Feeding behaviour
Common carp is active in feeding when the water temperature is over 18–20 oC. Though common carp tolerates high water temperature (around 28–30 oC) the optimum temperature of growing is between 20 and 25 oC. During those periods when the water temperature is lower than about 15–16 oC feeding of common carp becomes less and less intensive. Feeding practically stops if the water temperature sinks under about 8 oC. When the water temperature is under about 5 oC carp hibernates in groups in the mud of deeper waters.
The period when the water temperature is over 18–20 oC depends on the climate under which the carp is produced. Therefore, the actual total gross period required for the production of 1– 3 kg of large table fish may vary between about 1 and 3 years, while the net growing period for the same size of table fish will not be more than 0.8–1.3 years (Table 1 and Figure 7).
Digestive system
When the larva hatches, its mouth and digestive tract are not developed, and thus it is not capable of external feeding. Therefore, after hatching, the larvae feed from the yolk sack.
At about 15–30 days after stocking (Figure 8), feeding larvae grow into advanced fry (Table 1). At this stage, all of the organs except the gonads are already developed (Horváth, Tamás and Tölg, 1984; Horváth, Tamás and Coche, 1985a; Lavens and Sorgeloos, 1996).
It is only at the advanced fry stage that the digestive tract is entirely developed. By this time, the qualitative development of both the mouth and the digestive tract is completed (Lavens and Sorgeloos, 1996). During the early development of the digestive tract of common carp, the larvae do not have the needed set of enzymes to digest the ingested food. After filling of the swimbladder and the intensification of exogenous feeding, the trypsin activity rises. Later (within about 15 days), chymotrypsin activity appears and amino peptidase activity then increases (Lavens and Sorgeloos, 1996).
The mouth of the common carp is relatively large and opens in an accordion-like fashion, enabling the fish to dig in the mud of the bottom. There are two pairs of barbels, one pair on the upper lip and the other pair at the corners of the lower lip. They function as feelers for searching for food. There are 5–5 molar-like pharyngeal teeth which serve to grind the consumed food and feeds (Froese and Pauly, 2011). Still, grinding or at least crushing of grains used in aquaculture feeds is highly recommended. The common carp has no stomach; therefore the ingested food arriving from the mouth passes directly into the intestine.
The overall intestinal length relative to the total body length is related to the feeding habits of the fish species under consideration. In herbivorous fishes, this ratio is greater than in carnivorous species. This is mainly due to the fact that carnivorous species possess a stomach in which most of the digestion takes place. In case of omnivorous fish species such as common carp, the actual length of the intestine depends on, among others, the food and feeds consumed during the early stages of life (Hepher, 1988).
Natural food
Common carp is a typical peaceful omnivorous fish which consumes a range of different natural foods, including planktonic crustaceans, insects (including their larvae and pupae), the tender parts and seeds of water plants, and also fish eggs and larvae, as well as smaller fish (Table2.1, Table2.2 and Table5.1). It is important to note that common carp is a flexible and opportunistic feeder that can switch from preferred to alternative diets according to the food availability (Hoole et al., 2001).
By the end of larval development, the yolk sack is mostly consumed, and the mouth and digestive tract of the young fish is ready to feed from the environment. The natural food of larvae at first feeding (4.8–6.2 mm) must be small, not larger that 60–70 μm (Table 2.2), as this is the food size which the young fish can swallow easily. Later, as the developing larvae grow, the size of the natural food will also increase (Table 2.2) (Horváth, Tamás and Tölg, 1984; Horváth, Tamás and Coche, 1985a; Lavens and Sorgeloos, 1996).
Within the first 15–40 days after hatching, the young fish feeds exclusively on zooplankton but will also readily ingest floating supplementary or balanced feeds provided that their size is small enough to swallow.
During the next stage, the fish grows up to become a fingerling, its feeding habits and food spectrum also changing. As the fish grows, it feeds more frequently from the soft bottom; bottom feeding becomes more and more pronounced when the size of fish is larger than about 50 g (Table 2.1).
In certain cases, collected live food is given to the larvae or fry of common carp when circumstances and economic conditions justify this (Table 2.2). The use of cultured live food in the tank and cage culture of larger age groups in the large-scale production of carp for the table is hardly practiced. The collection of natural food (e.g. worms, snails, different insect stages) may only be feasible in sporadically practiced, small-scale and backyard tank and cage-culture systems. However, if this practice is taken up by too many families in a neighbourhood, the availability of collectable fish food and other environmental conditions will soon limit such activities.
