.jpg)
NRC Biodiversity Officer Lisa Forester surveys hornwort beneath Lake Swan.
Don McKenzie, Biosecurity Senior Programme Manager for the Northland Regional Council, says the Council hopes to eradicate the aquatic weeds hornwort and egeria (an oxygen weed) from the 17.4 hectare lake, also known as Lake Swan, on the North Kaipara Head.
Mr McKenzie says the two species already form large, dense mats in the 5.5 metre deep lake which are smothering native plants and affecting an important habitat for fish, plants, birds and other species.
“The Regional Council, representatives from Te Uri o Hau, local landowners and community members have also become increasingly concerned about the risk of hornwort and egeria spreading to other pristine dune lakes in the area.”
He says the Thursday 14 May planned release of 850 grass carp is seen as the most cost-effective and environmentally sustainable option to eradicate the two aquatic weeds.
“Grass carp only eat plants, are unable to breed in New Zealand waters and have already been used to successfully manage aquatic weeds in other New Zealand lakes.”
Mr McKenzie says dune lakes are especially important as a habitat for nationally threatened and regionally significant species of fish, plants, birds, and invertebrates and Northland has some the best examples of these ecosystems in the country.
He says aside from the threat to other lakes, the Regional Council is also concerned the existing dense growth of hornwort and egeria in Lake Roto-otuauru may cause the eventual collapse of other underwater plants in the lake. This in turn could provide ideal growth conditions for algae, which could relatively quickly dominate the lake and also adversely affect other aquatic life.
The roughly $50,000 potential solution – funded by the Regional Council - will see the grass carp, typically about 25 centimetres long, trucked up from Warkworth to the lake. Representatives from Te Uri o Hau, the community and school are expected to be present during the release.
“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 McKenzie says Northland has a large number of lakes and they are vulnerable to the impacts of invasive aquatic pests.
“The most common way for aquatic weeds to spread is by human activities and these weeds will grow from small plant fragments. We encourage all users of freshwater areas to check, clean and dry all of their gear to help stop the spread of aquatic pests.”