by
N.I. Nikoljukin
All-Union Research Institute of Marine Fisheries and Oceanography
Moscow, U.S.S.R.
Sturgeons (Acipenseridae) are known to interbreed under natural conditions, giving rise to viable and sometimes fertile interspecific and intergeneric hybrids. Hybrids have been described from crosses of various combinations of almost all species of the family. L.S. Berg in his well-known book “Freshwater fishes of the USSR and the neighboring countries” (1948) enumerates the hybrid forms of this family from the following crossings:
Huso dauricus × Acipenser schrencki
(kaluga × Amur sturgeon)
H. huso × A. nudiventris
(beluga × spiny sturgeon)
H. huso × A. güldenstädti
(beluga × sturgeon)
H. huso × A. stellatus
(beluga × stellate sturgeon)
A. nudiventris × A. stellatus
(spiny sturgeon × stellate sturgeon)
A. ruthenus × A. güldenstädti
(sterlet × sturgeon)
A. ruthenus × A. stellatus
(sterlet × stellate sturgeon)
A. güldenstädti × A. stellatus
(sturgeon × stellate sturgeon)
A. baeri × A. ruthenus
(Siberian sturgeon × sterlet)
This list does not include the whole variety of acipenserid hybrids that can occur in nature, as for instance the hybrid between beluga and sterlet (H. huso × A. ruthenus). The capacity of Acipenseridae to interbreed resulted in higher variability and formation of different varieties.
The first artificial hybridization of Acipenseridae was accomplished by F.V. Ovsyannikov, who as early as 1869 fertilized the eggs of sterlet (A. ruthenus) by the sperm of sturgeon (A. güldenstädti) and stellate sturgeon (A. stellatus), and drew attention to the importance of research in this direction. Later, however, experiments on hybridization of Acipenseridae were discontinued for a long time. Investigations were resumed in 1949 in the Volga River, in line with the research programme of VNIRO (All-Union Research Institute of Marine Fisheries and Oceanography). The task set was to combine in the hybrid organisms the properties of such large anadromus Acipenseridae as the beluga (H. huso) and the sturgeon (A. guldenstädti) with the properties of the freshwater sterlet. It was assumed that products of the cross between sterlet and sturgeon or beluga, with increased adaptive properties and heterosis, would be distinguished for their economic value as compared to the parental species. Since hybridization involved the sterlet, which attain sexual maturity earlier than other Acipenseridae, it was assumed that the hybrids produced would differ favourably in this respect from late-maturing anadromous Acipenseridae. Experimental research confirmed the soundness of these suppositions. It is expedient to make use of the heterosis of the hybrid forms of Acipenseridae when rearing them in freshwater ponds and reservoirs.
Though Acipenseridae are suited for commercial breeding in ponds they have seldom been used for this purpose in the past. Sterlet were successfully bred in ponds for many years, but this practice has now been nearly abandoned because of the slow rate of growth of this fish.
Of interest in this respect are some hybrid forms which adapt themselves more easily to pond conditions that are unusual and hardly suitable for Acipenseridae. Besides, the favourable characteristics of hybrids based on heterosis, viz., rapid growth, greater viability, and early sexual maturity may be advantageously used.
Among the acipenserid hybrids studied, the most promising in respect to fisheries is the hybrid between beluga and sterlet, which favourably combines the valuable properties of both parental species: the rapid growth rate of beluga and the early maturity of sterlet.
Even in ponds of low fish productivity, where the potential speed of growth of the hybrid could not manifest itself to the full, it reached the weight of 500 g by the end of the second summer, while sterlet under the same conditions grew much slower. In one of the ponds of the Teplovsk fish hatchery (Saratov region) which has greater food reserves, the hybrid fingerlings reached the mean weight of 74 g in one summer, the yield of fish being 580 kg/ha. In the other ponds where the hybrids were cultured along with carp, the fish production did not exceed 300 kg/ha, the growth of the hybrid being slower as they could not compete successfully with carp for benthic food.
The potential rate of growth of hybrids is quite high when sufficient food is available, one-summer fish growing to an average weight of 0.5 kg. This potential for rapid growth can be used advantageously by rearing them in ponds to marketable size with the help of intensive artificial feeding. Within two growing seasons it is possible to produce marketable hybrids weighing nearly 1 kg.
