Deemed an "undesirable fish species" in New Zealand, grass carp are a herbivorous fish native to Asia and have been used in parts of the country to control aquatic weed growth. They commonly grow to more than 50cm long and can weigh 10kg.
Grass carp are artificially bred and grown on fish farms licensed by the Ministry for Primary Industries (MPI), with a permit required to move them and the provision they be contained in the body of water they have been released in.
The species has been released in a wide range of locations in the North Island, but its distribution is more limited in the South.
Otago Fish & Game Central Otago operations manager David Priest said it was suspected the carp came from authorised releases in contained ponds near Queenstown.
The fish may have escaped during spring floods into Lake Hayes and swam down the Kawarau River into Lake Dunstan.
"Thousands of anglers have been enjoying fishing for trout these summer holidays and harvesting wild kai from our southern lakes fisheries," Mr Priest said.
"The last thing we want is a disease entering highly valued fisheries."
In New Zealand, grass carp could not readily breed outside of captivity, so there was no real risk of establishing a self-sustaining population in the wild, Mr Priest said.
"We appreciate the angler notifying us of this unexpected catch. Any further sightings of grass carp in Otago waterways should be reported."
One of the major concerns over the use of grass carp in New Zealand was their potential effect on trout fisheries in lakes, Mr Priest said.
Grass carp released into contained waters had the potential to host infection and disease, which could then be carried and transferred to other waters or species, he said.
The MPI most recently authorised the release of 100 grass carp to artificial ornamental ponds near Queenstown in February 2023.
They have been used for aquatic weed control in a select number of contained ponds in the area since 2013.
