Currently, the sturgeon population of the Volga-Caspian Sea is in critical condition. The moratorium introduced in Russia on the commercial fishing of sturgeon in the Caspian basin is gradually undergoing the remaining part of the population, the artificial reproduction of species that allow the production of its stocks. Hatchery juveniles, released in the natural habitat, obtained from broodstocks formed at the enterprise and kept for a long time in aggressive condi-tions under the constant influence of stress factors.
An unstable hydrochemical regime, an unbalanced diet, and high stocking densities have a negative im-pact on the physiological state of breeders and, subse-quently, on their offspring (Geraskin et al., 2004) . The most pronounced changes are observed in bio-chemical parameters, including in the lipid composi-tion of gonads and oocytes (Mizenko, 1979; Shatunovsky, 1980) [2, 3].
Oocyte lipids have a higher degree of unsaturation than fats in muscles and other fish tissues. Polyun-saturated fatty acids in the body of fish are involved in the adaptation of the body to environmental condi-tions, are involved in the synthesis of endohoromones that regulate the processes of growth, maturation, and reproduction (Ostroumova, 2012) . In this regard, the study of the lipid and fatty acid composition of sturgeon oocytes under various conditions of keeping will allow us to assess the state of spawners, the qual-ity of offspring, and correct the process of feeding and keeping them.
Material and methods of research
The research was carried out for a number of years at the fish-breeding enterprises of the Astrakhan region during the period of the fish-breeding campaign and in the laboratories under Astrakhan State Technical Uni-versity. The objective of the research was to analyze unfertilized oocytes obtained from starlet females stimulated with gonadotropic drugs. Oocytes were obtained by cutting the oviduct (Podushko, 1986) . The lipid composition of oocytes and fish tissues was studied by conventional methods (Bligh and Dyer, 1959; Rzhavskaya, 1976; Alekseev et al., 1981) [6–8]. The fatty acid composition of lipids was determined by the thin layer chromatography.
Research findings and discussions
The tissues of sturgeons kept in fish cages are characterized by a high supply of phospholipids and triglycerides. Fish kept in closed water supply at
a constant water temperature and their feeding activi-ty were quite high. This provided a high rate of metabolic processes and, accordingly, the accumu-lation of lipids in the muscles (Fig. 1).
Fig. 1. The composition of lipids in the muscle tissue of sturgeon kept under various conditions, %:
а – fish cages; б – recirculating water installation (RWI)
The research findings indicate that the lipids of sterlet’s oocytes are mainly represented in the tri-glycerides. A similar situation is observed in other sturgeon species. The concentration of lipids in the oocytes of fish kept in a recirculating water installation (RAS) was 82.09 ± 1.2%, while that of fish kept in cages was 64,57 ± 1,13% of the total lipids. On comparative basis, the concentration for fish kept in cages was slightly lower. Oocytes of Russian sturgeon females reared in RAS were characterized by the level of triglycerides 80.09 ± 1.12%, beluga sturgeon 79.71 ± 1.11. With cage keeping, a slight decrease was noted – by 2% in Russian sturgeon females and less than 1% in beluga.
Apart from higher amount of triglycerides in fish kept in RWI, a decrease in the level of phospholipids and cholesterol was observed. The opposite result was observed in the oocytes of starlet kept for a long time in unstable hydro chemical conditions - cages in a nat-ural reservoir. The maintenance of the spawner under conditions of unstable temperature conditions led to an increase in the synthesis of phospholipids and cholesterol in the case of a decrease in water temperature, or to digression with a short-term increase in temperature. Thus, phospholipids are used to maintain energy metabolism in fish kept in cages at different periods of the annual cycle (wintering, spawning, etc.) (Ostroumova, 2012) .
