by
A.G. Konradt,
Senior Scientific Worker
The problem of modern fish culture is first of all a problem of intensification, i.e., the problem of obtaining maximal output per area unit of a water body. Since we undertake the task of getting a certain type of output, namely fish, we shall use the term “output” to mean fish output, without implicating special problems of hydrobiology dealing with productivity of water bodies in the broad sense of this word.
Our aim is to secure the maximum fish flesh per unit of water body area. But it is impossible to increase the density of fish population in a water body without limit. At first the fish output will grow in proportion to the abundance of the reared fish population but then the average weight of the fish will drop and, if the rate of stocking is further increased, the water body productivity and fish output will begin to fall. The main cause of the decline of the water body productivity at too high a rate of stocking is the shortage of food. Eventually the intensification of fisheries can be effected only on the basis of building up the food supplies in a water body or on the basis of a better utilization of food available.
When rearing such fish as carp, this problem is solved by introduction of artificial feeds into a water body. However, feeding of fish makes all the production processes more complicated and the final product more expensive.
From the standpoint of fisheries, it is not expedient to rear only one species of fish. In such a case, a great quantity of feed organisms which are not used by this particular species are wasted. Subsequently, the water body goes through a long series of complicated changes which result in formation of food organisms. Dead animals and plants fertilize the water body with biogenous substances which serve as the basis for the development of the bacterial flora and phytoplankton. The latter, in turn, are used as food by zooplankton and other aquatic organisms which are eaten by fish in the long run.
So, before the initial organic substance is utilized by fish, it undergoes many changes. It is natural, that each of these transformations entails considerable losses, wastes of substance and energy. The direct utilization of food by fish is much more effective. It will be more effective when fish use the green mass of aquatic plants, especially phytoplankton. And, going back to the problem of a rational fish species composition of the same water body, one concludes that it is expedient to stock water bodies, not only with plankton and benthophages, but also with plant-eating fish. For it is these that can utilize the considerable standing crop of aquatic vegetation not used by other species, Ctenopharyngodon idella Val., and Hypophthalmichthys molitrix are representatives of the Chinese plain river complex they are famous for their valuable marketing qualities. Ctenopharyngodon idella Val. is a fast growing big fish which feeds on higher aquatic plants.
Hypophthalmichthys molitrix Val. is another rapidly growing planteating species which feeds on phytoplankton.
Aristichthys nobilis Rich. can be referred to as only partly plant-eating fish. Its chief food is zooplankton but its intestine contents generally include about 30 percent phytoplankton.
All the three specimens of plant-eating fish grow to large sizes and in feeding and warm-water bodies they reach the weight of 4 to 8 kg as early as the age of three years. The weight of even two-year-old fish can be 2 to 3 kg and, therefore, they can well serve the purpose of acclimatization in natural water bodies and rearing on special fish farms.
What are the prospects of introducing plant-eaters such as C. idella, H. molitrix and A. nobilis in with fish of local water bodies? The first results of their acclimatization in natural waters have now been obtained in the U.S.S.R. Thousands of H. molitrix and A. nobilis were released into the Kuban estuary system. The individual weight of the stocking material was rather high and averaged 600 g. The fish had been reared on special fish farms. Two years later check fishing was carried out in these estuaries. It was found that during such a short period A. nobilis had reached the average weight of 9 kg and H. molitrix 4 kg. Hence, the natural water reservoir suited plant-eating fish and a large supply of phyto- and zooplankton ensured the rapid growth of the fish tested.
In another region, in the warmwater Kara-Kum Canal, aquatic vegetation was extremely abundant and blocked the water flow which entailed high cost annual clearing work. Some years ago yearlings of C. idella, H. molitrix and A. nobilis were stocked into the Kara-Kum Canal. Now the canal is not overgrown with aquatic vegetation.
The growth rate of C. idella in this warmwater canal is very high. In 18 months it reached the weight of 3 to 4 kg. The adequate development of phytoplankton provides for the high growth rate of H. molitrix and A. nobilis. These two illustrations substantiate the possibility of utilization of natural waters for feeding of plant-eating fish.
Some attempts at acclimatization of fish in new areas have failed. In those cases, when fry of light weight (from 200 to 500 mg up to several grams) were released into natural water bodies, the subsequent check catch findings were negative. Hence, it is absolutely evident that indigenous fish, particularly predators, inhibit plant-eating fish so much that the latter cannot stand the competition and perish.
