Due to the deterioration of the conditions of natural reproduction of populations of industrial fish species, aquaculture is becoming increasingly important in Ukraine as a crucial factor in meeting the needs of mankind in protein food (Archibisova and Suslov, 2018). As fish and fish products occupy a significant place in human nutrition, fish receive about 40% of protein foods of animal origin mainly from aquatic organisms (Jennings, 2016).

Ukraine has all the opportunities for efficient fisheries, increasing domestic production through the development of fish farming, which over the past 5 years, in contrast to the dynamics of industrial catches is characterized by certain stability and provides annual production of 20 – 25 thousand tons of fish products. At the same time, there is a steady trend to increase the share of freshwater grazing aquaculture products in a total catch in inland waters, which has increased to 30 – 35% over the past 10 years (Kozakevych, 2017; Mushtruk et al., 2020).

In modern conditions of fish farming mainly polyculture of carp and herbivorous fish is used, which provides a balanced consumption of the whole complex of forage organisms and the formation of maximum fish productivity. The use of herbivorous fish cannot be limited to growing them only in ponds, these species are no less promising for reservoirs and other complex reservoirs. The technology of commercial fish production in small reservoirs due to grazing aquaculture involves obtaining the bulk of fish products by optimizing the use of natural feed resources, optimally selected species composition of introducers, which allows us to consider this area as a resource-saving and environmentally friendly technology (Orel, 2020).

One of the promising objects of cultivation in reservoirs of different types is a hybrid of silver and bighead carp because it has a rapid rate of weight gain until the end of the growing season. High plasticity in the choice of food objects (phytoplankton, zooplankton, detritus, and in the ponds, also – the remnants of the dusty fraction of carp feed) allows hybrids to have much less competition than other species (Marenkov, 2018) (Figure 1).

Hypophthalmichthys is a valuable commercial fish with a scrumptious taste. The meat is tender and delicious, belongs to the group of medium-fat (Buchtová and Ježek, 2011; Jia et al., 2018). In terms of calories, it is inferior to grass carp and common carp, but its value increases with age, with increasing linear size of fish, the meat becomes fatter. The fat content of meat varies from 4.5 to 29.0%, a significant amount of it accumulates on the internal organs of fish. A large amount of fat in the body of the silver carp allows it to adapt to starvation in winter and early spring.

Due to the peculiarities of the diet of silver carp meat contains amino acids, polyunsaturated fatty acids of omega- 3 and omega-6, with regular consumption, they help prevent the development of malignant tumors, nervous disorders, improve heart function, strengthen blood vessel walls, lower cholesterol and blood pressure in patients with hypertension (Ferguson et al., 2020)

The research was conducted in the spring, summer, and autumn periods from 2017 to 2019 in ponds based on the training-research-production laboratory of fish farming of the National University of Life and Environmental Sciences (TRPLF NULES of Ukraine) of Ukraine, village Nemishayevo, Kyiv region (zone Polissya); State Enterprise "Experimental Farm" Nyvka " of the Institute of Fisheries of the National Academy of Agrarian Sciences (SEEF "Nyvka" IF NAAS) of Ukraine, Kyiv is located on the border of the zones (it is on the river Nivka that the Forest- Steppe is divided - to the south and Polissya - to the north); Bila Tserkva Experimental Hydrobiological Station of the Institute of Hydrobiology of the National Academy of Sciences (BEHS IHB NAS) of Ukraine, Bila Tserkva (Forest-steppe zone), Kosiv, Kyiv region (Forest-steppe zone) and Velykoburlutsky, Kharkiv region (Forest-steppe zone) reservoirs.

After caught in the winter pond No 101 SEEF "Nyvka" IF NAAS of Ukraine, the value of the length and body weight of annual fish in 2017 were (min-max values): 116.0 – 204.0 mm, 12.50 – 90.80 g, and in 2018 – 160.0 –188.0 mm and 37.0 – 60.0 g (Table 1) (Makarenko, Shevchenko and Sytnyk, 2018b).

The length of annual fish in 2017 after catching in the winter pond № 2 TRPLF NULES of Ukraine fluctuated within (min-max values) – 101.0 – 150.0 mm, with a variation in bodyweight – 8.10 – 30.70 g, and in 2018 – 90.0 – 111.0 mm, 5.90 – 11.40 g (Table 1).

The length and body weight of annual fish after catching in the winter pond No 14 BEHS IHB NAS of Ukraine in 2017 were (min-max values): 101.0 – 137.0 mm and 6.50 – 20.70 g, and in 2018 – 102.0 – 136.0 mm, 9.0 – 21.0 g (Table 1).

The difference in the annual growth of a hybrid of silver and bighead carp (Figure 4) usually persists in their later lives, especially in the presence of competition between them for food, which is scarce (Zebral, et al., 2018; Mion et al., 2018).