Since 1966 the efficiency of breeding marketable stocks of hybrids in some fish farm ponds of the Soviet Union is being reviewed. For this purpose VNIRO has worked out “Directions on pond breeding of marketable hybrids of beluga and sterlet by intensive feeding” (1966). The directions give special attention to the bio-techniques of feeding young hybrids and to the adverse effects of growing carp and hybrids together; it is also pointed out that fish ponds should not be overgrown with filamentous algae.
The favourite food of very young hybrids is larvae of Chironomidae, called bloodworms. Later, those that have inherited predatory instincts from beluga start feeding on larger animals such as tadpoles, small frogs and any small inactive fish. Fingerlings of hybrids can attain average weight of 90–100 g in ponds with abundant food reserve, whereas in the sea, where the standing crop of benthos is many times higher than in ponds, e.g., in the Taganrog Bay of the Azov Sea, they average 500 g. Besides using benthic invertebrates, hybrids in the sea feed on numerous slow-swimming bottom fish, mainly Gobiidae, which are not to be found in ponds.
To supply Acipenseridae with a sufficient quantity of food in ponds, supplementary artificial feeds should be introduced on feed tray. Since all Acipenseridae in nature feed only on animal food they should be fed on food of animal origin too: scrap fish or fish of little value, wastes of fish and meat processing plants, various invertebrates such as Gammaridae, etc. It is desirable that all these products be given fresh or frozen, and if this is impossible, these feeds can be preserved with sodium pyrosulphate. The conversion ratio (food per unit of weight gain of fish) may be assumed as equal to 7.
The stocking rate of the young may vary, approximately 15 000 to 20 000 individuals per ha, depending on the natural fish productivity of the pond and on the possibilities of regularly introducing supplementary food, starting, at least, from the second half of the growing period.
The stocking rate of yearlings is 2 000 individuals per ha and two-year-old hybrids are fed intensively during the whole growing period. If the rate of stocking is higher feeding should assume still more importance, since the capacity for natural fish productivity of ponds decreases. The rearing practice should then acquire the character of tank fish culture.
To raise fish production in ponds, it is advisable to rear hybrids together with herbivorous fishes of the family Cyprinidae, provided they are of the same age group. The stocking rates of hybrids, mentioned above, may be retained, but it is desirable to lower the usual stocking rate of herbivorous fishes by approximately 30 percent.
In the autumn of the second growing season, two-year-olds weighing not less than 800 g are delivered to the market. A number of hybrids that have not reached marketable size are left to grow in the pond for a third summer.
Due to increased viability, hybrids winter well in the usual carp ponds; the mortality rate, even among one summer fish, being quite insignificant.
The tentative biological standards, worked out by VNIRO and tabulated below, may serve as a general guide for producing a marketable crop of hybrids of beluga and sterlet.
| Rearing time | Stocking rate per ha | Yield | Fish productivity kg/ha | |||
| Number in thousands | Weight (g) | Percent | Number in thousands | Average weight (kg) | ||
| First summer | 20 | 3 | 50 | 10 | 0.08 | 800 |
| Second summer | 2 | 80 | 80 | 1.6 | 0.8 | 1 150 |
The table presents minimum yield indices which may be obtained even in non-specialized carp-type fish farms.
By way of illustration the results of rearing hybrids in the Aksay fish farm in the Rostov region are of interest. In 1965, pond no. 3, with area of 0.1 ha, was stocked with 3 700 young hybrids weighing 4 g each. The natural food resources were extremely poor; the standing crop of benthos averaged 0.13 g/m2. The young were fed mainly on minced fish. The yield (survival) of young of the year, with average weight of 62 g, was 58.4 percent of the number of fingerlings stocked. Fish productivity equalled 1 330 kg/ha. Two-year-old hybrids in the farm averaged 0.9 kg the maximum individual weight being 1.8 kg.
In 1967 an experiment was undertaken in culturing mixed age groups of one-summer fish and three-year-olds. Pond No. 4 of 0.1 ha area was stocked with 1 000 fry of 4 g mean weight and 213 two-year-olds with mean weight of 700 g, the total stocking rate being 12 130 individuals/ha. As a result, fingerlings of average weight of 85.2 g (yield - 72 percent), and three-year-olds of average weight of 1.94 kg (yield - 80 percent) were produced. Fish productivity of the pond was 570 kg/ha for fingerlings and 2 200 kg/ha for three-year-olds, the total productivity being 2 770 kg/ha.
