Aquatic weed hornwort in Lake Heather. Photo: Rohan Wells, NIWA.
The Northland Regional Council is working closely with the Bushlands Trust and the Department of Conservation to eradicate the aquatic weeds hornwort and egeria (an oxygen weed) from the eight hectare lake.
Peter Wiessing, the Council’s Kaitaia Area Manager, says the two species already form large, dense mats in the 5.6 metre deep lake.
“These weeds are smothering native plants and affecting an important habitat for fish, plants, birds and other species. The release of these 400 fish will cost the Council about $17,000 but is the most cost-effective and environmentally sustainable option to eradicate these weeds.”
He says the 25 centimetre long carp – which all going well will be trucked up from a Warkworth supplier and released into the lake on Tuesday 29 June – are unable to breed in New Zealand waters, eat only plants and have a good track record of aquatic pest plant control in Northland.
Roughly 12 months after a similar introduction of grass carp to Lake Swan, on the North Kaipara Head last year, most of the egeria and about 40% of the hornwort has already been removed.
Mr Wiessing says places like Lakes Heather and Swan are important habitats for a vide variety of fish, plant, bird and invertebrate species and Northland has some the best examples of these ecosystems in New Zealand.
“However, the Regional Council, the Bushlands Trust, and the local community have become increasingly concerned about the risk of hornwort and egeria spreading from Lake Heather to other lakes in the area.”
Aside from that threat, Mr Wiessing says the Regional Council is also concerned that unchecked, the scale of the hornwort and egeria problem in Lake Heather could lead to the eventual collapse of other plants living in the lake.
This in turn could provide ideal growth conditions for algae, which could relatively quickly dominate the lake, affecting water quality for both recreational and stock use as well as adversely affecting other aquatic life.
He says representatives from Bushlands Trust, the local iwi, community and school are all expected to be present during the upcoming release, as well as staff from the Regional Council and Department of Conservation.
“If the eradication of hornwort and egeria is successful, we intend to remove the fish from the lake in several years’ time, allowing native plants to re-establish.”
Mr Wiessing says Northland has a large number of lakes and they are vulnerable to the impacts of invasive aquatic pests, which can establish and spread rapidly.
“These weeds will grow from small plant fragments and unfortunately the most common way they’re spread is through people and activities like boating and fishing.”
“We encourage all users of freshwater areas to check, clean and dry all their gear to help stop the spread of aquatic pests. It’s important to be alert for aquatic life that looks different and report it immediately to the Regional Council or MAF Biosecurity NZ,” he says.
Grass carp release Lake Swan, May 2009. Photo: David Tate, Mahurangi Technical Institute.