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Gray and Blair Jamieson say their carp can be confused with koi carp, which are deemed a pest species, while grass carp are classed as a restricted species, meaning any release needs consent from the Department of Conservation.
Under the Freshwater Fisheries Regulations 1987, koi carp are considered a ''noxious fish'', therefore making it illegal to possess, breed or release them.
They are considered an ''unwanted organism'' under the Biosecurity Act 1993 and it is illegal to release, spread, sell or breed them.
Both fish belong to the Cyprinidae family and are indigenous to Asian river systems.
What are the differences between the two?
According to the National Institute of Atmospheric Research (Niwa), grass carp are large, robust fish with large scales, commonly exceed 500mm in length and 10kg in weight, have an almost cylindrical body with flat head and round abdomen, and can live for 15 to 20 years. They are grey on top, with a yellowy-white belly and silvery green sides with lighter coloured fins.
Koi can also grow to great sizes. New Zealand stocks are derived from brightly coloured ornamental Japanese koi and can exhibit a calico pattern of black, red, orange, gold and white blotches.
When looking for food, plants and animals, koi churn up the beds of waterways, causing environmental deterioration and turbidity.
Grass carp, as the name suggests, are strict herbivores.
A Ministry for Primary Industries-funded study, carried out by Niwa, has shown grass carp can be used as an effective means of controlling invasive plant species in waterways, when the fish can be contained to the site.