How to use grass carp to help control aquatic weed growths in water bodies.

Grass carp (Ctenopharyngodon idella) were introduced to New Zealand in the 1960s to help control aquatic weed growths in water bodies. 

They are now artificially bred and grown on fish farms licensed by the Ministry for Primary Industries. They are very unlikely to reproduce in New Zealand rivers because of their very specific breeding requirements. 

If you want to use grass carp you must apply to move freshwater species. Possessing them without approval carries a penalty of $5000.  

Why use grass carp for weed control?

Grass carp are used for weed control because they: 

  • readily eat a wide range of weed species
  • are hardy and adaptable to a wide range of environmental conditions
  • can digest 60-70% of the nutrients in the aquatic plants they consume
  • are unlikely to breed in New Zealand
  • can be effective for a long period of time over large areas
  • can completely eradicate aquatic weeds.  

What do grass carp eat? 

Grass carp have preferences for certain aquatic plants and will eat their preferred species first, before eating less favoured plants. There are few aquatic plants in New Zealand that are unpalatable to grass carp when their preferred species are not present. 

Grass carp do not distinguish between native and introduced species of aquatic plants. Of the introduced oxygen weeds Hydrilla verticillata, Elodea canadensis and Lagarosiphon major are most preferred, but Ceratophyllum demersusm and Egeria densa are also palatable. Grass carp also eat native species such as Potomageton, Myriophyllum, Nitella and Chara

Azolla species, raupo (Typha orientalis) and water lilies (Nymphaea) are least preferred by grass carp. 

How much do they eat? 

The feeding behaviour of grass carp is strongly influenced by the water temperature: 

  • at 25C they will eat almost any weed species available
  • between 20-23C they feed intensively (100% of their body weight per day in young fish)
  • at 14C they will eat only preferred species
  • at about 10C they will almost cease to feed. 

Grass carp preferences 

Grass carp will tolerate a wide range of temperatures and oxygen concentrations. They prefer: 

  • shallow waters (under 3 m), but generally avoid water less than 30 cm in depth
  • fresh water – they have low salinity tolerances. 

Differences in the water chemistry between sites can also influence their preferences for certain plant species.  

Potential environmental effects 

The effects of grass carp grazing will vary depending on: 

  • whether the target plants are preferred or non-preferred species
  • the biomass of plants
  • the size and number of fish that are introduced
  • whether the water body has historically supported plant growth
  • the plant dependant species which occur at the site. 

Effects on trout 

  • Removal of plant species that grow on the shore or close to the shore may expose fingerling trout to predators (but it may also increase the habitat for juvenile trout which are free swimming)
  • If water transparency is decreased it may lead to a reduction in feeding ability
  • There can be competition between grass carp and trout – though this is generally limited because carp prefer warmer feeding waters than trout, and trout are carnivorous where carp are mainly herbivorous. 

Effects on other fish and aquatic life

  • Weed removal may affect fish species that require weed beds for spawning
  • Survival of smelt eggs in lakes may be increased by weed removal near shorelines and stream mouths
  • Other species (including native species) may be affected by an increase in predators or a reduction in invertebrate prey as a result of weed removal and loss of cover
  • No related effects have been observed on eel populations.  

Effects on waterfowl 

  • Mallard, grey duck, grey teal, shoveler, New Zealand scaup and black swan all feed on aquatic and emergent plants. The black swan is the most herbivorous and will be the most affected by weed removal
  • Complete weed removal would probably force these species to forage elsewhere. 

Effects on water quality 

  • There may be a reduction in water clarity if aquatic plant density is greatly reduced or eliminated. 

How many grass carp should I use? 

Use of grass carp is renowned for resulting in the total removal of aquatic plants. If you are planning to use grass carp for weed control you will need to assess whether complete plant removal is acceptable. 

Stocking rates vary considerably and calculating how many grass carp to stock will depend on a range of factors such as the type of water body, the aquatic weed species, water temperature etc. 

The rates given below are based on a standard fish size of 250 mm fork length (measured from the tip of the snout to the fork in the tail fin). Vegetated hectare is the area of the water body that has aquatic plants growing in it. These rates are a guide only. 

For aquatic plant removal: 

  • 3-5 years – 20-30 grass carp per vegetated hectare.
  • 2 years – 50-100 grass carp per vegetated hectare.

Note: A higher stocking rate is normally used in agricultural drains than lakes and ponds as drains usually have more vigorous weed growth. 

Measures for containing grass carp 

It is extremely important to contain grass carp in the water body you release them to.

Security measures are a balance between allowing fish passage for native species and preventing the escape of grass carp. Generally a screen mesh size of 35 mm is used to contain grass carp of 250 mm (fork length). Approval is not normally given for fish smaller than 250 mm. 

Mesh screens of 35 mm do not usually interfere with the migration of native fish. 

See fish passage management in New Zealand for more information on fish passage management for our native fish. 

Email the DOC Hamilton Office for more information on the measures you will need to take.  

Applying to use grass carp 

See apply to move freshwater species for more information. 

More information 

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