Long experience shows that fish acclimatization is effective, subject to preliminary rearing in nursery ponds. Preliminary rearing of plant-eating fish until they grow large enough is a must. The weight and size of stocking material may vary widely, depending on the type of water body intended for the stocking. However, fry weighing less than 25 g should not be used for stocking purposes.
What happens to plant-eating fish which attain sexual maturity under new and unusual conditions? Is their natural propagation possible under such conditions or is it necessary to release additional lots of grown fry with a view to feeding? It was not long ago that the opinion was shared by many that plant-eating fish cannot propagate in new water bodies. The conditions of our lake river systems seemed to differ too much from those of the natural habitats of the species in question. It is well known that fish are very conservative, as far as propagation conditions are concerned. In the course of their adaptation to new life conditions, fish become very sensitive to changes in spawning conditions. As a rule, natural propagation is impossible if spawning conditions do not meet the fish requirements, even to a slight extent.
Numerous illustrations of this may be provided. Construction of a hydropower station on one of the northwestern rivers prevented a large group of whitefish from migrating for spawning. In the tailwater of the river there were no spawning conditions and within 15 years the population of whitefish dropped from several hundred thousand caught when the river was not controlled, to several hundred individuals which had adapted themselves to propagation under the new conditions. It is well known that psammophilous and lithophilous fish are very sensitive to the spawning substratum. The Acipenseridae propagate under certain water current conditions and on specific bed soils.
Plant-eating fish are also very sensitive to propagation conditions. In their natural habitats they propagate in the upstream part of the river during high water time. Unlike the European plain rivers, the Asian rivers overflow as a result of snow melting in the high mountains and this happens when the river water temperatures are rather high. Therefore, according to the Chinese specialists, sudden changes in the water level and the current speed but not the high temperature of water, are prerequisite for spawning of plant-eating fish. As is known, one of the main factors in spawning conditions for most fish of the European continent is the water temperature required by a given species.
Propagation of plant-eating fish under specific spawning conditions caused doubts as to the possibility of the independent natural propagation of these fish in new waters. Fortunately, those doubts did not come true.
The natural propagation of the plant-eating fish in both regions where they have been naturalized is an established fact.
The propagation of H. molitrix and A. nobilis in the Kuban River has not yet been proved, for on one of its tributaries upstream there is a hatchery engaged in artificial breeding of C. idella, H. molitrix and A. nobilis. The H. molitrix and A. nobilis fry caught in the lower reaches of the river may be from the hatchery production. It is not likely but still may be the case. Therefore, their propagation in the Kuban River is not an established fact.
The propagation of plant-eating fish in the water system linked by the Kara-Kum Canal is much more evident. In recent years many millions of plant-eating fry have been caught in these waters. Such quantities far surpass the extent of the artificial fish rearing undertaken on the canal. The rearing pond arrangement is such that the fry cannot escape back to the canal. Therefore, the natural propagation of plant-eating fish in this area is unquestionable.
The possibility of plant-eating fish propagating under hydrological conditions which greatly differ from those of their natural habitat may be considered proved. This statement requires some clarifying for both of the regions mentioned above are characterized by the high water change in the river level during the period when water temperatures are rather high. The difference in the period of peak floods in these rivers and in the natural habitats of C. idella, H. molitrix and A. nobilis is slight.
It is quite probable that plant-eating fish will find suitable conditions for rapid growth in some other plain lake-river systems but will be unable to propagate there by themselves.
Various large water reservoirs built on the plain rivers in the European part of the U.S.S.R. may be referred to as favorable for growing these species. In these cases, when the flow in a water body is very slow, large quantities of plankton algae develop. We know some reservoirs where the standing crop of phytoplankton amounts to several hundred thousand tons in summer. Therefore, the stocking rate of C. idella and H. molitrix may be such as to ensure the consumption of a considerable amount of the phytoplankton standing crop but not all. However, plant-eating fish, although fully supplied with food in these reservoirs or similar land-locked waters, will not find normal conditions for propagation there. Reproduction will be impossible and the stocks will have to be replenished regularly.
Therefore, stocking with plant-eating fish in some waters should go along with systematic compensation for losses of adult fish by means of artificial propagation and fry rearing in special nurseries.
How should artificial reproduction of plant-eating fish be done in these special nurseries? There is no doubt that plant-eating fish can attain sexual maturity in ordinary fishbreeding ponds. Therefore, the nursery ponds do not require any special facilities to establish a faster or constant speed current. The ponds should have facilities for an emergency supply of fresh water in case of an extremely high (over 35°C) water temperature or a considerable oxygen deficiency. In ponds in the southern regions of the U.S.S.R. the three species of plant-eating fish attain sexual maturity at the age of three years and eggs can be obtained at the age of four. In regions with a more rigorous climate (north of latitude 50°), the period of male maturing is longer and takes up to five or seven years. It has been ascertained that specimens which attain sexual maturity for the first time produce low quality eggs, often very few eggs and sometimes remain immature after a pituitary injection. Females which have received a pituitary injection but have not laid eggs die in large numbers. Therefore, it is advisable to secure eggs from repeatedly matured specimens.
