EXPERT TOPIC
SILVER CARP
EXPERT TOPIC Silver carp
by Rebecca Sherratt, Features Editor, International Aquafeed
The silver carp (Hypophthalmichthys molitrix) is a subspecies of the Asian carp, primarily found in China and eastern Siberia.
T
he silver carp is a frequently farmed species in the aquaculture industry, second only to the grass carp (Ctenopharyngodon idella). In some countries the silver carp is considered an invasive species. Connected waterways have resulted in silver carp now being found in over 88 countries, primarily used to assist with water quality control, as well as for aquaculture purposes. Silver carp reside in large rivers and are known for being resilient fish, able to tolerate low dissolved oxygen (3mg/l) and salinities up to 12ppt. Silver carp can be distinguished by their small eyes that are set below their mouths. As the name suggests, they are silver in colour, with darker pigmentation occurring along their backs. Their bodies are club-shaped and their abdominal ridge is distinctly knife-shaped. In their native habitat in the east, silver carp generally mature at between four and eight years old, although in North America they have been noted to mature at as young as two years old. Silver carp can live for up to 20 years and a female will lay up to five million eggs-per-year. Farming of silver carp first began in China in the 1950s, before rapidly gained popularity in Europe. Year on year, farming of silver carp has seen a sharp increase. In 1990, 1,520,469 tonnes (t) of silver carp was farmed globally. This number increased to 4,099,666t one decade later and again increased to 5,300,735t in 2016.
Diet and spawning
This species of carp are phytoplanktivorous filter feeders and can filter particles as small as 4µm. The silver carp’s epibranchial organ assists in the trapping of small particles by secreting a mucous. The filtration rate of silver carp varies depending upon the fish’s size and internal temperature. Similar to many species of carp, the silver carp has no stomach, so is assumed to feed itself on phytoplankton, zooplankton and detritus at a continual rate throughout the day. Spawning occurs in late spring and summer at temperatures of between 18-30°C. After spawning, eggs will absorb water and swell, in order to remain at the bottom of the water, until hatching. Induced spawning is a common process carried out with silver carp, with broodstock stocked by weight at 1,5002,250kh/ha and a female-to-male ratio of approximately 1:1.5. The pituitary glands of broodstock silver carp do not secrete a sufficient quantity of hormones to induce natural propagation in ponds. To overcome this, broodstock are injected with estrogenic agents such as Luteinising Release Hormone (LRH) and Human Chorionic Gonadotrophin (HCG) to induce the fish to secrete gonadotrophic hormones. To ensure the effectiveness of induced spawning, fish must spawn in the daytime. One must also take into consideration the season, temperature, weather conditions and gonad maturity if they wish to ensure optimised results when inducing spawning. Silver carp fry are especially delicate and so expertly controlled intensive systems are required 52 | September 2020 - International Aquafeed