THE PRESENT APPROACH TO THE PROBLEM OF NEUROHORMONAL CONTROL OF THE FISH SEXUAL CYCLE AND TECHNIQUES OF HORMONAL INFLUENCE APPLIED IN FISH CULTURE

by

N.L. Guerbilsky
Professor of Leningrad University

The method of pituitary injections has been used in our country for fish rearing purposes for nearly 30 years. This method, which ensures the controlled development of the spawning state, has been evolved on the basis of analysis of the mechanism of the system which is chiefly responsible for the sexual cycle and spawning control. Generally speaking, the reproductive system in all fish is similar but at the same time it is highly adaptable due to the propagation process adaptations of numerous and diverse species. (See Fig. 1)

This simplified diagram shows the organs of the reproductive system and their functional relations during the period when females and males become spawners. A profound analysis of these complex interrelations, as a means of realization of the integrated development of the internal and external parts of the organism is a fascinating and urgent problem.

The development of gonadotrophin and the entire neurohormonal mechanism of the reproductive system control: exteroceptors-analysers, the hypothalamus with vegetative nuclei and their neurosecretory cells, the hypothalamal pituitary neurosecretory path, the pituitary gland, sex glands, their interoceptors and afferent relations, the central nervous system, and spawning behaviour is an indication of aromophosis, which is an essential factor in the evolution of the vertebrates, fish and Cyclostoma in particular.

The idea of controlling the spawning state development is based on this system and is extremely simple.

Fishery management experience has shown that spontaneous spawning state development is not determined only by the temperature and hydrochemical conditions. The combination of conditions required for the development of the spawning state by a given species comprises some specific features characterizing the “spawning situation”. Convincing illustrations of the above statement are: application of fresh sod in case of a delay in spawning as a result of the sitting of the pond beds, soft vegetation is very popular in carp rearing; adequate spawning of Vimba vimba natio carinata in running water ponds with pebble bed soils (the Kuban vimba and chalcalburnus nursery and the Don sturgeon and vimba hatchery); mass spawning of orpha (Leuciscus idus m. orpha) within a very short time following the treatment of the pond with bunches of fresh green juniper branches and other cases. When some elements of the required combination of conditions are available, it is the exteroceptor response to a number of specific features of spawning grounds required by a given species, that starts the sequence of phenomena characterizing the spawning state development.

Such imitation of spawning ground conditions, used as a method of the spawning state development control, without additional influencing, does not meet commercial production requirements because this procedure cannot be effectively planned as far as fish output indices and calendar terms go. Moreover, use of this method of reproduction of wild fish such as the Acipenseridae and many bony migratory and semi-migratory fish in natural water bodies involves complicated construction measures and does not meet the economic requirements.

To obtain eggs and sperm of such fish, the method of pituitary injections has been developed and is being successfully applied in fish culture. For instance, all eggs are obtained only by this method on the sturgeon farms on the lower Volga, Ural, Kura, Kuban, Don, Dnieper and other rivers (Guerbilsky, 1941, 1947).

The method of pituitary injections has become especially important in fish culture because of dam construction. As a result of this construction, many rivers have undergone such changes that propagation of some migratory species is either inhibited considerably or even halted to such a degree that the formerly effective natural propagation has to be substituted by the operation of fish farms.

Application of the method of pituitary injections allowing us to shift the terms of spawning or to cause a mass spawning of a considerable number of fish for production purposes, is beneficial for pond fisheries as well. Furthermore, this method may be widely used in fish acclimatization processes. It facilitates considerably fish management acclimatization measures, fish hybridization work and, finally, selection of various pond culture species.

When practicing pituitary injections, we use the hormone which is indispensable for each fish to develop the spawning state, whatever the water body where it propagates.

For all fish, as well as other vertebrates, this hormone is a common element indispensable in the functioning of the reproductive system.

When guiding the development to the spawning state, we use this same hormone but we start the process with the injection of the hormone, skipping other initial stages, i.e., the influence of the spawning ground conditions and all consequent phenomena, the gonadotrophic function of the pituitary gland of the recipient included.

The reaction brought about by this kind of treatment is the transition of the fish from the fourth stage of maturity to the fifth one. Therefore, our main interest lies in the range of the pituitary gonadotrophin influence and in some essential details of the pituitary injection application in fish culture associated with it.

The effect of any influence on the organism depends, both on the nature and intensity of the influence, as well as on the state of the organism responding to this influence. Therefore, the effectiveness of pituitary injections depends, first of all, on the coincidence of the time of treatment with the suitable period of the sexual cycle.

