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. The 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 1,500-2,250kh/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 to maximise survival rate. Fry rearing will take place for approximately 15-20 days post-hatch, followed by the fingerling stage, wherein fingerlings are reared for between three-to-five months. Fingerlings will then be reared into market-size fish, the full culture cycle taking around two years to complete.

Farming and harvesting

Farming of silver carp gained significantly popularity in China in the 1950s, following advancements in artificial breeding. Silver carp became an especially popular cultured species in China due to low costs associated with fertiliser and the silver carp's short rearing period. These unique advantages to farming silver carp also led to an increase in farming in western waters in the following decades.

In the modern day, silver carp are usually farmed through polyculture methods with other species of carp such as grass carp, bighead carp (Hypophthalmichthys nobilis) and common carp (Cyprinus carpio). Due to their status as filter feeders, farmers will often use manure to produce the plankton required to feed silver carp. The amount of fertiliser used by farmers will generally vary, depending upon the season. Commonly used types of fertiliser include animal manure, grass silage and oilseed cake. Ponds will be fertilised approximately 5-10 days before stocking.

When harvesting silver carp, utmost care must be paid to ensure the fish are handled gently and carefully. Injury and mortality rates during summer and autumn harvesting can often be higher than those in winter or spring, due to increased water temperatures. This leads to fish consuming larger amounts of oxygen and, as a result, they cannot handle being confined together for extended periods of time. Humid and rainy days should also be avoided when harvesting. It is recommended to turn on aerators to increase the dissolved oxygen content when undergoing any activities that may distress the fish.

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