Species name: Ctenopharyngodon idella (Valenciennes, 1844)
Synonyms: Pristiodon siemionovii, Sarcocheilichthys teretiusculus, Ctenopharyngodon laticeps, Leuciscus tschiliensis, Leuciscus idellus, Leuciscus idella, Ctenopharyngodon idellos, Ctenopharygodon idella
Common names: Grass Carp, White Amur
Etymology: Cteno = comb, pharyngodon = teeth (i.e. comb like cutting surface of the pharyngeal teeth). Idella = distinctive.
Description: Body is cylindrical and olive to brassy green in colour while the ventral surface is silver to white below. Adults have been recorded at a maximum size of 45kg and 150cm in length. They have been confirmed to live for as long as 21 years and anecdotally may live longer than this again though the average life expectancy is around 15 years.
Distribution: originally from Asia found from China to Eastern Siberia it has now been introduced to many countries around the globe for the purpose of biological vegetative control and aquaculture.
A Brief History within New Zealand: Grass Carp were first imported into New Zealand in the early 1960’s as a biological control method for the rapidly spreading aquatic pest weeds within New Zealand by several government agencies. Early and repeated spawning failures within the lab meant that trials were hampered by a lack of production and only a few sites were tested until the 1980’s. Since 1992, with the disbandment of the Fisheries Research Division, management of Grass Carp in New Zealand has fallen on private companies, which is where Aquatic Weed Management comes into the picture.
Ecology: Adults tend to live in lakes, ponds and slow moving backwaters of large Rivers. The species tends to prefer slow moving or standing waters with submerged vegetation.
Biology: The species is tolerant of a wide range of temperatures from 0˚C to 38˚C though feeding is severely depressed at the lower end of the scale. It can tolerate water salinities up to 15ppt and oxygen levels as low as 0.5mg/l for short periods of time. Globally they are one of the most important aquaculture species being widely cultured and eaten.
Grass Carp will eat a number of aquatic weed species both native and exotic, generally feeding on submerged aquatic plants they will also eat terrestrial grasses when they become submerged. Typically they will have a preference order in terms of palatability eating the most palatable weed species first and foremost, moving on to the less palatable weed species once the most palatable species becomes scarce.
In the New Zealand context they are an ideal species as they will consume the vast majority of the invasive aquatic weed species found throughout the country. As a rule of thumb any Pond, Lake or River containing large volumes of aquatic weed is probably polluted with one of New Zealand’s invasive aquatic weeds as the New Zealand native species do not dominate ecosystems to the same extent that the exotic aquatic weed species do.
Life Cycle and Reproduction: Spawns across gravel bottoms of river beds in the wild. Eggs are pelagic (floating within the water column) drifting down stream to hatch, taking up to 3 days to do so. The larvae use the floodplains of their native region as a nursery prior to moving back up stream.
This reproductive strategy/ method, is the primary reason why the species is not capable of breeding within New Zealand. The species is not known to spawn in lentic habitats (still or not moving such as lakes) and require flowing water. Most of New Zealand’s Rivers are too short, while where the Rivers are long enough there are too many barriers such as hydrodams, a lack of floodplains and the water never gets warm enough for a long enough duration to trigger spawning.
Source: Our Grass Carp are cultured in a Biosecure facility where they are cultured to the acceptable release size of 250mm. This size has been selected as being the most appropriate for release as the fish can be easily contained within the release site using standard stormwater covers and additionally means that if smaller fish are ever found (however unlikely this may be) then a natural breeding event may have occurred.
Given the species presence within New Zealand is nearing 50 years and fish were first released into the wild around 40 years ago this is highly unlikely.
Use: Grass Carp and Silver Carp as biological control agents are an effective method of control for Aquatic Pest Weeds and Nuisance Algae’s within New Zealand. In fact they are frequently a more economic method for control than any of the alternatives such as manual harvesting or the use of chemical herbicides. In addition the on-going cost of managing a water body where Grass Carp have been stocked is negligible and when compared to the use of mechanical harvesters, suction dredging or chemical herbicides where repetitive applications or removals are required (typically every summer) comes out economically favourable for the use of Grass Carp.