The longer this period, the more opportunities for application of the method of pituitary injections. Therefore, it is necessary to consider in our case the peculiar features of gametogenesis and sexual cycle of the fish concerned.

The substantial dependence of the processes occurring in the organism on natural seasonal phenomena is especially manifest in fish and all other animals with a relatively unstable temperature of the body.

One of the most vivid manifestations of these relations is the seasonal cycle of the reproductive system.

The study of fish ovogenesis has disclosed the stages of this process.

The more or less smooth process of the ovocytes growth, due to the build-up of the cytoplasm and nucleus mass (“protoplasmic growth”), is followed by a qualitatively different and relatively faster growth process, due to the origination and accumulation of yolk (“trophoplasmic growth”).

On attaining the ultimate size due to the trophoplasmic growth, the ovocyte is able to convert to a qualitatively different state characterised by pre-ovulation nuclear processes and (in the ovocytes of the majority of fish) by transformations in the aggregate state of trophic inclusions. These processes terminate in an ovulation. A remarkable adaptive feature, essential for adequate propagation, is the ability of fish ovocytes, to become relatively stabilized whenever the conditions required for the transition from the protoplasmic growth to the prophoplasmic one and from the state of the completed trophoplasmic growth to preovulation nuclear changes are lacking.

At other phases of ovogenesis such a delay may upset the development of ovocytes. This phenomenon accompanied by the mass atresia is observed, for instance, in fish barred by dams on their spawning migration run and at spawning temperatures in case preovulation changes start in ovocytes. Ovulation and spawning, however, do not occur since other factors of the required condition complex are lacking.

The growth of ovocytes in fish spawning in spring is completed shortly before egg shedding begins. Hence, the preovulation nuclear processes and ovulation follow immediately after the ovocyte growth completion. However, the delay in the ovocyte development at the completed trophoplasmic growth stage is characteristic for many fishes spawning in spring. This is another important adaptation, because such fish can spawn in spring immediately, as soon as the required natural conditions develop. This, as is known, takes place at different times depending on the meteorological characteristics of the year.

Such fish retain the ability to become spawners within a short period, subject to suitable conditions, throughout the whole winter. For instance, loach females taken from water bodies in any of the winter months after a pituitary injection at a temperature of 15 to 20°C, start spawning within 45 to 50 hours and preovulation processes in ovocytes, ovulation and the subsequent development of the progeny proceed normally (Tchernyshev, 1941).

Hence, the pre-spawning stage with some fish may last for a rather long time.

The process of fish spermatogenesis, similarly to other animals, differs from ovogenesis greatly.

However, during fish spermatogenesis, delays in the development at some stages of the sexual cycle may be also incident. For instance, the milt normally maintains for a long time the state prior to the next cycle of spermatogenesis. This state is characterized by a prolonged stage of primary spermatagonia. Another typical delay in the sexual cycle is the state of the completed spermatogenesis, the milt ampullas being filled with mature sperm. The milt of quite a number of fish spawning in spring preserves the above condition throughout the whole winter preceding the spawning.

It has been proved on males as well as females of many fish that males in such milt condition can develop the spawning state as a result of pituitary injections at any time in winter, subject to suitable spawning temperatures required by a given species.

This data is absolutely indispensable for an adequate conception of range of the gonadotrophin influence of the pituitary gland. Irrespective of whether we deal with fish characterized by a partial or one-time spawning, the state of the ovocytes of the older generation and the completion of the spermatogenesis process in the milt ampullas, respectively, are the main indications of the completion of the fourth stage of maturity.

The most characteristic feature of the ovocyte development of the older generation (at the fourth stage of maturity) is their polarization, usually occurring only upon the completion of vitellogenesis. The ovocyte polarization consists of the nucleus travelling, together with the parts of cytoplasm free from yolk, towards the micropyle so that the future position of the animal pole of the ovocyte is determined by the micropylar foramen towards which the nucleus is traveling during the ovocyte polarization. The eccentric position of the nucleus indicates that the egg is maturing. When the nucleus is displaced completely, it is seated immediately under the micropyle in the protoplasmic disc at the animal pole.

On the basis of our previous experience gained in the work with the Acipenseridae, we can state that the central position of the nucleus in the ovocytes of the older generation of these fish coincided in our experiments with the negative results of the pituitary injections, thus the polar differentiation of the ovocytes of the older generation being criterion for the use of pituitary injections.

