THE BIOLOGICAL BASIS OF REPRODUCTION OF MIGRATORY FISH WHEN THE RIVER IS CONTROLLED

(in the instance of Acipenseridae)

by

F.I. Vovk,
Candidate of Biological Science
The Volgograd Department of GosNIORKH

There are three species of the Acipenseridae that migrate from the Caspian Sea up to the Volga River to spawn. They are: the beluga (Huso huso Linne), the Russian sturgeon (Acipenser güldenstädti Brandt) and the sevruga (Acipenser stellatus Pallas). The fourth species of the anadromous sturgeon, Acipenser nudiventris Lov, also enters the Volga River but very seldom.

The anadromous Acipenseridae form more or less individual local stocks in the Caspian Sea, their ranges being concentrated around certain spawning rivers. The most abundant stocks of Huso huso Linne, Acipenser güldenstädti Brandt and Acipenser stellatus Pallas spawn in the Volga. The yields of the Volga stocks amount to 70 to 80 percent of the total catch of the Acipenseridae in the Caspian basin. All this makes evident the importance of the Volga in the reproduction of this most valuable fish of the Caspian.

The Volga discharge control, especially with the Volgograd dam (the Volgograd hydroelectric station named after the 22nd Congress of C.P.S.U.) upset the long established conditions for natural propagation of the migratory Acipenseridae and the problem of reproduction of their stock has become extremely urgent. Long-term research on the biological basis of Acipenseridae reproduction has made it possible to outline and partially carry out a large number of measures directed at not only making up for the damage caused by the construction of the dam but also for increased reproduction to ensure the annual output of 40,000 to 50,000 t.

The Acipenseridae are a perennial, slowly growing and late maturing fish. The beluga attain the age of 100 or more years, the Russian sturgeon 50 to 60 years and the sevruga live as long as 30 years.

The average age of sexually mature adult Acipenseridae caught in the Volga is much lower than these age limits. It is 12 to 27 years for the beluga, 10 to 20 years for the Russian sturgeon and 8 to 17 years for sevruga.

Older fish are usually very rare. Since males mature earlier than females, the age composition of a spawning population is shifted by two or three years towards younger age groups, the average length and weight of a fish also being less accordingly.

The maximum weight of a beluga, one of the biggest species of the Acipenseridae, may be as much as one ton and more. Its average commercial weight is 80 to 100 kg in the range of 35 to 300 kg. The average weight of a Russian sturgeon is about 15 kg (males, 12 kg; females, 20 kg). The average weight of sevruga is 8 to 9 kg (males, 7 kg; females, 12 kg).

The beluga attain sexual maturity at the age of 12, the Russian sturgeon at 8 to 9 and the sevruga at 7 to 9 years. The process of maturing of one generation lasts from 7 to 10 years or longer. Maturity depends not only upon the age of the fish but upon their size as well. Thus, mass maturing of male sturgeons occurs when they are 120 cm long, the weight being 11 kg although the age of fish of this size varies from 7 to 17 years or more. The female matures at the age of 9 to 20 years when the body becomes as long as 140 cm and the weight is 18 kg.

The Acipenseridae do not spawn every year but after considerable time intervals. The interval between the first and second spawnings range from three to six years. It is shorter with males.

The duration of the periods between spawning seems to depend upon the degree of exhaustion of the fish in the course of spawning migration and spawning, as well as upon the length of its stay in the Volga after spawning.

The structure of the Acipenseridae population in the Caspian is compound and peculiar. Until the reproduction was upset, the Acipenseridae population in the sea had included fish of many dozens of generations.

According to the age and sexual maturity, the population can be divided into several groups.

The sexual immature percentage of the population makes up about ten generations. Approximately the same number of generations is composed of older fish that serve as recruit stock for the spawning population going from the sea up the river. The rest of the immature fish of these generations stay in the sea as a reserve.

The sexually mature percentage of the population (commercial stock) consists of a dozen of the remaining generations which go through repeated maturing cycles. It is this group that forms the spawning population which leaves the sea annually for a certain period.

Sturgeon can be used as an example to show the relationship between the abundance of these groups of the population and the standing crops (See Table 1).