A similar trend of differences in biological parameters was observed in older age groups (biennials) of a hybrid of silver and bighead carp. In the autumn during the catch in the pond No 2 SEEF "Nyvka" IF NAAS of Ukraine indicators of length and weight of biennial fish in 2017 (min-max values) – 315.0 – 363.0 mm, 320.0 – 552.0 g, in 2018 – 283.0 – 362.0 mm, 212.0 – 463.0 g; in the pond № 1 TRPLF NULES of Ukraine in 2017 – 293.0 – 327.0 mm, 235.0 – 363.0 g, in 2018 – 264.0 – 398.0 mm, 175.0 – 414.0 g; in the pond No 10 BEHS IHB NAS of Ukraine in 2017 the indicators were in the range – 217.0 – 310.0 mm, 90.0 – 287.0 g, in 2018 – 266.0 – 378.0 mm, 171.0 – 458.0 g (Table 2).

In comparison with the hybrid of silver carp, the average standard weight of biennials grown on farms producing aquaculture products is white silver carp: in the Polissya zone – 300 g, forest-steppe zone – 350 g, variegated silver carp, respectively 350; 400 g (Vdovenko, 2012; Vdovenko, 2013).

Figure 5 shows the ratio of the average values of linear growth in length and weight of biennials of a hybrid of silver and bighead carp.

In the spring during the catching in the winter pond No 119 SEEF "Nyvka" IF NAAS of Ukraine indicators of length and weight of biennials fish in 2018 fluctuated within the following limits (min-max values) – 285.0 –363.0 mm, 429.0 – 804.0 g, in 2019 – 225.0 – 360.0 mm, 178.30 – 754.60; wintering pond No 1 TRPLF NULES of Ukraine in 2018 – 251.0 – 333.0 mm, 238.0 – 633.0 g, in 2019 – 236.0 – 298.0 mm, 209.0 – 485.0 g; winter pond No 13.

BEHS IHB NAS of Ukraine in 2018 – 191.0 – 302.0 mm, 111.0 – 438.0 g, in 2019 – 155.0 – 236.0 mm, 63.30 –227.90 g (Table 3).

The ratio of the average values of linear growth in length and mass of biennials of a hybrid of silver and bighead carp is shown in Figure 6.

According to experimental studies in 2017 – 2018 in TRPLF NULES, it was found that from the stocking of annual fish to the catch of biennials, the rate of linear growth was the highest – 194.52 mm, 165.58%; 204.28 mm, 204.85%, respectively, and the rate of weight growth was found to be significantly higher in SEEF "Nyvka" IF NAAS of Ukraine – 372.80 g, 939.0%; 336.97 g, 663.9%. In 2018 – 2019, after wintering in biennials from the pond BEHS IHB NAS of Ukraine, compared with biennials, the weight of fish decreased (– 126.82 g, – 52.86%), due to wintering of fish. In other experimental ponds (SEEF "Nyvka" IF NAAS of Ukraine and TRPLF NULES of Ukraine) after fish wintering, an increase in linear and weight indicators was observed. This may be because the ponds were caught in September-October and the data was also obtained during this period. When transplanting fish into winter ponds, it could still feed on detritus for some time and thus increase in length and weight. A significant difference in the size and weight of fish indicates a difference in living conditions in the ponds (Table 4).

Fish production of ponds for grazing fish retention is determined exclusively by the state of the natural feed base of ponds, the availability of fish feed organisms, the effectiveness of their use by various objects of polyculture (Liangzberg, 2016; Makarenko, 2018; Passos, Vidal and D’Anatro, 2019).

To characterize the foraging of a particular pond in a certain period, an indicator equal to the sum of potential fish productivity for phytoplankton and zooplankton was used (P/V – the coefficient for phytoplankton was taken as 100, zooplankton – 20, forage coefficient: 50 and 7, respectively). As a characteristic of growth, we used the indicator of absolute weight gain with differentiation by farms, age groups, and years (Table 5). The result of regression analysis is presented in Figure 7 and Figure 8.

The analysis of the obtained data showed that the dependence of the weight gain of the hybrid of silver and bighead carp (ΔM) of younger age groups on the potential productivity (P) on the feed base is satisfactorily (coefficient of determination is 0.64 – 0.66) described by the equation ΔM = a * Pb, where a = 0.001 – 1.182; b = 1.85 – 2.57.

It was found that grown in pond fish farms hybrid of silver and bighead carp had a higher rate of linear and weight growth, compared with the parental forms kept in similar conditions of pond farms (Sukhenko et al., 2019).

Today, much attention is paid to the study of aquatic bioresources of large reservoirs of the Dnieper cascade, small reservoirs of southern Ukraine, and other artificial reservoirs. (Klymenko and Statnyk, 2012; Makarenko, Shevchenko and Sytnyk, 2018a; Liu et al., 2017; Tereshchenko et al., 2016; Tsaryk et al., 2020; Petruk, 2013).

