Council applies to release grass carp in Rangitikei waters

Grass carp eat most water plants, live 15 to 20 years and can grow to 30kg in size. Photo / supplied

Introducing two Asian carp species into four council water bodies could be a low-cost way to clear nuisance weeds and prevent algal blooms, Rangitīkei District Council strategy and planning manager Blair Jamieson says.

The council has applied to the Conservation Minister to release grass and silver carp into Dudding Lake, the two Tutaenui reservoirs and Taihape Oxidation Pond.

If the minister agrees, a working group that includes Rangitīkei District Council, Horizons Regional Council, Niwa and Cawthron Institute staff will decide whether the releases are made.

Grass carp eat most types of water weed. Nuisance weed can also be limited by mechanical harvesting and herbicide spraying. But both have to be done regularly and can leave sediment and rotting vegetation in the water.

Silver carp are filter feeders that eat phytoplankton - tiny floating plants. These include the cyanobacteria/blue-green algae that cause toxic algal blooms at Lake Dudding in early summer.

Both carp species are from the long, warm rivers of the equator. Grass carp are not known to breed in New Zealand, and silver carp could only possibly do so in the Waikato River.

Another carp species, koi carp, is a pest fish that degrades water by creating sediment and reducing oxygen. It was probably introduced accidentally and authorities are trying to get rid of it.

Jamieson spent six years working for New Zealand Waterways Restoration, a business that deals with carp. His father owns it and it's the only source of grass and silver carp in New Zealand.

One third of its work has been removing koi carp, he said, and 20 tonnes can be taken from a lake in just one weekend.

Some say removing carp completely is difficult. But Jamieson says it can be done in a long process involving netting, then electro-fishing, possibly baiting and finally spearing.

Grass and silver carp are used to improve water bodies at 200 sites across New Zealand, including Auckland stormwater ponds and Hawke's Bay lakes.

Proposed stocking rates here would be 32 grass carp per hectare for Lake Dudding, one grass and 20 silver carp, initially in a cage, for the Taihape pond, and a ratio of 15 grass to 50 and 75 silver carp for the water supply reservoirs.

All would be confined to their water bodies by mesh fences.

Carp are the most commonly eaten freshwater fish in the world. But Jamieson said their flavour depends on the quality of the water they come from. They are not farmed for food here.

Neither grass nor silver carp eat other fish or insects.

If they are released, Jamieson will be able to monitor their progress, and said his expertise would save the council money. The fish would be bought from his father's business.

They would be a minimum of 45cm long - too big to be taken by any predator.