TABLE 1. RELATIONSHIP BETWEEN THE ABUNDANCE OF THE CONSTITUENT PARTS OF THE RUSSIAN STURGEON POPULATION AND THEIR STANDING CROP

The most abundant population group is the juvenile, the reserve is much less numerous and sexually mature adults are very few. As far as weight goes, the reserve group is leading, next comes the juveniles and then the recruit stock. The remaining part forms the smallest group both in number and weight. The percentage of commercial stock is strikingly low in the general Acipenseridae stock. To insure higher catches of sturgeon on their migration routes up rivers, an enormous stock of sexually immature and repeatedly spawning fish must feed in the sea. The vast Caspian Sea with big food supplies can feed large populations, capable of yielding 40,000 to 50,000 t of fish annually.

With the population structure typical of the Acipenseridae, it is impossible to develop considerable commercial stocks of the fish in small bodies of water, such as reservoirs.

Development of the commercial stock of the Volga Acipenseridae is not only dependent on the food supplies of the Caspian but also on the extent of their propagation in the fresh water rivers. The Volga and its full-watered tributaries and a large spawning range could replenish the Acipenseridae stocks of the Caspian.

All progeny descending from the Volga to the sea is under government protection. Fishing with netting of all kinds is prohibited. The remaining sexually immature fish which stay in the sea between two spawning migrations are also under protection. Such measures serve to maintain the abundance of the stock and also contribute to maximum production of fresh caviar, the most valuable product of the Acipenseridae.

In the postwar years the commercial stock of Acipenseridae has included exclusively the spawning population which is caught in the fore delta areas and in the delta of the river, i.e., on the routes of migration up the river. Commercial fishing for Acipenseridae is completely prohibited in the Volga itself. The volume of commercial fishing is controlled by special regulations which ban fishing completely at certain seasons when the fish run up the river. During the mass run of the Russian sturgeon and sevruga their fishing in the mouth of the Volga is banned for two months (June and July). Only part of the spawning population is allowed to go up the river, their number being determined by the reproduction requirements.

Less than half of the Russian sturgeon and sevruga spawning population stock and one fourth or one fifth of the beluga stock are allowed to pass.

After spawning the fish return to the sea, their number considerably lower. Some of them are caught during the descent and some die natural deaths. Consequently, the remainder that reach the sea is comparatively small and no more than one fifth of the original spawning population.

The recruitment of the Acipenseridae spawning population, i.e., fish spawning for the first time, considerably outnumbers the remainder of the repeated spawning fish.

Such a ratio of the recruitment and the remainder affects other elements of the spawning population structure, particularly its size and sex composition. When reproduction is normal, the size composition of the spawning population is rather uniform. For instance, the average length of the male sturgeon body is 120 cm year in, year out and females, 140 cm. It can be accounted for by the fact that the overwhelming majority of the spawning population is the recruitment stock which mature upon reaching the above size.

Permanent and considerable prevalence of males is quite normal for the spawning population.

Their prevalence is due to their larger share in the remainder of the population (up to 70 to 80 percent) as a result of a higher mortality rate of the female fish during the spawning season.

However, the sex ratio of the recruitment stock does not remain stable either. A certain relationship has been established between the sex composition of the recruitment stock and the abundance of its individual generations.

If a highly abundant generation joins the spawning population, the males will prevail during the first years due to their earlier maturing. Later, in a certain number of years, the abundance of the female fish in the recruitment stock starts to grow. The dynamics of the sex composition, based upon the maturing of males and females at different age, is a reliable indication of fluctuations in the generation abundance and is used in forecasting the dynamics of the commercial stock.

The Acipenseridae spawning migrations up the Volga go through certain stages. The first to enter the mouth is sevruga (from April through August). Their peak run occurs in May. Next comes the Russian sturgeon, with the peak of their run in July. Last to pass up the river is beluga, their peak run taking place in October.

Sevruga enter the river when spawning conditions are ripe and reach the spawning state soon. Spawning over, they immediately start migrating seaward. They stay in the river for no longer than four months. A very small number of sevruga, mostly females, can be encountered there in autumn.

The migrations of the Russian sturgeon and beluga are much longer. The overwhelming majority migrate for about two years. When the spawners enter the mouth of the river, they are still sexually immature (third stage). As they go up the river, they attain the fourth stage of maturity by autumn. They hibernate near their spawning grounds in this state.

In spring, when the water temperature is as high as 8°C, the beluga and the Russian sturgeon (at 12°C) begin to spawn. They keep descending to the sea through the whole summer. Some sturgeon may stay in the river even for a still longer time.

A small fraction of the spawning population of the Russian sturgeon and beluga is late in entering the river. When winter sets in, they stop running and again go on in early spring when already at the fourth stage of maturity. Since they have fallen behind the main spawner school, they do not run far up the river but spawn in the lower pools of the Volga and under somewhat different conditions: during the abatement or at a stable low level of water in river bed spawning areas but not those flooded in spring, the water temperature being higher. When spawning is over, they like sevruga, descend immediately to the sea. The fish hibernating in the river and spawning in early spring are usually referred to as “winter fish” while those spawning in the year they enter the river as “spring fish”.

