Species name: Hypophthalmichthys molitrix
Common names: Silver Carp, Silver Amur
Etymology: Hypo = under, opthalmos = eye, ichthys = fish (i.e. under the eye). Molitrix = approximately grinder ( in reference to the pharyngeal grinding apparatus ).
Description: Body is laterally compressed and deep, with a ventral keel extending from isthmus to anus. It has a large head with a small eye positioned on the ventral (bottom) side of the head. The body is Olive to Silver in colour it has no barbels. Adults have been recorded at a maximum length of 105 cm and may reach weights of 50 kg though fish larger than 25kg are very uncommon. They may live for many years and certainly in New Zealand, have been recorded living as long as 20 years.
Originally native to most Pacific Drainages from Asia found from China to Eastern Siberia it has now been introduced to many countries around the globe for the purpose of biological control of algal blooms and aquaculture.
A brief history within New Zealand:
Silver Carp were first imported into New Zealand in 1969 by the Hawke’s Bay Acclimatisation Society from Malacca. The first import consisted of 35 fish which were placed in quarantine until cleared. A second shipment occurred in 1970 where 1000 individuals were imported of which only 94 Survived. All of New Zealand’s stocks are derived from these two imports consisting of 129 Individuals in total, which were then bred from. In September of 1978 field trials began with this species in Lake Orakai to marked success.
Adults tend to live in Large Rivers with marked fluctuations in water levels. They are stocked into many other Rivers and other still bodies of Water such as Lakes, Ponds and Dams where they will live but have never been recorded breeding.
The species is tolerant of a wide range of temperatures from 6˚C to 30˚C though feeding is severely depressed at the lower end of the scale. It can tolerate water salinities up to 7ppt at the most though they are relatively sensitive to low oxygen levels.
Silver Carp are known to feed on a wide range of phytoplankton and zooplankton species. They are ‘typical’ planktivores in this regard, utilising their gill rakers to filter food particles from the water column. They are known to feed on the following ‘algal’ species; diatoms, dinoflagellates, chrysophytes, xanthophytes, some green alga’s and Cyanobacteria (Blue-Green Alga’s) as well as detritus, conglomerations of bacteria, rotifers and small crustaceans.
Why use Silver Carp?
In the New Zealand context they are considered and ideal species to control nuisance algal blooms in eutrophic waterways such as Cyanobacteria (Blue-Green algae) which can pollute a waterway and can lead to a toxic build up and potentially fish kills.
On the right Lake Orakai before Silver Carp and After Silver Carp. Silver Carp were added to the left of the barrier and had managed to control the algal bloom within 3 months.
Life Cycle and Reproduction:
Adults spawn in rapids across shallow gravel beds in Rivers or Tributaries when the water level increases by 50 120cm from its base flow. Conditions for spawning include high water flows 0.5 – 1.7 m/s, temperature of 22-28˚C and very high oxygen concentrations. The larvae drift downstream and use the floodplain lakes as a nursery prior to migrating back up stream.