Downstream migrations of early juvenile fish are typical for most species of ichthyofauna and are an adaptive mechanism of settlements. The purpose of this work was to study the features of the downstream migration of early juvenile fish of the lower reaches of the Oka River, to assess the quantitative indicators and species structure of migrants. The materials were obtained during the work carried out in the period from May to June in 2022–2024. Ichthyoplankton cone nets with a mouth area of 0.2–0.25 m2 were used as tools. Trapping was carried out at three deep levels and was timed to coincide with the main phases of the day, depending on the degree of illumination. According to the results of the study, the slope of early juvenile fish is confined to the surface horizon, the maximum concentrations of ichthyoplankton are noted in the morning, the minimum in the evening. A number of specific features of migration activity and vertical distribution of individual species in the conditions of a riverbed stream in the lower reaches of the Oka River are highlighted. In the course of observations of the downstream migration, it was found that the substitution of dominants is observed in the structure of ichthyoplankton. Species composition of catches of sloping juvenile fish of the lower reaches of the river The Oka is quantitatively inferior to the survey results of sections of the middle and upper reaches. The lower reaches of the river are characterized by a low proportion of rheophiles in the catches of ichthyoplankton traps and a relatively high proportion of limnophiles. The concentration of ichthyoplankton in the riverbed zone of the lower reaches of the Oka River in 2022–2024 years was higher than in the upstream sections of the river, which is explained by the wide variety of topical conditions. Coastal biotopes are more productive than riverbed zones in terms of spawning and determine a high concentration of phytophilic species in the riverbed zone.

The study was conducted at the Central Fish Farming Enterprise in Wasit Province, Iraq, from 1 January and 5 April 2024, during which the effectiveness of hormonal preparations: pituitary extract, Ovasis, Spawn Pro, was analyzed in stimulating carp under controlled conditions. The results showed that Ovasis achieved the highest fertilization rate of 94% and the highest hatching rate of 92%. The pituitary extract followed with a fertilization rate of 92% and a hatching rate of 89%, while Spawn Pro had a fertilization rate of 90% and a hatching rate of 89%. Ovasis + pituitary showed fertilization and hatching rates of 83–84%. Spawn Pro + pituitary was the least effective, with a fertilization rate of 78% and a hatching rate of 81%. From an economic perspective, Ovasis was the leading preparation at a cost of 3850 rubles, followed by Spawn Pro at 3650 rubles, Ovasis + pituitary at 7465 rubles, and Spawn Pro + pituitary at 7285 rubles. The pituitary extract was the most expensive and economically ineffective, costing 40 000 rubles for 220 kg of females. The study concluded that Ovasis is the most effective in terms of fertilization, hatching speed, economic feasibility, and lower cost compared to the pituitary extract.

The purpose of this work is to increase the rate of development and growth of Arctic char juveniles, a valuable promising specy for both commercial cultivation and artificial reproduction. The basis of the new method of the increasing cultivation biotech efficiency is the use the artificially saline biostimulating medium. For maximum stimulation of the juveniles growth rate, they were grown at a commercial farm in table salt solutions of 0.5–1.5‰ concentration, where 2 most important and widely used factors of the same effect were additionally included — optimal temperature and planting density, according to the basic principle of the natural impacts complexity. The maximum weight values (428±23 g) were obtained in a group with pre-sorted leaders of the lowest planting density (27 pd/m3 ) with that compared to more compacted (119±15 g) and control in fresh water (93.8±0.52 g) both in the same planting density (500 pd/m3 ). For the first time, an increase in the growth rate of Arctic char juveniles was observed when using even a slightly saline medium (0.5–1.5‰), below the threshold of critical salinity (4–8‰). The main advantage of growing juveniles in this combined medium — the growth rate increase to 26.97% was revealed, by comparative analysis. The industrial table salt solutions of low concentration are proposed to use in combination with known methods for optimizing the juveniles planting density (as well as temperature regimes, sorting) to accelerate the growth rate of juveniles that comparable to the effect of critical salinity in closed water supply systems of fish farms for yearround rearing.

The research was carried out in the Konakovsky department of the Freshwater Fisheries branch of the VNIRO Federal State Budgetary Budgetary Institution (VNIIPRH) and is a continuation of previously performed work [2]. The effect of cryopreserved and native sperm on the development of germ cells in domesticated forms of Siberian sturgeon and sterlet in the second year of cultivation in the spring and summer seasons is shown in comparative terms. The analysis of fish farming data in the summer continues to show a correlation of body weight with the mass of the left, right and gonad length. In the spring season, the sex of females and males is determined by anatomical differentiation, and in the summer visually by the state of the gonads at autopsy. When using cryopreserved sperm (experiment) and native (control), the development of germ cells is similar in Siberian sturgeon and sterlet. In the summer, as in the spring season, regardless of body weight, histological analysis of germ cell development shows a regular process of sequential gametogenesis. In the spring season, the females of the Siberian sturgeon and sterlet have the first stage of gonad maturity, and mitotic reproduction of oogonia takes place in the nests of the egg plates. In the summer season, Siberian sturgeon and sterlet have the II semifat stage of maturity, which characterizes the cytological level of gonads. Males of Siberian sturgeon and sterlet in the spring and summer seasons are characterized by a similar pattern — anatomical differentiation of sex. In the gonads of the first stage of maturity, spermatogonies multiply, which shows a regular spermatogenesis. In the experimental and control groups of Siberian sturgeon and sterlet, many blood cells, histiocytes, are present in the parenchymal tissue of the gonads. They carry out resorption processes, fulfilling the protective function of the body, absorbing and freeing the gonad from unnecessary and outstripping cells in the body, including ensuring synchronicity at certain stages of germ cell development.

During the implementation of selection activities to develop a new highly productive breed of carp, a study was conducted on the production characteristics of the first-time spawning producers of the third generation of carp hybrids and backcrossed carp with an improver breed (Moscow scaly). As a result, the reproductive indices for working and relative reproductive capacity in backcrossed carp females were distinguished by higher values than in the third generation of carp hybrid females. Analysis of fishbreeding and biological parameters during pond rearing of five-year-olds and their offspring — yearlings of the studied carp groups revealed an insignificant difference in their average weight, but at the same time, a higher fatness coefficient according to Fulton in backcrossed carp, as well as their effective plastic metabolism in comparison with hybrids, which was confirmed by the obtained values of the feed coefficient. The exterior indicators for the running index in backcrossed carp had relatively lower values than those of hybrids, therefore, as a result of backcrossing of carp hybrids with the improver breed, a more pronounced high-backedness was observed. The use of the mass accumulation coefficient, reflecting the weight gain of carp over a certain period of time, made it possible to establish its synchronous change in the values of two age groups of carp (yearlings and five-year-olds) of carp hybrids and backcrossed carp during the vegetation period of the studies. The highest indicators for the mass accumulation coefficient were noted for both five-year-olds and yearlings of the two groups of carp in the period from June to July, which also corresponded to the intensive development of the natural food supply in the nursery ponds during this period of time.