Carp (Cyprinus carpio) is the most important fish species kept in Czechoslovak ponds. At present it represents 90–91 percent of pond fish production. Carp has been cultivated in ponds for several centuries, and has adapted itself remarkably well to local conditions. Its biotechnology is well managed even in the required breeding details. Carp is characterized by outstanding economic properties such as fast growth rate, and early sexual maturity, which result in the fish reaching a market size in a relatively short time, as well as a high resistance to unfavourable environmental effects. These are the basic prerequisites to further intensify its cultivation and increase its popularity on the Czechoslovak market and prospects for export.
The time required for the carp to reach consumption weight is three years in Czechoslovakia, which is due to unfavourable climatic conditions. The growth period, i.e., the time with a temperature of 10°C minimum, begins only in the first week of May and usually lasts until the end of September. In most cases, particularly in the most important pond area - southern Bohemia - the period is not longer than 160 days per year. Another factor exerting an immediate influence on the time required to achieve the consumption weight is the consumer's requirement of 1 500 g or more. It is only in some regions that a two-year rearing period is partially used as a result of new processing techniques (i.e., frozen cut portions).
The individual weight of carp ranges within the following limits:
autumn of the first year of life (5 months old) - 30–80 g;
autumn of the second year of life (17–18 months old) - 500–800 g;
autumn of the third year of life (29–30 months old) - 1 300 – 2 000 g.
The spawning of carp is performed in the course of May and at the beginning of June in special small ponds (Dubravius' ponds) in small groups, mostly in the ratio of 2 ♀♀ : 4 ♂♂, but also 1 ♀♀ : 2 ♂♂. The spawning ponds (Fig. 7) are small reservoirs covering 50–150 m2, usually in a line under a larger pond from which they are supplied with prewarmed water. Otherwise, due to their small size, the temperature in these small ponds would show considerable fluctuation and might be hazardous to the preparation of the breeders. The bottom and banks of the ponds are overgrown with grass which enables the deposition of spawned eggs. The ponds are filled with the breeders 24 hours before stocking (never sooner). They serve exclusively for the spawning and incubation of eggs. The hatched fry are taken from the ponds without any decrease of the water level on the third day from hatching, the latest time being the fifth day.
At the present time efforts for increased efficiency of the use of the spawning ponds have required the construction of so-called universal spawning systems. These systems meet all biological needs for the spawning of all phytophile species (pike, carp, tench and wels (Silurus glanis) and make it possible to use the spawning system throughout the spring season.
Czechoslovak fish farmers also successfully developed and introduced the method of carp stripping. The fertilization rate is 85–90 percent, the losses being lower than 25 percent in the course of incubation (Fig. 8).
When the hatched carp fry start to receive exogenous food, without having completely consumed the reserves in the yolk sac, they are carefully released in fry ponds. Experiments have demonstrated that particular emphasis must be paid to the prevention of fluctuations in temperature during transportation and release. Czechoslovak carp breeders use the following two procedures for the rearing of fry: “forced” rearing, and “normal” rearing.
Carp fry of 3–5 days old, are released in shallow ponds without mud (1–2 ha, average depth 50 cm). In some cases the bottom of these ponds is partly covered with legumecereal mixtures prior to filling. When gradually flooded they secure smooth development of the natural food - zooplankton (functioning as green manure). One hectare of water surface of fingerling ponds prepared in this manner is stocked with 400 000 to 600 000 sac fry. Within five to six weeks of rearing the fry reach the size of 4–5 cm. At this stage they are caught and released in fingerling ponds.
Particular importance is attached to securing smooth development of natural food, the sufficiency and size-availability of which are crucial factors underlying economic results. Organic manures used to fertilize the ponds are dung-water, dung, and poultry litter. The manuring rate depends on the results of biological and hydro-chemical analyses performed in the laboratories of every farm. Good results were achieved in rearing sac fry in ponds in which ducks were kept for one to two months prior to the release of sac fry.