The muscle tissue of fish kept in recirculation plants was characterized by a large amount of saturat-ed and monounsaturated fatty acids, in comparison with the tissues of cage reared fish. This is due to their use as an energy source. An increase in the content of polyunsaturated fatty acids in the muscles of fish from cages indicates adaptive expenditure and an increase in lipid fluidity due to an increase in double bonds and
a decrease in fat saturation. During the maturation of reproductive products from muscle tissue, lipid transport occurs and some fatty acids do not accumu-late in oocytes. The analysis of monounsaturated fatty acids showed their smaller amount in the tissues of cage fish. A high level of polyunsaturated fatty acids in such fish indicates their adaptive expenditure. The fluidity of lipids increases due to light increase in double bonds and decrease in fat saturation.
The lipid status of fish is also affected by feeding. Thus, sterlet, which consumed only natural food,
was dominated by more than 20% polyunsaturated fatty acids. About 15% are essential acids of the ω3 class (Fig. 2).
Fig. 2. The amount of fatty acids in the composition of total lipids of sterlet tissues, %
Comparing fish from the replacement group grown under industrial conditions and caught from the natural environment, it was revealed that in the total lipids of fish kept under temperature control conditions, the level of phospholipids was higher and amounted to 41.3%. In sterlet from the natural popu-lation, this value did not exceed 37.3%. The phos-pholipid fraction of sterlet grown from caviar was distinguished by a high content of fatty acids of the linolenic series – 17.4–18.5%.
In the RWI, starlet specimens were kept under conditions with slight changes in water temperature during winter-spring period (7–10 °C), while in the cages, water temperature dropped to 1–3 °C during the winter period. As a result of this temperature, food consumption for fish kept in RAS was quite high indi-cating a high rate of metabolic process, high energy required for movements and formation of reproductive systems. This intensive feeding led to the accumulation of lipid resources in the body.
On the contrary, for fish kept in cages, a decrease in water temperature led to use of more energy for metabolism process and formation of reproductive products, which, in turn, led to a decrease in the level of lipids in the body.
Thus, the earlier temperature condition subjected to fish affected the content of the main lipid fractions (phospholipids and triglycerides) which are actively involved in not only energy of metabolism process but also the reproductive systems. On the other hand, sta-ble temperature condition for the gas regime in RAS did not affect the lipid metabolism of fish. Reserve fats and phospholipids were consumed in accordance with fish body requirements and for fish kept in cages, fat metabolism was more intense, which contributed to an increase in the proportion of phospholipids in the muscles.
Findings from the research indicate a close rela-tionship between the content of polyunsaturated fatty acids in the tissues of aquatic organisms and environ-mental conditions. An increase in the content of poly-unsaturated fatty acids in the muscles of fish from cages indicates more energy use and increase in lipid fluidity due to an increase in energy use during metabolism and reproductive processes and a decrease in saturated fats. During maturation of the gonads, lipids are transported from the muscles, while at the same time, some fatty acids are mobilized from the body, but do not accumulate in oocytes (Lovern, 1942, Malkolm Lav, 1976) and are probably consumed in the metabolic process [9, 10].
The amount of saturated and monounsaturated fatty acids in the muscles of fish kept in RWI was higher than in the tissues of fish kept in cages, which is due to their use of energy sources (Kreps, 1981; Shulman and Yuneva, 1990; Nordgarden et al., 2003; Weber at al., 2003; Murzina et al., 2012; Kalchenko et al., 2013) [11–16]. The fatty acid composition of lipids in fish oocytes kept under different conditions was somewhat different. The provision of oocytes obtained from females in RWI with monounsaturated fatty acids was higher than oocytes from fish kept in cages (Fig. 3).
Fig. 3. Fatty acid composition of unfertilized sterlet oocytes contained in different conditions
This may be explained by the fact that monoun-saturated fatty acids, having a high oxidation rate, were spent to a greater extent on maintaining energy needs and therefore they did not accumulate in the oocytes of fish from cages. In this case, the accumula-tion of polyene fatty acids (40.16-49.3% of the total fatty acids) was the most active.