Newly hatched Acipenseridae larvae leave the spawning ground without delay. The duration of their descent to the sea may be extended, depending on the location of these grounds, so that the fry reach the sea feeding areas at different ages. The spawning areas of sevruga were scattered from the town of Saratov down to the Volga mouth and the fry migrated seaward during the whole summer.

Formerly the spawning areas of beluga stretched to the upper reaches of the Volga and the Kama Rivers and later as far as Kuybyshev. The fry, however, never stayed in the river for long and descended in the course of one year. It is only the fry of the sturgeon that descend slowly at different ages. When they have descended from the lower spawning areas they are fingerlings already, when from the upper ones they are two or three years old, some individuals being even older.

When the Volga discharge control became effective and the construction of the water reservoir cascade was completed, the natural propagation of the Acipenseridae was inhibited to a considerable extent.

Especially great changes have taken place since the formation of the Volgograd reservoir. The spawning areas have been considerably reduced. Some of them have been completely lost, being in the zone of the water reservoirs, others (Saratov-Kuybyshev reach) have been cut off.

The only spawning area available to the fish now is the area stretching from the tailwater of the Volgograd dam to the Volga delta. The general conditions for Acipenseridae spawning in this part of the river have also grown much worse. Due to the lower and shorter water discharge from the Volgograd reservoir in spring, the spring spawning grounds used by the sturgeon are flooded late and for too short a period of time. Daily and weekly fluctuations in the water level, as well as a considerable drop following the spring discharge of water, often bring about drying of some spawning grounds and destruction of the eggs laid there.

The temperature conditions in the tailwater are especially unfavorable for the Acipenseridae propagation. In spring and in the first half of summer (April to July) the water is colder. The total heat decreased by 120 to 200 day-degrees. On the contrary, in the latter half of summer and in autumn (August to November) the total heat increased by 300 to 320 day-degrees, as compared with average value of several years prior to the river control. These changes have brought about a 10 to 15 day delay in the spawning of the Russian sturgeon and beluga. Spawning is now faster and takes place at a low (threshold) temperature, at that. The embryo of the egg also develops at the lowest temperature permissible, this resulting in a higher percentage of nonviable and deformed larvae.

But the high water temperature in the latter half of summer adversely affects the ripening process of sexual products in the winter Russian sturgeon and beluga which gather in the foredam area. A long-term influence of high temperature causes pre-maturing of a considerable percentage of the spawners, so that in autumn males are observed shedding milt. Over-maturing of sexual products can be observed among the spawners subjected to a long-term influence of higher water temperature. Over-ripening, along with the delay in spring spawning, causes the pathological development of sexual products which often ends in ova resorption. Frequent injuries to the spawners influcted when near the dam, as well as the exhaustion of the fish due to their activity late in autumn and in winter are also conducive to this phenomenon. Other environmental factors are of importance, such as the rate of flow, water level fluctuations, etc. They contribute to a higher natural mortality of the spawners, females in particular, and to a lower viability. The unfavorable conditions in the Volgograd dam tailwater not only affect the spawners but also result in a higher natural mortality of the Acipenseridae progeny when migrating downstream at the early stages of development.

Due to the above, the efficiency of natural spawning has decreased and the scale of reproduction has been reduced. The annual Volga progeny of the Acipenseridae spawning in the tailwater by no means secures keeping up the commercial stock of the Caspian. In order to intensify the natural propagation of the Acipenseridae it was decided to expand the spawning range in the headwaters of the Volgograd hydroelectric station with a view to using the still existing spawning areas in the pool of the Volga from Saratov to Kuybyshev. To achieve this, the large-scale transplantation of the spawners caught in the tailwater was carried out by means of live fish containers. Such transplantations are conducted in August and September, being timed to the period of the winter sturgeon peak run. During six years (1959–64) 145,000 individuals were transported and released into the Volgograd reservoir 100 km away from the dam of the hydroelectric station, 125,000 of them being marked with tags fixed to the radius of the thoracic fin. After the fish elevator was put into service, another 78,000 Russian sturgeon and several thousand sevruga were transplanted into the Volgograd reservoir within four years (1961–64).

A total of 223,000 sturgeon spawners (over 3,000 t) were released into the upper spawning areas from 1959 to 1964.