The supplementary use of standard feed mixtures for carp fry is also common practice. The daily feed rations should not exceed 5 percent of the assumed weight of the stock. The total feed intake per fish should not exceed 5 g in the course of these five to six weeks.
When carp fry are reared by this “forced” method, higher gains are achieved in the first year of life: the individual weight of carp yearlings is about 100 g.
When using the normal method, the sac fry are taken from the spawning ponds and released in the fingerling ponds for the whole growth period. For this reason we use a much smaller stock: 40 000 to 100 000 individuals per ha. The usual losses reach 50 percent. Very careful attention must be paid to the smooth development of natural food (combined organic-mineral fertilizing); it is also necessary to start supplementary feeding of fry in due time. The present fish culture standards require wetted feed mixtures to be used up to the time the fry reach the size of 4–5 cm. Granulated feeds produced on the basis of a uniform formula are offered after this time. At the beginning the fry arefed two to three times daily; at later stages the feeds are delivered once or twice a day. The amount of the natural food in the ponds is taken into account when preparing the batches, i.e., with respect to the results of the analysis of biological samples. Other factors taken into consideration are water temperature, growth of fry and daily checks on feed consumption. The total consumption of feed per individual must not exceed 100 g of standard mixture (measured over a whole rearing season). The feeders are tableshaped, the surface of which is about 2 m2. Usually they are located in about 30 cm of water; later in the autumn they are sunk to greater depth because of decreased temperature. The feeders must be easy to dismantle to prevent complications during their emersion for regular cleansing and disinfection. The number of feeders must correspond to the density of stock, one feeder being sufficient for 5 000–8 000 fry.
The fingerling ponds enabling good hibernation of carp yearlings can be used also as wintering ponds; otherwise at the end of September the fry are taken from fingerling ponds and released in wintering ponds. In Czechoslovak fish culture, the ponds best serving this purpose are deep ponds without mud, not larger than 5 ha in size; wintering ponds of this type must be summer-dried at least for some time prior to filling. This practice is carried out for two purposes: sanitation of the environment and the provision of favourable production prerequisites, i.e., the availability of maintenance rations in the wintering pond as long as the fry are capable of receiving food, otherwise undesirable starvation results, thus reducing the food reserve of the fry. Before winter storage the content of fat in muscles should be 3 percent or more. The stocking rate of the wintering ponds depends on local experience, on individual weight, health and general condition. The density is usually 100 000 to 150 000 individuals per ha. Efforts are made to prevent fry mixing from different fingerling ponds.
In recent years winter storage of fry in special small ponds covering areas up to 0.2 ha has been achieved with good results (in essence, this is the local application of a system developed in the Soviet Union). The average depth of these ponds is 1 m. The bottom is of silt-loam structure, without organic remains. Sufficient supply of water rich in oxygen is also a prerequisite. The exchange must take place in the interval of 7–10 days. The density of the stock is about twice as high, 200 000 individuals per ha. Unlike ordinary fry wintering ponds, these ponds are stocked only by the end of October. Intensive supplementary feeding is carried out if necessary.
Before carp fry are released in wintering ponds, the stock is subjected to thorough health control. This is done whenever restocking takes place. This task is performed at the headquarters of the State Fisheries Farms by a unified network of veterinary stations (local, district and regional) that have veterinary surgeons and specialists in ichthyopathology. The required veterinary and breeding measures are prescribed and carried out on the basis of the results of this health control. Mandatory veterinary control is also carried out in the case of various kinds of transfers of fish from one place to another (e.g., between farms of the State Fisheries Enterprise). The main objective of these strict health regulations is to arrive at maximum sanitary conditions in Czechoslovakia as soon as possible. Czechoslovak fish culture is exposed to the hazard of a number of diseases due to increasing intensification of production and advancing pollution of surface waters. For instance, the losses caused by infectious dropsy were reduced to an economically tolerable degree in recent years due to the above-mentioned measures.
