A trial using carp to control aquatic weeds is proving a success.

The trial funded by the Ministry of Primary Industries is showing grass carp could be used to eradicate early infestations of aquatic weeds – preventing damage to other plant species in the water.

Carp are being trialled as a aquatic weed control method.

MPI senior science advisor Andrew Bell says using grass carp to remove the invasive weeds from waterways is an effective biological control.

Andrew says the recent successful trial was carried out at Waikato's Lake Karapiro treating the widespread pest weed hornwort.

'This novel approach was so effective that MPI now has it in the toolbox for use should the ‘unwanted organism' hornwort be found in South Island lakes.

'Local iwi supported the project and are interested in the results and the potential to use grass carp as a biological control tool. They recognise that grass carp may have a role not only in biosecurity responses when hornwort occurs in new locations, but also as a potential invasive weed management tool in hydro dams and other sites in their areas.”

Since being introduced to New Zealand in the late 1960s from Malaysia, grass carp have also been successfully used for the control of large infestations of other invasive weeds.

They are unable to breed in the wild in New Zealand, unlike koi carp, which breed well and have become a pest species.

The study was carried out for MPI by National Institute of Water and Atmospheric Research (NIWA) scientists.

Principal scientist of aquatic plants Dr John Clayton says as part of the study, the team built six large enclosures and placed them within hornwort beds in Lake Karapiro.

'The cages contained different numbers of fish in order to establish the most effective stocking rate required for rapid removal of nuisance weed within the enclosed area.

'Three to five fish per enclosure were found to be effective and very fast, with weed in the enclosed areas removed in nine weeks.”