Another significant criterion, equally applicable both to the Acipenseridae and to bony fish, is the condition of the nucleolar apparatus in the ovocyte nucleus. During the whole period of the protoplasmic and trophoplasmic growth, the ovocyte nuclei are characterized by a great number of nucleoli tightly contiguous to the nucleus membrane. The travel of the nucleoli towards the nucleus center and their dissolution in caryolymph is a very important and characteristic initial indication of the transition of the nucleus from embryonic bubble stage to meiosis. This phenomenon is extremely important for application of the method of pituitary injections. As a direct experiments showed, preovulation processes, beginning from the travel of the nucleoli, occur under the influence of the pituitary gonadotrophic hormone, i.e., fall within the range of its effect.

As mentioned above, the main process during this period is the transition of the nucleus to the metaphase of the first meiotic division. The entire complicated process, which is inadequately termed by some authors as “the dissolution of the nucleus”, should be broken up into the following phases: travel of the nucleoli from the nucleus periphery to its central region; polarization and dissolution of the nucleoli; emergence of filiform chromosomes; isolation of the whole set of chromosomes from the composition of the embryo sac into a generative nucleus of a considerably small size; transition of the generative nucleus to the metaphase of the first meiotic division; the first meiotic division, and emergence of the first reduction body suspension of meiosis in the stage of metaphase or anaphase of the second division, which generally occurs after the ovulation and ends upon penetrating of a sperm into an ovum. This pattern of the ovum maturing process can vary in particulars but on the whole, it is typical of all fish.

To ensure effective application of pituitary injections and to forecast the moment of maturing precisely enough, it is essential to have an idea of the duration of this process occurring under the influence of the gonadotrophic hormone. Special experiments were carried out to elucidate this problem. (T.I. Faleeva, 1953); (B.N. Kazansky, 1953).

Simultaneously with pituitary injection, by means of a metal probe or through an opening, samples were taken of the ovocytes of the sturgeon females. Then similar samples were taken from the same female every five hours during the period of maturing after the pituitary injection until the completion of the ovulation. On the basis of the experiment, it was revealed that, at the temperature of 18 to 20°C during the first five hours after the pituitary injection, the nucleoli disappear simultaneously, with chromosomes emerging in the nucleus in the form of pairs of crossed, slightly nodular threads. At the same time the rear edge of the nucleus becomes scalloped and this section of the nucleus displays an accumulation of plasm, almost devoid of yolk. During the following five hours, as the nucleus volume decreases, the accumulation of protoplasm free from yolk near the nucleus increases, so that 9 to 10 hours after the pituitary injection treatment the nucleus volume drops about 300-fold. (B.N. Kazansky, 1953). Subsequently, this transformed nucleus undergoes changes typical of the prophase which immediately precedes the first meiotic division. Fifteen hours after the injection one can observe the metaphase of the first division of maturation and 20 hours later - 1.5 to 2 hours prior to the completion of the ovulation - the nucleus develops the state of metaphase of the second division of maturation. There is no doubt that the nuclear processes described occur due to the gonadotrophic hormone of the pituitary both in nature and in the experiments.

As mentioned above, the rate of these processes depends on temperature - and, within a certain temperature range, it is in direct proportion to the temperature. The correlation between the duration of the preovulation nuclear processes and ovulation and the duration of embryogenesis (the duration of the embryo development till hatching) at a given temperature is based on this and is of great importance for scientific and fishery management purposes. Recently T.A. Detlaf and her collaborators have ascertained that the duration of the period lasting from the injection to the complete development of the fifth maturity stage by the sturgeon is from 17 to 23 percent of the time required for the completion of the embryogenesis at a given temperature. This has provided the authors an opportunity to recommend convenientcalculation tables which help determine the most expedient time for opening females and obtaining eggs at sturgeon hatcheries. This permits us to examine much fewer females, to raise the production quality and to make the planning of the work easier. (T.A. Detlaf and S.E. Zubova, 1962).

When dealing with bioactive substances, such as the gonadotrophic hormone, dosage is of vital importance. This has caused considerable difficulties which cannot be considered to be overcome until the present moment. While the exact determination of dosage is quite feasible when dealing with chemically pure preparations, synthesized hormones, for instance, dosage of preparations which comprise various by-substances apart form the required hormone is complicated and difficult.

