 14 August and Friday, 17 August. Fishing lakes at the reserve will be closed to public use from Thursday, 9 August when warning signs will be put up. 

The signs, at main entrances to the reserve, will instruct the public not to swim, gather food (including fish), or drink water from the lakes or Otukaikino Stream, near the dog exercise area, while the permitted application of a herbicide is underway.

Dogs should be kept out of the fishing lakes and the Otukaikino Stream.

The fishing lakes will re-open on 23 August when the signs will be taken down.

In July, the City Council and Environment Canterbury met with stakeholders Fish and Game, and the Take a Kid Fishing organiser, the Kids Fishing Charitable Trust, to discuss the Boffa Miskell report’s findings. All parties agreed chemical treatment was the most effective option. They also asked for the weed to be physically removed from the lakes.

The Council will arrange mechanical harvesting using a weed boat six weeks after the spraying when the chemical has had time to take effect.

Council Head Ranger Coastal and Plains Team Robbie Hewson says the aim is to physically clear the weed mass to make the lakes fishable in time for the Take a Kid Fishing event.

“We’ve worked really hard to find a solution that will allow a popular family fishing event to go ahead as planned, while also tackling an invasive weed that poses a real long-term threat to our waterways.”

Environment Canterbury's Regional Biosecurity team is working closely with Christchurch City Council staff on the project. The regional council holds the permission to use the herbicide Endothall to control aquatic pests and will carry out water quality monitoring before and after spraying to ensure all environmental requirements are met. 

ECan Principal Biosecurity Advisor Laurence Smith says by working together, the Councils are achieving benefits for the community, including the continued recreational use of the lakes at The Groynes, and protecting the aquatic environment from the impact of lagarosiphon at the lakes and natural biodiversity values downstream.