Breeding of common carp
As the common carp breeds naturally in confined waters, several methods of propagating the species have been developed in different areas. The simplest allows uncontrolled breeding in communal ponds, with shallow marginal areas covered with grass or aquatic vegetation which serve as substrates for their adhesive eggs. A more advanced method uses special spawning ponds for spawning, hatching and larval rearing. The most familiar type of carp spawning pond is probably the Dubisch pond, named after the Silesian fish farmer who developed it. It is a square or rectangular-shaped shallow pond (8–10m2), generally surrounded by a reed fence for protection from chill in temperate climates. It has a peripheral 40–50cm deep ditch, the rest of the pond being only 20–30cm deep. In the centre of the pond is a sloping spawning area covered with meadow grass. The Hofer type of pond is a variation of this, without a peripheral ditch but with a harvesting ditch near the monk. In a carp farm, a number of such spawning ponds may be built to spawn an adequate number of fish, when the temperature conditions are suitable. Before the spawning season, the ponds are dried and, if necessary, treated with lime to eradicate unwanted organisms. The ponds are filled when the water is sufficiently warm (above 18°C) and selected brood fish are introduced at the ratio of up to six males to three females. They usually spawn within 24–48 hours. The brood fish are removed after spawning and the eggs are left to hatch in the pond. Within a week after hatching, the fry are removed to nursery ponds for further rearing. In present-day carp culture, efforts are made to exercise greater control, so as to achieve a higher percentage of spawning success and hatching rates.
The spawning season for common carp in temperate climates is in the spring, when water temperature rises above 18°C. By the manipulation of environmental conditions and the use of selected races or strains, farmers have succeeded in extending the breeding season to suit particular culture requirements. In tropical climates, it has been possible to breed common carp at any time of the year.