For pituitary injections we use acetone-treated pituitaries ground to powder. It is made of fatless and completely dried pituitaries of fish. The share of hormone pure gonadotrophic in the whole mass of the powder, prepared as suspension and injected by means of a syringe into a special muscle of a fish, is not known. It is especially difficult to find an answer to this question because the concentration of the gonadotrophic hormone in the fish pituitary does not remain the same during the year and varies both within the seasonal cycle and the whole life of a fish. Therefore, expression of the optimum dosage of the acetone-treated pituitary powder in weight units, may lead to serious errors and failures in experiments and production.

In such cases the endocrinologists apply test-objects which permit the use of biological units to measure the preparation activity. For instance, our laboratory uses loach units to measure the fish gonadotrophic hormone (Kazansky and Nusenbaum, 1947; Kasansky, 1949). The unit is assumed to be the amount of powder (in milligrams) which is sufficient to cause ovulation in a loach female of average weight in the fourth stage of maturity at room temperature. Having developed such methods of testing, or in other words, the methods for determination of the preparation activity, we can easily convert biological units into weight units. For instance, when determining the minimum effective dosage of pituitary of the wild carp, it turned out that the dosage was 0.2 mg per one female loach weighing 35 to 45 g at a temperature of 16 to 17°C. Thus, one milligram of the powder made of the acetone-treated pituitary of the wild carp represented five loach units of the gonadotrophic hormone.

The experience with pituitary injections application shows that excessive doses are especially harmful (See table 1).

In the course of the application of the method of pituitary injections in carp pond fisheries, certain results were obtained which are essential for understanding the functional mechanism of the spawning state development and for further perfection of this method in fishery management practices. The experiments carried out in 1941 in the Novgorod region (Chernyshov, 1941), in the Estonian S.S.R. (Guerbilsky and Nusenbaum, 1949) and in the Latvian S.S.R. (Kazansky, 1950) proved that the application of the injection method in carp culture permits us to cause simultaneous spawning, subject to favorable temperatures, at the desired time and to plan all fish culture work connected with fry rearing (Chernyshov, 1941). Later the dose of pituitary was determined empirically within the limits of spawning temperatures (25 to 30 loach units for females and 10 to 15 for males). One mg of dry powder made of acetone-treated pituitary of the wild carp in the fourth stage of maturity (i.e., in late autumn, winter or spring, prior to spawning) contained from 3 to 5 loach units and bream pituitaries contained 3 to 4 loach units. With this dosage spawning commenced 20 to 36 hours after the injection (Kasansky, 1950).

Table I

Effect of various doses of pituitary on the quality of eggs of the Kurinsky Sturgeon (S.V. Bank, 1949). Water temperature 13.3 to 15.2°C.

(note: All ten females were collected in the course of one day, delivered to the fish farm and treated with an injection at the same time and were kept under all other equal conditions).

Czechoslovakian fish culturists have attained considerable suceess in the application of the pituitary injections, with a view to securing early mass carp spawning in spawning ponds. Janecek (1953) cites the best dosage determined empirically of one and one half pituitaries per female. High doses (four pituitaries) produced a pathological result: the fish kept swimming violently for ten hours but did not spawn, the carp organism developing considerable pathological changes as a result.

Havelka cites good results but points out various abnormalities in the spawning process and also emphasizes the problem of dosage under “hypophysation” application conditions (Havelka, 1952). Such well-know Czechoslovakian fish culturists as Kostomarov and Hochman pay special attention to selecting the precise dosage and underline the detriment caused by excessive doses (Kostomarov, Hochman, 1957). On the basis of comprehensive investigations, these authors believe that the optimum dosage is 0.7 to 0.8 mg of acetone-treated pituitary per 1 kg of the female live weight.

The investigation conducted by Czechoslovakian authors Cernaev and Bena is of particular interest (Cernaev, Bena, 1954). They observed an accelerated development of the spawning state by the mirror carp “nests” under the so-called “combined stocking” conditions, i.e., when the treated males were stocked into the spawning ponds together with intact females and the treated females together with intact males. In this case we observe distinctive interaction of the sexes when becoming spawners and a manifestation of the leading role of the nervous system in this process.

There is still another methodological aspect to be considered. Generally, when applying pituitary injections, we administer the whole dose of the hormone at one time but this is not the way it occurs under natural conditions. The build-up of the acting gonadotrophic hormone takes place gradually as it is released from the pituitary gland, so that its effect is prolonged. If the organism is ready to develop the spawning state, this difference is probably of no significance. When intending to do this with regard to an organism which is not quite ready, when the polarization of the older ovocytes is not yet completed or if the development of this process in various ovocytes of the same generation is asynchronous, then it is quite possible that partial injections, i.e., administration of the dose by portions, may be quite beneficial.