The tagging allowed tracing the behavior of the transplanted fish. It was established that the sturgeon spawners released into the reservoir tend to resume their upstream migration at the regular rate of 17 to 20 km daily. By winter-time they spread along the Volga River near the spawning areas of the Saratov-Kuybysheve stretch. They spawn in spring and then descend to the Volgograd reservoir. The Acipenseridae descend to the tailwater mostly in the second year after their release (the spawning year), fewer in the third year and only a few individuals descend in the fourth year. When in the sea, the tagged spawners are encountered and caught mainly in the northern part of the Caspian, less often in the central part and still less in its southern areas.

Numbers of the descending young fish per catch show that the efficiency of natural propagation on the spawning areas of the headwater has remained the same as it used to be. It has been observed that the relative abundance of the descending fry before and after the Volga discharge control was started, is directly proportional to the number of the spawners participating in the spawning.

These measures aimed at furthering natural propagation have proved to be very effective. The utilization of the upper spawning areas has made it possible to replenish the commercial stock considerably. Such measures are also very important because they have allowed definition of the role played by water reservoirs constructed on spawning rivers in the migratory fish reproduction.

The regular migration routes of the sturgeon fry hatched in the spawning areas of the headwater have been destroyed. Descending from their spawning grounds, the fry enter the water reservoir where they stay for quite a long time. Such a delay in the migration affects the growth, the behavior and the survival rate of the fry. Before the discharge control measures, the sturgeon fry which descend to the sea were, for the most part, fingerlings.

By winter-time their average weight was 5 to 6 g. The weight of the fingerlings descending now from the reservoir to the sea has increased 8 to 10 times, their average weight being as high as about 50 g. On the whole, it takes the sturgeon fingerlings three years to descend from the Volgograd reservoir. The gain in weight during the first year is 53 g, in the second year to 177 g, 250 g in the third and 300 g in the fourth.

The weight gain decreases in subsequent years. The average weight of the descending fry is about 230 g.

The older age and the higher weight gain of the descending fingerlings contribute much to their survival during their downstream migration to the sea. Larger and more viable fingerlings have a better chance to avoid predators than small fingerling. This is especially true for the fingerlings hatched in the tailwater of the Volgograd dam.

Due to utilizing the spawning areas of the headwater, the damage caused by the reproduction by the dam construction has been made up for to a noticeable extent. However, the high effectiveness of this measure is, unfortunately, temporary.

When the Saratov hydroelectric station is completed, it will be effective no longer, since those valuable spawning areas will disappear. Natural propagation will be preserved only along a short stretch of the Volga tailwater, from the city of Volgograd to the mouth of the river. It is evident that there is no possibility of solving the problem of Acipenseridae reproduction under conditions of inadequate natural propagation.

This is why the general scheme for the development of Caspian Acipenseridae reproduction treats natural propagation as a matter of secondary importance. The emphasis is laid on the development of large-scale artificial propagation. Only in this way may it be possible to provide enough Acipenseridae in the Caspian Sea to fully utilize the available food supply.

Hatchery rearing has been adopted as the basis for the artificial propagation of the Acipenseridae. Hatcheries are planned for construction in the lower pools of the rivers. Soviet fish culturists have worked out the fundamentals of the biotechnics of artificial propagation. The method of pituitary injections for stimulating the maturing of spawners is part of its foundation. Egg incubation is carried out in special incubation apparatus. Round tanks of VNIRO (BHMPO) type are used for advanced larvae rearing. Then the fry are reared in nursery ponds until they attain the standard weight.

The last stage is releasing the fry into the river or transporting them to their feeding grounds in the river mouth areas of the sea.

BIBLIOGRAPHY

Alyavdina, L.A., 1954 Spawning conditions for anadromous sturgeon fish below Stalingrad. Trans. of Saratov section of the VNIRO, Vol.3.

Babushkin, I.Y. and M.P. Borsenko, 1951 Caspian sturgeon fish. Moscow. Pishchemproisdat.

Batichkov, G.A., 1963 Some results of the mass tagging of the Volgo-Caspian sturgeon in the Volgograd water reservoir and below the hydroelectric power station dam. Sturgeon culture in the water reservoirs of the U.S.S.R. Moscow, Edition of the Academy of Science of the U.S.S.R.

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Ginsburg, Y.I., 1965 Influence of the dike in the Volga River on the spawning of anadromous sturgeon fish and the biology of their juvenile stages. Trans. of the Volgograd Section of the GosNIORKH Vol.2.

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