Of specially great significance is the unimpaired ability of the hybrid of beluga and sterlet (different genera) to reproduce. This is a very rare phenomenon in intergeneric hybrids. The onset of sexual maturity in the hybrid is early; in males - at the age of 3–4 years; in females - from the age of 6–7 years. The size of eggs of the hybrid is nearly the same as that of the stellate, but smaller than Huso huso.
The fecundity of the hybrid and the high flavour qualities both of the flesh and of the caviar increase its value in commercial culture. The possibility of producing second and subsequent hybrid generations opens prospects of selective breeding of new varieties of Acipenseridae.
Thanks to heterosis and the heightened adaptive plasticity, the sexual products in the hybrid broodstock mature even when they are bred and reared in ponds throughout their whole life cycle, never entering a river or a sea. The fish cultural problem of producing progeny of Acipenseridae has thus been solved, even raising them entirely under pond conditions. Here the positive role evidently has been played by the method of distant hybridization. However, numerous attempts to achieve this with thoroughbred Acipenseridae have not been successful so far.
The high viability and adaptability of the hybrid is illustrated by a batch of fish that for a long time (about 6 years) starved in the ponds to such a degree that they not only did not gain weight during the growing season, but many of them lost weight, could in the subsequent years when fed well produce both sperm and ripe eggs.
The unusual viability of the hybrid made it possible to use the method of stripping eggs, by incision, from live females that had received hypophyseal injection, after which the incised abdominal wall was sutured. The operation was well endured by the hybrid females. By this technique, I.A. Burtsev (in 1967) obtained eggs for fertilization and production of the second hybrid progeny from 5 females averaging 8.3 kg. The eggs from one female weighed 1 kg. The females which were operated on were returned to the pond of the Aksay fish hatchery. Examination of the females in October, when transplanting them to wintering pond, showed that they were in good condition; the sutures were completely grown over and hardly noticeable as pale stripes. The females not only survived but gained considerable weight during the growing season, from 1.7 to 3.6 kg. Thus the operation did not have any adverse effect on the subsequent condition and growth of the females. The technique of successfully obtaining eggs from living females, applied for the first time in sturgeon culture, might enable the repeated use of individual brood fish. This makes it possible to select females by the quality of their offspring, which must play a significant role in selection work.
The second brood of hybrids of beluga and sterlet (F2) was first obtained in the Rogozhinsk sturgeon hatchery, located in the Don River delta. The first was produced in 1966, and a considerably greater number (33 000 fry, reared to average weight of 3.5 g) was produced in 1967. The fish production indices (percentage of fertilized eggs, survival of young in reservoirs and ponds) proved in the majority of cases to be not lower, but at times even higher, than in the hybrid F1.
Nearly all the young F2 offspring were transported to the Don fish rearing plant in the Donetsk Region for pond culture of the young-of-the-year and for carrying out selection experiments. Some of the young produced from the eggs of each of the five females were introduced separately into five small ponds, the one-summer fish averaging 79.4 g, the maximum individual weight being 250 g.
In rearing one-summer fish of hybrid F2 and silver carp (Hypophthalmichthys molitrix) together, the total fish productivity of one of the ponds equalled 1 300 kg/ha.
Morphologically, both hybrid F1 and F2 take an intermediate position between the parental species, but in fact there is a much greater variety in F2 individuals. As compared to F1, it especially varies in body colouration. The individual variability in F2, of some countable traits and measurements, is also higher.
It is of interest to note that cytogenetically both broods resemble each other and the parental species: the modal number of chromosomes in all of them is 60, varying somewhat greater in F2 than in F1 and the parental species. This follows from the comparison of the variation sets. Consequently, the hybrid is characterized by a heightened variability both morphologically and cytogenetically (Nikoljukin, 1966).
The possibility of producing new breeds of the beluga x sterlet hybrid should find application in the production of new breeds with stable heredity, especially the pond breed of Acipenseridae. On the other hand, the possibility of producing progeny from hybrid broodstock reared under pond conditions opens prospects of organizing specialized sturgeon farm ponds covering all the processes of fish culture: from obtaining fertilized eggs to producing a marketable crop without obtaining fry hybrid from sturgeon hatcheries.
Due to the wide range of adaptability of the beluga x sterlet hybrid, it can dwell in purely fresh waters ponds, or in brackish water bodies such as the Proletarsk reservoir, or in salt waters like the Azov Sea (Nikoljukin, 1964).