The experiments of this kind contribute to lowering production losses and are of scientific and practical interest. It is the gradual (in two stages) administration of the powder suspension of acetone-treated pituitaries that has proved to be an extremely important variant of the method when dealing with the White Amur and the Hypophthalmichthys molitrix Val. Fish culturists in China (mainland) and Konradt came to the above conclusion independently, while rearing these species on pond fish farms (Konradt, 1961). He theoretically substained the following pattern of influence on White Amur and Hypophthalmichthys molitrix Val: if the permissible dosage of the pituitary substance for White Amur and Hypophthalmichthys molitrix Val spawners is 14 to 16 mg per female weighing 8 to 10 kg, it should be administered by portions. At first, a very small dose of 2 to 3 mg per female is required for leveling the gonad conditions. This dose is too small to induce ovulation in fully mature females. In less mature females, this dose can induce preovulation nuclear changes in the ovocytes within a short period of time and prepare the fish organism for a positive reaction to the next injection of the permissible dose of the hormone. We recommend administration of this dose 24 hours after the preparatory injection.

The method of pituitary injections is used in many countries. For instance, the American specialists (Atz and Pickford) emphasize the possibilities of the wide use of this method. (Atz and Pickford, 1959).

The method of pituitary injections has been tested by long-term production practice and has become routine in fish culture and fish acclimatization work. It is very important in sturgeon raising, the basis for intensification of the semimigrating fish reproduction farms, a method successfully applied in carp culture and also in fish selection and hybridization work.

However, the experimental work on hormonal control of the fish sexual cycle and on controlled spawning is not completed at this stage. It is still continuing in order to overcome some difficulties arising when the pituitary injection method is applied. It is evident from the above that the precise dosage of the preparation under production conditions poses considerable problems. We should also add that, as application of the method in fish culture develops on an ever-growing scale, the demand for pituitaries increases along with the involved difficulties in their mass procurement. Therefore, the tendency of looking for a substitute of similar effect, production and dosage of which could be carried out under industrial conditions at pharmaceutical plants, is quite justified.

In 1947 our laboratory, in collaboration with the endocrinological staff of the Central Institute of Obstetrics and Gynecology, worked out methods for diagnosis of early pregnancy with the aid of loach as test objects. The loach ovocytes attained maturity and females ovulated after an injection of four ml of urine of a pregnant woman, the females shedding eggs 24 to 72 hours after the treatment. The highest percentage (98.57) of positive reaction occurrence was recorded with the fish treated with the urine of the women in the 7th to 12th week of pregnancy. Injections of urine of 223 healthy non-pregnant women at the ages of 17 to 40 did not produce a single case of positive reaction. (Guerbilsky and Kirshenblat, 1947). This is because urine blood, serum and various tissues of pregnant women contain chorial gonadotrophin which is the acting agent of such injections. Later Kirshenblat used this reaction for the quantitative determination of the chorial gonadotrophin content in tissues, blood and urine, the quantity of this hormone being expressed in “loach units” (Kirshenblat, 1950). For the first time we received choriogonin manufactured at the G. Richter chemical works in Budapest, owing to the courtesy of A.A. Neifach. The staff of his laboratory (The Institute of the Animal Morphology, A Sc U.S.S.R.) uses this preparation generally in obtaining loach eggs in winter for the purpose of cytological and embryological investigations. The first series of our experiments was carried out on loach and ruff females in March-April 1964. The results are shown in Table 2.

The following conclusions were drawn on the basis of these experiments:

1. Choriogonin, as well as acetone-treated fish pituitaries, is suitable for artificial maturing of loach and ruff females from the fourth to the fifth maturation stage and for obtaining mature eggs from them.

2. Subject to favorable results obtained in the leading fishery enterprises in spring and autumn, the organotherapeutic preparation-choriogonin, dosed in mouse units, will serve as a good aid for fishery management in dealing with spawners (Guerbilsky, 1964).

In 1964, in cooperation with our staff members P.E. Garlov and A.A. Bojev, we conducted similar research on zander, wild carp, bream and Vimba vimba at the Aksaisk-Don hatchery. The experiments were a success in the case of zander only. The three specimens of carp did not respond to the treatment at all, despite considerable doses, for instance, 1,500 mEq in the case of bream. But zander females developed from the fourth maturation stage to the fifth stage with 250 to 500 mEq at 10° to 14°, the lower threshold of the dosage effectiveness being around 150 mEq.

