Environment

NRC Biodiversity Officer Lisa Forester surveys Hornwort beneath Lake Swan. Photo: NIWA More than 800 grass carp were released into Kaipara’s Lake Roto-otuauru (also known as lake Swan) in a bid to wipe out two invasive weeds threatening some of the nation’s most precious dune lakes.

It is hoped that the carp will eradicate the aquatic weeds hornwort and egeria (an oxygen weed) from the 17.4 hectare lake.

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.

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.  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.

Watch video of grass carp release

Mahurangi Technical Institute filmed the release of the carp and created a video which can be viewed on YouTube.

Read media release:
'Grass carp to be released into Kaipara lake' (8 May 2009)

Acknowledgements

We would like to acknowledge the support of the Pouto community, Pouto School, Te Uri o Hau, the Bishop family, and Mahurangi Technical Institute.