Throughout the winter season the wintering ponds are regularly controlled. Careful attention is paid to the chemical condition of the water source. The wind-driven defreezers, sometimes called Paulát's defreezers (Fig. 9) after their designer, are effectively used by pond managers. The movement of the wind-wheel is transmitted by a vertical axis to a screw located horizontally under the water surface. The inclination of the blades is adjusted so that the rotatory movement may drive warmer water from the bottom to the surface; this destroys the uniformity of the ice cover and produces a ventilating hole which is further enlarged as a result of the movement of air and wind. The mechanism is mounted on four floats securing good stability even in high wind. At present the wind movement is supplanted by electric power (Fig.10) so that the mechanism can also work on windless days.
A motor-driven aerator has proved to be highly efficient in emergency cases. The aerator is a portable aggregate whose power supply is provided by an air-cooled two-stroke engine (in some cases by an electric motor). The shaft of the engine is extended and the screw which is mounted on its end reaches under the water surface. The inclination of the blades is adjusted so that vacuum may be produced in rapid movement around the screw. The vacuum is constantly removed through the hollow casing of the shaft which has inlet openings over the water surface and discharge holes in the immediate proximity of the screw. The rapid movement of the screw distributes the air bubbles pumped under the water surface, hence providing for the enrichment of water with oxygen.
As to other equipment currently in use in Czechoslovak pond fish culture, we should mention circular and chain power saws (Figs. 11–12) for making ventilation holes in the frozen pond water surfaces.
Despite all these measures, fish losses still occur in the course of winter storage. For yearlings losses in number are about 10 percent; in two-year old fish about 5 percent. In favourable winter storage the weight loss does not exceed 5 percent. In well-prepared fingerling ponds we achieve even weight gains.
At the end of March or in the first half of April, most of the fingerling ponds are fished and the stocks are released in rearing ponds. These are smaller ponds, with shallow water, the size of which usually does not exceed 30–50 ha. The calculation of the stock is based on the water area of the reservoir and on the value of natural production as well as production achieved as a result of feeding (the amount of feed is divided by the absolute feeding coefficient). Furthermore, the planned individual gains (difference between weight at releasing and weight when fished) and the assumed losses are taken into consideration. The same method is also employed for the calculation of the stock in the finishing ponds where the two-year-old stocks reach commercial size in the course of the third growth period. The average loss encountered in the rearing of carp at the age of two years is about 10 percent. In the third year it is not larger than 2–3 percent.
The manuring rate which exerts immediate influence on the development of natural food is governed by the results of hydrochemical analyses. The following values are considered as optimum: alkalinity: 1.5 – 2 mval., phosphorus content: 0.2–0.3 mg of PO4/l, nitrogen content: 2 mg in 1 NO3, etc. Both organic (farmyard composts, green manuring) and inorganic manuring (now predominant) are used in Czechoslovakia. Mixed manures are considered advantageous from the point of view of economy and management. This is true particularly of the NP type of manures (the N:P2O5 ratio being approximately 2 : 1). Mineral nitrogen fertilizers are also currently used. In combination with phosphorus fertilizers, the use of N-compounds increased fish production by 180 kg per ha on an average.
In the second and third years of life carp is supplementarily fed granulated mixtures. The batches are determined on the basis of water temperature, stock density, occurrence of natural food, oxygen content and consumption rate of the applied feeds. Feeds are given in the second half of the growth period (July to September) when 75–80 percent of the total annual ration is consumed.
In the rearing of stock and commercial fish, natural food plays an important role. Among other things, this is demonstrated by the fact that in their second year of life feed consumption per individual fish does not exceed 1 kg of feed, with an individual gain of about 500 g. The important role of natural food in carp nutrition is manifested in a high biological value and good condition of commercial fish produced in Czechoslovak ponds. It is for this reason that the carp can compete well with fish reared mainly on industrial feeds enriched with vitamins and antibiotic additives.
