Carps
The main carp species cultivated in the world are primarily seven in number and are often grouped on the basis of their natural geographical occurrence: the so-called Chinese carps, which include the grass carp, Ctenopharyngodon idella, the silver carp, Hypophthalmichthys molitrix, and the bighead carp, Aristichthys nobilis, and the so-called Indian major carps, which include catla, Catla catla, rohu, Labeo rohita, and mrigal, Cirrhinus mrigala.
The seventh species is the common carp, Cyprinus carpio. Taxonomically, carps belong to the family of Cyprinidae (order, Cypriniformes).
Grass Carp
Grass carp is a natural inhabitant of the Flatland Rivers of China, and the middle and lower reaches of river Amur in Russia. The fish has been introduced world wide into over 50 countries since beginning of the t wentieth century.
In many countries, the main purpose of its introduction, in addition to culture, is biological aquatic weed control in natural waterways, lakes, man-made lakes and irrigation channels. Grass carp, like other Cyprinids, has a toothless mouth but has specialized pharyngeal teeth for rasping aquatic vegetation. The patterns of teeth structure of grass carp are adapted to the feeding habit. Large fishes are able to masticate the leaves of tough land plants such as fibrous grasses.
Intestinal length relative to standard length from 1.6 to 2.7 in adults (0.5 in larvae). Digestion of fiber in the grass carp is said to be incomplete and about half the food material ingested is excreted as feces, which, supposedly, can support directly or indirectly a large biomass of other species of fish in the fishpond.
The natural food of grass carp fry is first protozoa, rotifers and nauplii and then cladocera and copepods, with benthic algae when fry reaches 20-25 mm length. Then, the natural diet becomes phytoplankton and minute algae and, for fish above 30 mm, the natural diet becomes macrovegetation.
The preferred weeds according to the different regions of the world are: Wolffia, Lemna, Spirodela, Hydrilla, Najas, Ceratophyllum, Potamogeton, Vallisneria, and Myriophyllum. Under culture conditions, grass carps are often given substances like cereal brans, oilcakes, and silkworm pupae, and kitchen refuse as supplementary food.
In natural waters, grass carps reach a weight of 200 to 650 g at the end of the first year and, after 4 years the weight may be 4 to 5 kg. In the Yangtze River, fish weighing more than 20 kg have been caught. When cultured, growth is a function of many factors (stocking rate, feeding quality and quantity, competition with other fish when in polyculture conditions…).
The range of daily growth of grass carp may be from 2.8 to 9.8 g.day -1 depending on culture conditions and climate environments. Age at which grass carp attains maturity varies greatly according to climate and environment factors, mainly temperature: from 1–2 years in tropical Asia to 8–10 years in cold-temperature countries (Russia). Relative fecundity of grass carp varies from 80 000 to 120 000 ova per kg of female body weight (Table 4).
Table 4. Relative fecundity (number of ova per kg of female fish) in the main cultivated carp species.
The fish breeds during monsoon months in its natural habitat, the rivers, but does not spawn naturally in the stagnant waters of ponds and tanks. Induced spawning is required under culture conditions.
Silver Carp
Silver carp naturally occurs in the river systems Yangtze, West River, Kwangsi, and Kwangtung in South and Central China, and in the Amur Basin in Russia and the species has been introduced into many countries for aquaculture purpose.
The feeding habit of silver carp is mainly zooplankton, rotifers and nauplii of copepods at the young stages and expands as the fry grow to include copepods, cladocera and phytoplankton.
Larger fry and adults feed on Flagellata, Dinoflagellata: primarily phytoplankton and secondarily zooplankton.
The fish shows specific anatomical and morphological modifications in correlation with its dominant phytoplanktivorous feeding habit. The length of the gut of the adult fish is 15 times the body length.
The gills of silver carp have a complex net work and profusion of closely set gill rakers, allowing the fish to filter small size algal cells (down to 30 µm in diameter).
Under culture conditions, silver carp may reach 1.8 kg after 2 years, 4.6 kg after 3 years and usually, growth declines after the third year. The average growth rate is 6.3 g.day -1 during the 3 first years of culture. As for grass carp, the age at first maturity of silver carp depends mainly on temperature. For example, in North China, the sexual maturity is attained after 5–6 years (weight: 2–5 kg) while in South China maturity is achieved only after 2–3 years (same individual body weight).
The average relative fecundity of Silver carp is slightly higher than for the grass carp. The fish breeds naturally during April-July in the flowing waters of its natural habitat, the rivers of China, but do not spontaneously in ponds and tanks where induced breeding is required.
Authors:
Lionel Dabbadie and Jerome Lazard