The procedure of getting sexual products does not differ essentially from artificially obtaining eggs from spawners of any other fish. It is difficult to handle large plant-eating fish, so any devices facilitating catching and holding the fish and the egg squeezing should be used on the widest scale possible.
Unlike the carp, plant-eating fish cannot propagate in artificial spawning ponds. Eggs and milt are obtained from them only after a preliminary pituitary gonadotropin injection, according to the method of Professor N.L. Gerbilsky. Propagation of vertebrates in stimulated and controlled by the system of endocrine glands. Hormones, biologically active substances, produced by these glands provide for the normal functioning of the various systems of the organism. The main endocrine gland is the pituitary. This gland is responsible for the secretion of the so-called gonadotropin which brings about the spawning state. Under normal (natural) conditions the gonadotropin of the pituitary is released into the organism in response to the signals provided by the nervous system when the spawning conditions are ripe.
The hormone is released into the blood and reaches an effector, the gonad, which undergoes changes and makes the fish spawn. As was stated above, no signals are received by the pituitary when correct spawning conditions are not present, so the gonadotropin is not released and spawning does not take place.
But what would happen if the gonadotropin is injected prior to the nervous system signals? If the administered hormone reaches the gonad, the whole process will take place exactly as it does in the case of natural spawning. The ovaries will ovulate and the fish will be able to spawn under conditions which would be very different from those of natural spawning. It is necessary to provide the same water temperatures as required for natural spawning, because ovulation will not occur when gonadotropin is released into blood under highly abnormal temperature conditions and also because eggs laid in such conditions will not be able to develop normally.
How is gonadotropin obtained and administered? It is still impossible to synthesize gonadotropin and fish culturists use the natural hormone. Before spawning much gonadotropin is accumulated in the fish pituitary. If the pituitary gland is removed at this time and dried by means of acetone, the gonadotropin is preserved in good condition in a small clot of dry fatless substance. This preparation is convenient to use, as it can be stored in a dry and hermetically sealed vessel for a very long time. If the dried pituitary gland is ground in a small quantity of physiological saline solution and then injected with a syringe into the muscles of the fish under experiment, all the changes which cause spawning will occur.
The maturing period of spawners after the injection depends on temperature. At a spawning temperature of 20 to 22°C, it lasts 10 to 12 hours in plant-eating fish. Long-term practical experience gained in pituitary preparation investigations reveals that, after the pituitary injection, fish attain maturity only if their gonads have reached a certain stage of maturity. Otherwise the pituitary injection is not effective. Experience working with plant-eating fish also shows that the percentage of spawners which react favorably to the injection is very low among individuals that are equally mature, judging by their exterior appearance.
Such a reaction to the injection can apparently be explained by the non-uniform development of the gonads. Plant-eaters' spawning season lasts 2 to 4 months. Therefore, in the spawning stocks there are always fish with gonads still insensitive to the gonadotropin. Such variations usually occur at the final stages of gametogenesis. Preliminary preovulation processes take place in such gonads, when very small doses of gonadotropin are administered on the basis of these patterns. A somewhat different pattern of the pituitary injection has been suggested for plant-eating fish.
Initially, as a stimulant, females are treated with a dose 8 to 10 times less than the usual one bringing about maturity. The injection makes the preovulation processes of ovocyte maturity complete in the immature gonads of some females. Such a small dose of the gonadotropin cannot cause ovulation in those females which are ready for spawning. Hence, the preliminary stimulating injection of gonadotropin brings the female gonads to the same stage of maturity. Twentyfour hours are required for such equalization and 24 hours after the first injection females receive the second injection, the usual dose being 7 to 8 pituitary glands per female. At a water temperature of 20 to 22°C fish reach maturity after the second injection and eggs may then be obtained. Mature males usually begin to shed milt by the beginning of the spawning season without preliminary injections but to obtain the required quantity of milt, they as well as females, are always treated with one stimulating injection of gonadotropin 10 to 12 hours before egg collection. The usual dose in this case is not more than 1 or 2 pituitary glands per male. This pattern of pituitary injections into plant-eating fish, especially individuals which spawn for the first time, is much more reliable and effective than the usual one-time injections.