Positive results of similar experiments on zander were obtained the same year by our laboratory staff members I.A. Barannikova and I.I. Saenko on the Volga River.

The studies with regard to other species continued in 1965.

The investigations of another possible substitute for acetone-treated pituitaries are of great interest as well. This is the blood serum of mares, with foal preparation SZhK. The results of the application of this preparation on carp and trout pond farms are described by Bulgarian culturists Bratanov, Dikov and Danjchev (1963). The authors set themselves the following tasks:

1. Acceleration of egg and sperm shedding processes and fertilization during the first warm days in spring.

2. Obtaining more alevins of the first spawning with the same number of females and water area.

3. Creation of biological prerequisites for earlier maturing and early shedding of subsequent portions of eggs.

The following stimulants were used: SZhK from a mare in the second month of pregnancy (intramuscular injections of 5 ml; 700 to 1,000 mEq in 1 ml); preparation “Estrovet” (2 ml; 50,000 mEq and “Hyophysin forte” (2 ml intramuscular injections).

After treatment, the females were stocked into the spawning ponds. The beginning and duration of the spawning behavior were recorded. The final estimation of the results was performed on the 21st day after hatching by calculating the alevins. The advantages of one or another type of influence were evaluated by comparing with the check intact spawners which had spawned at the same time in other ponds. Here are some of the figures cited by the authors: in all three series of experiments the treated spawners produced more alevins than the intact ones; after the hypophysin injection it was more by 29.4 percent Estrovet by 33.6 percent and SZhK by 38.7 percent. Similar experiments in 1959 and 1960 yielded the same data. When it comes to the evaluation of these results, certain differences such as the considerable variation of the individual fecundity of the mirror carp females between the intact fish and the experimental series as well as within all the three experimental series were observed, and the results, therefore, are not convincing enough for the forming of an unprejudiced opinion on the effects of various hormonal influences.

Table 2. Artificial Maturing of Loach and Ruff Females (the 4th–5th stages) as a result of choriogonin treatment

Summarizing the results of their work, the authors conclude that the SZhK treatment produces the strongest effect and that, by means of hormonal injections, it is possible to reduce the duration of the spawning process, depending upon the climatic and physiological conditions and that this opens prospects for better organization of fishery management measures.

The experiments conducted by many authors in a number of countries and the integration process of the pituitary injection method into fish culture practice have contributed to the perfection of this method. They have extended the scope of its application and have provided new opportunities for its further development.

Some encouraging progress has also been made in attempts to find new hormonal preparations as substitutes for acetone-treated pituitaries.

Within the same period of time ichthyology accumulated new and valuable data on sexual cycles; gametogenesis and control of these processes in various fish species.

The studies of the localization of the gonadotrophic function in the pituitary gland of the bony fish (Kazansky and Persov, 1948) and in the Acipenseridae (Barannikova, 1949) are of considerable importance. Our knowledge of the range of influence of the pituitary gonadotrophin has been enriched.

Administering considerable doses of acetone-treated pituitary by portions, Sakun has shown that it is possible to bring about meiotic phenomena in salmon ovocytes prior to the completion of the trophoplasmic growth stage (Sakun, 1963). In the course of the experiments on macropodes, the role of the pituitary gonadotrophin in determining the complicated forms of the spawning behavior was investigated (Faleeva, 1963, 1965). The neurosecretion phenomena in the lateral vegetative nucleus of the hypothalamus have been studied and the synchronism of these phenomena, as the spawning period of the wild carp and mirror carp begins, has been established (Polyanov, 1951). The role of the hypothalamus pituitary neurosecretory system in determining the spawning migration of the salmonids and sturgeons has been studied (Barannikova, 1961, 1964, 1965).

All the investigations listed above and many other studies of functional bases for specific adaptations involved in fish propagation contribute to discovering new ways of controlling these processes which will remain an urgent task of fish culturists engaged in working out techniques of fish stock reproduction for many years to come.

The high abundance and extensive range which are the main indications of the species' prosperity, i.e., its biological progress, are based on the inherent hereditary systems of specific adaptations, as well as on those developed in the process of its evolution.

The understanding of these adaptations and their profound analysis form the theoretical basis of the technical control of the abundance and distribution of food fish for the purposes of fishery management.

BIBLIOGRAPHY

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Fig. 1 - Simplified scheme of fish organism transition in spawning period