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Management plan to control exotic pest aquatic weeds

As noted in the attached article, Rangitīkei District Council (RDC) has applied to the Department of Conservation for a 26zm3 Conservation Act approval to potentially release grass carp (aka white amur) into Lake Dudding as part of an OVERALL MANAGEMENT PLAN that seeks to address the issues of nutrient loading and exotic pest aquatic weeds. The approval has been sought by Council to contribute to an overall action plan, presently in development between RDC, NIWA, Cawthron, Horizons and other national experts.

In order to avoid any confusion and species misidentification, the following must be noted:

· The application is NOT for KOI CARP! There is often confusion, but please be aware that grass carp are NOT a pest species, and are regularly used as a biological control across the whole country with an approval process managed by DOC. DOC will not be approving a pest fish species for release; there are laws prohibiting the release of such species. Additionally, grass carp were approved by cabinet for biological control purposes in 1991.

· Grass carp are herbivores that only eat aquatic vegetation and CANNOT breed in New Zealand. There is over 35 years of New Zealand based research on this, noting that they are one of the most carefully introduced species into NZ; due to many mistakes being made in relation to other species in the past.

· Grass carps anatomical structure and feeding behaviour does not contribute to the stirring up of bottom sediments. It is the noxious KOI CARP that does this by vacuuming up sediment with their large extendable mouths - greatly increasing water turbidity.

· No evidence has been produced that grass carp have significant harmful effects on either the native biota or the introduced salmonoids (char, trout, and salmon). All studies suggest that the impacts of grass carp are far less damaging than weed removal either by mechanical draglines or by herbicides. Fisheries research has shown that native fish populations increase where grass carp present. This related to the opening up of space in the littoral zone of lakes, and to changes in food chain bases from weed bed faunas to benthic faunas. Additionally, the native freshwater mussel populations will be given an increase in habitat.

Fisheries Division of MPI (MFISH) produced the following position in 2011 from assessing the 35+ years of NZ research. It is from this, and the known conservation value of the site that an approval has been sought:

"From the results we have seen on the use of grass carp to control nuisance aquatic plants over many years, their effect has been mostly beneficial and we are not aware of any adverse effects resulting from the use of grass carp provided stocking levels are not excessive. While potential risks need to be assessed, we consider that there are benefits in using grass carp compared to other methods and these benefits should also be considered when processing applications. Other aquatic weed control methods such as the use of herbicides and mechanical removal of nuisance weeds result in considerable loss of aquatic life and can result in degradation of the habitat through rotting vegetation and disturbance of sediments. These methods do not require approvals from any agencies, provide temporary respite and require frequent applications over the warmer months. MFISH considers grass carp should be the preferred method for aquatic weed control, for sites where aquatic weed control is required and: the extensive growth of aquatic weeds interferes with the main functions of the site (eg stormwater management, water sports), carp can be contained within the site, and the water quality is suitable for survival of carp."

If you have any questions please feel free to email me at and I will do my best to provide you with information or put you in touch with someone who can.

Read the Whanganui Chronicle article here

Grass Carp

Grass Carp