The eggs are squeezed from mature females into a dry enameled basin and mixed thoroughly with the milt, with no water added. Two to five minutes after mixing the eggs with the milt, water is added then the mixture is allowed to sit a few minutes. The excess sperm is washed off and after partial swelling of the eggs they are placed in a Weiss apparatus for incubation.
About 50,000 eggs are put into one apparatus. The eggs of plant-eaters swell a great deal, the outer diameter varying from 4 to 5 mm. Eggs are vary light. Their specific weight is near I, therefore they are easily washed out of the apparatus even when the water current is not very strong.
A current of 0.5 to 0.7 1 per minute is rated for a standard Weiss apparatus with a capacity of 50,000 eggs.
The incubation apparatus is equipped with drain troughs and alevin traps are mounted beneath them. Each apparatus may be supplied with a separate alevin trap but one common trap may be installed at the end of the common drain trough of several. The traps are rectangular frames covered with fine gauze netting (No. 17 to 19). Hatched alevins are carried out of the apparatus into traps by a stream of water and are scooped out together with water. Alevins must stay in water.
Then alevins are put into liveboxes made of the same fine gauze netting and installed in the pond. The livebox dimensions are 1 × 0.75 × 0.5 m.
Such a livebox can accomodate 300,000 alevins. Alevins are kept in the livebox for 3 to 4 days, depending on the temperature, until they start to move and feed by themselves.
Afterwards they are released into rearing ponds. The alevin stocking rate should be 20,000 to 30,000 per ha. About 50 percent survive to become fingerlings.
When stocking, alevin numbers are estimated by volume on the basis of an average sample. For this purpose they are concentrated in a precisely measured flask. Then they are scattered evenly throughout the water and 2 or 3 samples of a certain volume (1.1 to 0.2 1) are taken from the alevin mass. The remaining alevins are released into a water body and each sample is counted. The exact data required for stocking calculations is obtained by means of arithmetical recomputation in terms of the initial vessel capacity.
In autumn, when water temperatures are lower, fry are collected in rearing ponds and are carried by livetank trucks to the water bodies intended for stocking with plant-eating fish.
BIBLIOGRAPHY
Aliyev, D.S., 1961 Experience in receiving breeds of the Far East plant-eating fish in Turkmeniya. “Questions of ichthyology”. Vol.I, N 1, pp. 650–58
Aliyev, D.S., 1961 Amur and Hypophthalmichthys molitrix in Turkmeniya. “Fish Culture and Fisheries”, N 5, pp. 14–16
Aliyev, D.S., 1965 “Meliorative” pond fish. “Nature”, N 8, pp. 56–60
Collected articles, 1963 Problems of the fishery utilization of plant-eating fish in water reservoirs of the U.S.S.R. "Acad. of Sci. of Turkmeniya Sov.Soc.Rep., Ashkhabad, 1963, bibliography 130 names.
Konradt, A.G., 1961 Conditions for rearing plant-eating fish in the fish farms of the Soviet Union. “Sci.-Tech.Bull. of the State Res.Inst. of Fresh Water Fisheries”, N 13–14, pp. 53–57
Konradt, A.G., 1962 Experience in growing Hypophthalmichthys molitrix Val. in the Leningrad region ponds. “Sci.-Tech.Bull. of the State Res.Inst. of Fresh Water Fisheries”, N 15, pp. 55–57
Stroganov, N.S., 1964 Growth and sexual maturation of Amur fish in the Moscow region ponds. “Questions of Ichthyology”, Vol.4, N 4, pp. 664–671
Veriguin, B.V., 1961 Results of the acclimatization works of the Far East plant-eating fish and arrangements for their further study and assimilation in new regions. “Questions of Ichthyology”, Vol.1, N 4, pp. 640–649
Vinogradov, V.K., 1965 Hatcheries for plant-eating fish. “Fish Culture and Fisheries”, N 2, p. 46
Vinogradov, V.K. and L.V. Erokhina, 1964 Hybrids of Hypophthalmichthys molitrix Val. and Aristichthys nobilis Rich. “Fish Culture and Fisheries”, N 5, pp. 11–13
Vinogradov, V.K. et al., 1963 Methods of receiving breeds of plant-eating fish. “Fish Culture and Fisheries”, N 6, pp. 9–11
Vovk, P.S., 1963 Results of the creation works of the spawner stocks of plant-eating fish in artificial water reservoirs of the U.S.S.R. “Transactions of the First Meeting of the Fish. Reserv. Studies of Georgia”, pp. 155–161
