The Auckland Regional Council is introducing grass carp into Lake Wainamu, near Bethells Beach in a bid to rid the lake of an exotic weed.
The invasive aquatic weed egeria (oxygen weed) has formed large mats in the lake and 270 grass carp are being released this week to eradicate it.
Egeria smothers native aquatic plants, disrupts recreational activities and can pose a risk to swimmers if they get tangled in the weed.
Grass carp, a herbivorous fish, have been widely used for weed control in New Zealand but are unable to breed in the wild.
"We investigated using aquatic herbicides and mechanical removal of the weed but using grass carp is more cost effective and environmentally sustainable," says Graham Surrey, ARC Freshwater Science Project Leader.
"We expect that the grass carp will eradicate egeria from Lake Wainamu within three to four years, at which point we will remove them from the lake and allow the native aquatic vegetation to re-establish."
A barrier net has been installed across the outflow of the lake to stop the fish from escaping. They have also been individually tagged so that ARC staff can monitor the fish population and mortality rates.
Egeria has been able to thrive partly due to the improved water quality that has resulted from the ARC removing more than 10,000 exotic fish from the lake since 2004. Both the egeria and the pest fish were introduced illegally.
"We have made a lot of improvements to Lake Wainamu over the years with initiatives such as our annual pest fish removal programme, but we need to continue the hard work," says Sandra Coney, Chair of the Parks and Heritage Committee.
"We want to keep the lake enjoyable, safe and clean for the public. Everyone can play a part by cleaning equipment before and after they use it in the lake, as this kind of weed is often spread by boats, fishing gear or machinery," she says.
The ARC has consulted with local landowners, Auckland and Waikato Fish and Game Council, Ministry of Fisheries and iwi and gained permission from the Department of Conservation (DoC) to introduce the fish.
Lake Wainamu is one of seven lakes regularly monitored as part of ARC's state of the environment monitoring programme and also one of the ‘special eleven' High Conservation Value waterbodies recognised in the Auckland Regional Pest Management Strategy. The ARC is working with other groups to protect these lakes and scientists sample Lake Wainamu six times a year.