In order to prevent frequent handling (and therefore possible injury, stress, and mixing of stocks) which can be the primary cause of the occurrence of some diseases, Czechoslovak fish culture workers also developed a method of pond stocking for two years. However, this method can be employed only if the production ponds afford safe winter storage between the second and third years of life and can bear an uninterrupted two-year production cycle. Most of the ponds are used in the ordinary manner, i.e., stocking and fishing every year, with winter storage in wintering ponds.
In the past ten years efforts have been made in Czechoslovak pond management to stabilize carp culture. This notion covers the trend towards the unification of carp culture from the morphological (type of scaliness, conformation) and growth (efforts for maximum use of the genetic basis of rapidly growing individuals in further breeding) points of view. Some physiological factors are also taken into consideration, such as rapid growth and early sexual maturity, reliability in reproduction, high fertility, economic utilization of feed, hardiness and, particularly, natural resistance to some diseases.
Each farm of the State Fisheries Enterprise is responsible for the constant improvement of its breeding material. There are no special fish breeding units or farms in Czechoslovakia. The Czech carp are adaptable to the specific conditions of the given region and are used as the basis for the breeding work. However, the methods of breeding are unified, i.e., they are developed by the Trust State Fisheries in cooperation with the Research Institute of Fisheries, Vodnany, for each 2–3 year period. The methods are always reviewed when this period elapses. The operation engineer is responsible for the performance of carp breeding work on each farm.
The main goals of carp breeding activities are the following:
- achievement of pure lines in the type of scaliness and the elimination of individuals serving as vectors of the lethal factor N;
- determination of the production properties of the filial generations of dominant and recessive homozygotes under various production conditions;
- development of methods of heterosis effect and their practical utilization in carp culture.
Thanks to rapid development in rainbow trout culture on a large-scale, breeding work has made considerable progress. The following factors are of particular interest to trout breeders:
- growth rate - achievement of early marketable size;
- economical utilization of feeds;
- individual early sexual maturity and high fecundity;
- resistance of some individuals to the negative effects of environment as a prerequisite for further intensification of the cultivation of this species.
Problems in fish polyculture requiring solution are: the increasing need for variety on the Czechoslovak markets, the need to secure sufficient stocks for river basins (particularly for artificial lakes) and water courses, as well as the possibility of exporting commercial fish, and in some cases stocking of additional species. The intensification of the cultivation of mixed species is a method to solve these problems. In essence, this is the problem of specialization or the proper combinations of fish species being kept not only under their typical climatic conditions but also, as far as possible, in adapted environments. Only in this way can we achieve production of these species representing 10–11 percent of the total crops of commercial fish every year.
The interest of fish breeders is now focussed on four species of predatory fish and on two non-predatory species: pike (Esox lucius), pike perch (Lucioperca lucioperca), rainbow trout (Salmo irideus), wels (Silurus glanis), tench (Tinca tinca) and whitefish (“Maréna”). In the near future this group can be extended to cover other species, both through direct introduction and gradual acclimatization. In the period 1971 to 1975 we intend to develop methods of rearing herbivorous fishes (including their reproduction) and to introduce these methods in fisheries practice. These are: grass carp (Ctenopharyngodon idella), bighead (Aristichthys nobilis) and houting (Coregonus peled) imported to Czechoslovakia from the Soviet Union in the middle of the 1960s.
Particular attention is paid to intensive cultivation of rainbow trout (Fig.13), both in small ground ponds and in flowing storage nets in artificial lakes. There is already a number of trout farms with annual capacity of 500 to 1 000 metric centners (50 000 to 100 000 kg) of commercial fish. The fish are fed granulated mixtures, now produced on an industrial basis also in Czechoslovakia. Good production results have been achieved with the cultivation of brook trout (Salvelinus fontinalis), its biotechnology being similar to that of rainbow trout.
