Grass Carp for Biocontrol and Food

Grass carp were first brought to New Zealand in the 1960’s because of their potential to control the growth of aquatic plants.

It’s claimed that they are one of the most cost-effective and environmentally sustainable options for eradicating aquatic weeds. Grass carp only eat plants, (eating up to their own body weight in plant material each day in summer). They are unable to breed naturally in New Zealand waters – addressing the fears of a takeover by non-native species. They have already been used to successfully manage aquatic weeds in many other NZ lakes.

The fish are being bred at Mahurangi Technical Institute’s facility in Warkworth. In recent years MTI’s director Paul Decker has formed a partnership with a local aquaculture business, NZ Waterways Restoration. Initially that partnership was for the supply of carp for weed control work but nowadays there’s a move to introduce the fish as a food source.

Grass carp, or white amur (Ctenopharyngodon idella) are originally from China. They are a different species from the common carp. They are called grass carp for the obvious reason that they feed on vegetation. They are not to be confused with koi carp which are a pest. The stock comes from fish bred from the original 12 to 15 imports from the 1960’s.

Grass carp can reach about 18kg. Silver carp, which eat only algae, can weigh slightly more. When grass carp have escaped, as they did in the Waikato River in the 1980’s, they failed to breed, Mr Decker says. That’s backed by the National Institute of Water and Atmospheric Research.

Grass carp do not breed easily in ponds, in their natural environment preferring swift moving water. Paul says they are effectively sterile in NZ conditions. This is where the MTI comes in. They have the technology to breed the fish in captivity.

This Warkworth/Orewa based company raises grass carp using the breeding technology developed at MTI. The company is run by Blair and Gray Jamieson.

Their business has been largely about controlling aquatic pest weeds, toxic algaes, and pest fish. Using grass carp was one of the tools in their arsenal.

NZWR also provides fish removal services, both for fish that have become invasive or for when upgrades are being made to a waterway. By removing pest fish such as koi & bullhead catfish they hope to to create optimum conditions for the re-establishment of New Zealand native flora and fauna.

Since August 2012 they have starting supplying NZ Land Based Acquaculture, a new grass carp venture in Nelson, based in Hope. While carp has traditionally been used for clearing weed in NZ , this business is about food and the aim is that many of the fish grown in the Nelson venture will end up in supermarkets.

Grass carp not only grow quickly but have a low requirement for dietary protein. They can be produced at low cost by feeding them with aquatic weeds, terrestrial grasses and by-products from grain processing and vegetable oil extraction.

It is a large fish without fine inter-muscular bones and so is acceptable to consumers in many countries and it very likely has good potential for development.

Farming grass carp offers one key advantage over other types of fish farming in NZ. Salmon and other fish farms rely on fish meal and fish oil processed from mainly wild caught fish as the main food source. Grass carp just need vegetation. (This doesn’t apparently have any impact on the level of Omega 3)

While many of the Jamiesons’ clients are councils, they also undertake clearing of water bodies for private owners. That includes South Head’s Lake Kereta which in 2008 was 75 per cent covered in hornwort weed up to 2m high. A recent NIWA report confirms 99.9 per cent of the weed is gone.

Gray Jamieson says there’s also growing interest in carp for cleaning up storage dams on dairy farms and irrigation ponds where weed has a big impact on efficiency.

Grass and silver carp come under the Conservation Department’s permitting system with permits generally costing up to $2000.

Although DOC does have some concerns about grass carp and the possible effect it might have on competition with native species, Forest and Bird is quoted as having no major issues with the farming of these fish.