The study of hydrobiocenoses in small reservoirs within the zones of Polissya and Forest Steppe is currently not comprehensive, its studies are isolated and do not contain a clear interconnection.

When comparing different groups of the hybrid of silver and bighead carp caught from experimental small reservoirs, there is also a significant difference in their size and weight characteristics. In the spring of 2018, during the stocking of the Kosiv Reservoir, the length and weight of annual fish was (min-max values) – 159.0 – 204.0 mm, 35.80 – 74.30 g, and in 2019 – 135.0 – 190.0 mm, 19.90 – 67.90 g. In the autumn of 2019, during the overfishing of the Kosiv Reservoir, the length and weight of triennial fish, fish stock (annuals) stocked in 2018, fluctuated within the following limits (min-max values) – 535.0 – 573.0 mm, 1493.70 – 1907.50 g; triennial fish, stocking of fish planting material (biennials) in 2019, varied – 577.0 – 640.0 mm, 2124.50 – 2722.60 g (Table 6).

The ratio of the average values of linear growth in length and weight of annuals and triennials of hybrid of silver and bighead carp is shown in Figure 9 and Figure 10.

In autumn, during the stocking of the Velykoburlutske Reservoir, the value of the length and body weight of young-of-the-year fish in 2017 was (min-max values): 118.0 – 157.0 mm, 13.30 – 28.80 g; in the autumn when catching biennials in 2018 – 385.0 – 434.0 mm and 592.0 – 884.0 g; in the summer when catching triennial fish in 2019 – 369.0 – 605.0 mm, 601.20 – 1754.20 g (Table 7). During the study pechenizka reservoir (Kharkiv region), it was found that the mass caught three years was 920 g. (Gogol, 2014).

The rate of linear-weight growth of different size-weight groups of a hybrid of silver and bighead carp from the Kosiv Reservoir has not been determined, as not enough information has been collected.

In the study of the Velykoburlutske Reservoir, it was found that during 2017 – 2018 the average linear increase (from young-of-the-year fish to biennials of a hybrid of silver and bighead carp) was 227.18 mm, the absolute increase – 302.74%; for weight growth, these indicators were respectively: 757.74 g and 3.858%. In 2018 – 2019, in triennials, compared with biennials, the length increased by 66.30 mm, 116.02%, and weight – 459.64 g, 159.09%, which may be due to the high feed base of the reservoir (Table 8) (Buzevych and Makarenko, 2020).

An important practical aspect of grazing aquaculture of herbivorous fish is to determine the optimal amount of planting material, in particular, it concerns stocking of reservoirs, which can be carried out in two strategic directions: mass stocking of young fish with low stock and less stocking with large planting material. One of the advantages of the first direction may be a higher growth rate of planting material in reservoirs (compared to ponds). This allows forming a more resistance to the conditions of a particular reservoir initial herd of aquaculture facilities (Raoult, Gaston, and Taylor, 2018; Buzevych and Buzevych, 2017; Khrystenko et al., 2012; Sukhenko et al., 2017; Zakharchenko et al., 2019; Didenko, 2007). To verify this factor, we conducted a comparative analysis of the rates of linear and weight growth of annuals and biennials of hybrids of silver and bighead carp grown in pond conditions and young-of-the-year – triennials in the reservoir.

It was found that in 2017 – 2018 in TRPLF NULES from stocking annual fish to the catch of biennial fish the rate of linear growth was the highest – 194.52 mm, 165.58%; 204.28 mm, 204.85%, and the rate of weight growth was found to be significantly higher in DG "Nyvka" IRG NAAS of Ukraine – 372.80 g, 939.0%; 336.97 g, 663.9%. According to the research in 2018 –2019, after wintering in biennial fish from the pond BEGS IGB of Ukraine, compared with biennial fish, the weight of fish decreased (– 126.82 g, – 52.86%), which is due to fish wintering. In the ponds of DG "Nyvka," IRG NAAS of Ukraine, and TRPLF NULES of Ukraine after wintering a significant increase in linear and weight indicators of fish was observed.

The dependence of the mass gain of a hybrid of silver and bighead carp (ΔM) of younger age groups on the potential productivity (P) on the forage base, which is satisfactorily described by the equation was established.

For the Velykoburlutsky Reservoir, it is shown that during 2017 – 2018 the average linear increase (from young-of-theyear fish to biennials of a hybrid of silver and bighead carp) was 227.18 mm, the absolute increase was 302.74%; for weight growth, these figures were respectively: 757.74 g, 3.858%. In 2018 – 2019, in the studied triennial fish, compared to biennial fish, the length increased by 66.30 mm, 116.02%, and the weight – 459.64 g, 159.09%, which may be due to the high feed base of the reservoir.