Thus, the development of thermal adaptation in fish is largely associated with the degree of lipid unsaturation (Kreps, 1981; Sidorov, 1983) [11, 17]. An increase in the proportion of polyunsaturated fatty acids in the eggs of fish kept in cages indicates a fairly high utilization of monounsaturated fatty acids. Of the polyene fatty acids, an important role in metabolism is played by docosahexaenoic acid (22:6ω3) which is most sensitive to changes in the physiological state. The 22:6ω3 is synthesized from 20:5 ω3 (eicosapentaenoic acid), which is involved in reproductive function.
A high level of saturated and unsaturated C18 fatty acids in oocytes is noted compared to low concentra-tion of C16 fatty acids. Of the ω9 fatty acids, oleic acid (18:1ω9) predominated, providing the permeability of cell membranes and being a powerful internal antioxi-dant. The ratio of essential fatty acids in the oocytes of both groups (RWI and cages) was approximately the same as indicated (Тable).
The level of fatty acids in the oocytes of sturgeons kept under various conditions
Fatty acid RWI, % Fish cages, %
Sterlet Russian turgeon Beluga Sterlet Russian sturgeon Beluga
Palmitic 1.41 1.12 1.65 1.61 1.23 2.10
Stearic 1.12 1.27 1.45 1.32 1.31 1.87
Oleic 2.12 1.92 1.5 1.86 1.96 1,92
Linoleic 0.31 0.63 0.4 0.63 0.86 0.78
Linolenic 0.46 0.44 0.34 0.64 0.52 0.66
Arachidonic 0.58 0.64 0.50 0.98 0.91 0.62
Eicosapentaenoic 0.23 0.08 0.12 0.61 0.52 0.31
Docosahexaenoic 0.41 0.38 0.2 2.15 1.25 0.9
However, the influence of environmental factors in cage conditions contributed to a higher accumulation of docosahexaenoic acid in oocytes. The proportion of 22:6ω3 fatty acid in this case was 12.78% of the total fatty acids. In oocytes of fish kept in RWI, these indicators were 3-5 times lower. Research findings indi-cate the special role of docosahexaenoic acid in the adaptation of fish to environmental conditions. Its accumulation in tissues with a negative change in environmental factors indicates an increase in the intensity of lipid metabolism.
The precursor of prostaglandins involved in the reproductive activity of the organism is arachidonic fatty acid (20:4ω6). During maturation of the gonads, its concentration in oocytes was higher than in tissues. Unfavorable conditions during the winter, negatively affected its synthesis of arachidonic fatty acid (20:4ω60) in the body of spawners kept in cages. In the oocytes of females kept in cages, the proportion of 20:4ω6 acid was 6.54% of the total fatty acids, while it was higher by 1.73% in the oocytes of females kept in RAS. Fish breeding and biological indicators of fish with low levels of arachidonic acid were low. The number of fish that matured in cages was 76.38%, maturation was more effective in RAS – 84.83%, with a high percentage of fertilization – 87.38%. The level of fatty acids in the oocytes of sturgeons kept under various conditions, %.
A decrease in water temperature when keeping sturgeon spawners in the pre-spawning period stimu-lates active synthesis and accumulation of docosahexaenoic (22:6ω3) fatty acid in the body. The level of arachidonic (20:4 ω6) acid in oocytes decreases by 1.5 times, which negatively affects the fish-breeding and productive state of Russian sturgeon females by 8.5%, beluga - by 12.7%, sterlet - by 7.5%. The percentage of fertilization is reduced by 4-17% in various sturgeon species.
From the analysis of the findings, it should be noted that the environment of keeping spawners affect the state of lipid metabolism. In one case, they stimulate the synthesis of polyene fatty acids, in particular, docosahexaenoic ω3 fatty acid, which is responsible for the adaptive abilities of the body, while on the other hand, they inhibit the arachidonic ω6 fatty acid necessary for ovulation.