In 1962 a hybrid was introduced into the Proletarsk reservoir, situated on the brackish Zapadny Manych River (tributary to the Don River). The zone of flooding of the reservoir's eastern part wholly covered the salt lake Gudilo, occupying over one-third of the whole reservoir. In late June the reservoir was stocked with 330 000 young hybrids with average weight of 7 g. In late August one and a half to two months after stocking, seine catches revealed that the young had survived and grown better than in ponds, the majority of hybrids having attained over 100 g.
In 1966, in the fifth year of life, many individuals attained the weight of 8 kg. This same year, mature males with ripe sperm were found. It may be expected that females will also start maturing in 1968.
Though the purpose of the introduction of the hybrid into the reservoir was mainly to breed the fish as a marketable product, mature spawners may also be used for artificial propagation at the sturgeon rearing station in order to obtain new hybrid offspring.
Hydro-electric power projects on large rivers such as the Don, have strongly interferred with the reproduction of Acipenseridae, barring their way to the most important spawning areas. The efficiency of natural reproduction of Acipenseridae, the area of which is confined to the lower reaches of the Don, has gone down to such a degree that the progeny of beluga and sturgeon, in most cases, cannot be considered to be of any significance. Reproduction of Acipenseridae has become possible mainly due to their artificial breeging in hatcheries.
Of great interest are the experiments being carried out at VNIRO for producing a new form of Acipenseridae by hybridization and acclimatization of this new form in the Azov Sea. The aim is to produce a hybrid with the rapid rate of growth of beluga and attaining maturity much earlier (Nikoljukin, 1964).
The introduction of the hybrid of beluga and sterlet into the Azov Sea is worthwhile because the heterosis of the first generation may be used for increasing the fish productivity of the sea with the valuable Acipenseridae. An even more important task is to create a new, early-maturing form able to reproduce more efficiently in the Don below the Tsymlyansk hydroelectric power plant.
The basis for obtaining this new form (breed) is the hybrid between beluga and sterlet, the young of which have been released in the Don delta since 1963. From here the hybrids have spread into the Taganrog Bay and the Azov Sea proper, where they grow rapidly, predating on other fish. The survival rate of the hybrid is high. Thus, in the autumn of 1964 young hybrids prevailed in the commercial catches in the lower reaches of the Don and in the Taganrog Bay, constituting 50 percent of the total catch of young Acipenseridae.
The young of the year feed mainly on Musidacea and fish, whereas two-year-old hybrids feed exclusively on fish (Gobiidae and Clupeonella sp).
Intensive feeding accelerates the sexual maturity of the hybrids. In the autumn of 1965, a 3 kg male aged 2+, with well developed tastes at maturity stage IV, was caught in the Don. This testifies that males become mature by about 3 years of age.
In 1967, in the fifth year of life, a more intensive spawning migration of males from the sea into the river was observed. By this time the hybrids had attained the weight of 13 kg. Partial onset of maturity may be expected in 1968.
When producing the new hybrid form, the following sequence of crossing is to be observed:
Beluga × sterlet (F1)
Beluga × (beluga × sterlet) (FB)
(Beluga × sterlet) × /beluga × (beluga × sterlet)/
The adult male spawners of the first hybrid brood are used for backcrossing with beluga females, whereas females of F1 are crossed with the males of the backcross (FB).
Of the three crossings, the first two (F1 and FB) have been repeatedly made by us, yielding positive results. The last (third) cross of the programme scheduled, will be made approximately in 1970–1971. In the progeny from crossing F1 and FB, the hereditary characters of beluga will prevail (5/8) over those of sterlet (3/8); and consequently they will evidently surpass F1 in the rate of growth.
By the crosses mentioned above, the heterosygosity and heterosis of the hybrid forms will be maintained at a certain level. The hybrid form from the last (interhybrid) crossing should be propagated.
When producing a new form, selection experiments are aimed, above all, at rapid growth and early sexual maturity.
Berg, L.S., 1948 Freshwater fishes of the U.S.S.R. and the neighbouring countries. U.S.S.R. Academy of Sciences Press.
Nikoljukin, N.I., 1964 Hybridization of fishes and its acclimatization. Transactions All-Union Research Institute of Marine Fisheries and Oceanography, 55(2), In Russian.
Nikoljukin, N.I., 1966 Some questions of cytogenetics, hybridization and systematics of the Acipenseridae. Genetics (USSR), (5)
