Grass Carp
Grass carp (Ctenopharyngodon idella)
Grass carp were first brought to New Zealand in the 1960s because of their potential to control the growth of aquatic plants. Unlike the other introduced fish brought to New Zealand, the potential value and impacts of grass carp were investigated in secure facilities prior to their use in field trials. In 1984, some fish escaped from a field trial in the lower Waikato River. Although these fish grew to maturity, grass carp have extremely specific spawning and rearing requirements, and these fish never established a wild population.
There has been much controversy surrounding the use of grass carp as a weed control agent in New Zealand. The uncertainty of whether they could establish wild populations led to the requirement that only sterile fish could be used in the wild. This requirement has since been relaxed, and it would appear that their use in suitable areas will continue to expand. So far, their main value in New Zealand has been to eradicate problem weed species from lakes, thus allowing native plant communities to re-establish. Although this may mean they have a role to play in lake restoration, their use for weed control in drains is still being investigated. Today, stocks of grass carp are maintained for breeding at a private hatchery north of Auckland.
Grass carp are a handsome, robust fish and commonly exceed 500 mm in length. Their backs are a dark bronze colour, but this gradually fades to silver on the belly. The margins of their large scales are outlined in a darker colour, giving them a crosshatched appearance. Grass carp have no barbels, but can be distinguished from goldfish by the short dorsal fin with only 7–9 rays.