THEORETICAL FOUNDATIONS OF AQUACULTURE - I.M. Sherman - 2011

3. THEORETICAL FOUNDATIONS OF FISH FARMING TECHNOLOGICAL PROCESSES

3.4. Specifics of Optimizing Fish Wintering

Fish are poikilothermic animals, a fact already highlighted above, which makes it unnecessary to delve deeply into this phenomenon. It is characteristic not only of fish but is typical of the entire fauna lacking a thermoregulation system, where the body Temperature of individuals depends almost entirely on the ambient temperature.

Moderate and high latitudes exhibit a fairly distinct change of seasons, which is naturally accompanied by fluctuations in air temperature and, consequently, the thermal regime of respective Water bodies. Through long-term phylogenesis, inhabitants of moderate and high latitudes have developed appropriate adaptation mechanisms, making this the norm for the species within its range. Corresponding ichthyocenoses of natural water bodies have thus formed, comprising stenobionts and eurybionts determined by the thermal regime. Due to large water volumes, marine and oceanic inhabitants are predominantly represented by stenobionts, whereas inhabitants of lake-river systems, characterized by smaller water volumes where thermal changes can be quite rapid, are primarily eurybionts.

Through the domestication of fish, humanity has achieved certain positive results, and today marine and freshwater fish species—including both eurybionts and stenobionts—are effectively cultivated.

Based on the foregoing, it is clear that optimizing fish wintering is closely linked to the thermal regime and the PHYSIOLOGICAL AND BIOCHEMICAL Reactions of the Organism to this phenomenon, The Significance of which can hardly be overstated.

In modern aquaculture, wintering is one of the most challenging stages in the technological process of both warm-water and cold-water fish farming. Modern fish farms, depending on the cultured species, operate as either warm-water or cold-water enterprises, which shapes the specific spatial and temporal characteristics of their technological processes. Cold-water fish farms are virtually unaffected by feeding pauses, resulting in an almost continuous year-round feeding schedule. This makes it rather problematic to view wintering as a distinct technological process separated from direct fish production, replacement stock rearing, and broodstock maintenance. For cold-loving fish species—which are predominantly predators that feed year-round—rations are adjusted According to the dynamics of the thermal regime. At the same time, cold-water species are oxyphils; against the backdrop of an appropriate thermal regime, they consistently require a high concentration of dissolved oxygen in the water combined with intensive water exchange to flush Metabolic waste products from the environment.

Unlike cold-water fish farming, warm-water aquaculture primarily cultivates herbivorous and omnivorous fish species. These are predominantly eurybionts regarding the thermal regime during the spring-summer and pre-spawning periods. However, in the autumn-winter period, they virtually cease feeding, reduce their mobility, and minimize Energy Expenditure to the lowest possible level. Energy during this time is derived from internal reserves accumulated by the organism during the preceding vegetation (feeding) season.

Modern pond-based warm-water aquaculture relies on the polyculture of common carp and herbivorous fish, whose feeding intensity and growth rates, all else being equal, are closely tied to water temperature. As the water temperature drops to 8 - 9 °C, these fish stop intensive feeding, drastically reduce their locomotor activity, and concentrate in the depressions of the water body's bottom. During this period, METABOLISM slows down, and their energy needs are met through previously accumulated fat reserves, which is accompanied by weight loss and a decrease in the condition factor. Concurrently, common carp, like other fish species, require oxygen for Respiration. This oxygen level diminishes as the photosynthetic activity of aquatic plants weakens, and with The formation of ice, oxygen diffusion from the air decreases sharply—an objectively negative factor for the fish.

Based on the above, as early as autumn, it is necessary to organize the winter maintenance of fish—wintering—one of the most complex technological processes in pond aquaculture, during which high concentrations of both fingerlings, replacement and broodstock, and (in a three-year rotation cycle) two-year-olds are established per unit area of wintering ponds.

The success of wintering often determines the overall economic outcome of a fish farm, as excessive fish mortality during the winter creates a severe deficit of stocking material. Practice shows that winter mortality rates for fingerlings often reach about half of all fish placed in wintering ponds, and in some farms, nearly all the stocking material occasionally perishes over the winter.

Depending on the farm's available facilities, overwintering is carried out in special wintering ponds adapted for this purpose, as well as in rearing and fattening ponds, cages, and pools, depending on the applied technology. The primary and general requirement for wintering ponds is to provide appropriate conditions for the overwintering fish, which primarily include sufficient depth, water flow, and satisfactory Physical and Chemical environmental parameters.

Overwintering outcomes depend not only on abiotic environmental parameters but also on the physiological condition of the fish, their body chemical composition, size, mass, and condition factor. During the winter, the mass of fingerlings decreases on average by 10 - 12 %, less frequently by 16 - 17 %, while their length, body height, and width decrease by 7.5 %. Two-year-old fish experience significantly lower losses — 6 %, and occasionally 7 - 10 % of body mass, with length, width, and height decreasing by 3.5 — 4.0 %; fat loss in fingerlings reaches nearly 50 % (836 — 1254 J), protein loss amounts to 17—30 % (418-627 J), and total energy expenditure ranges from 32 to 40 %.

Therefore, it is clear that fingerlings must be well-prepared for prolonged metabolism under actual starvation conditions, which is

achieved through the sufficient accumulation of reserve substances in the body during the vegetation period, with fat deposits playing the primary role. In fingerlings raised on natural feeds, the fat content should be 1.8 — 3.0 %, while for those on artificial feeds it should be at least 4 %, with protein content at 12 and 11 %, respectively.

However, in practical pond fish farming, criteria such as size, mass, and condition factor are more widely used to assess winter hardiness. These indicators are interrelated and characterize the General condition of the fish to a certain extent, making it possible to predict overwintering outcomes.

Under harsh winter conditions typical of the Polissia zone, scaled carp fingerlings prove to be the most resilient. This is partly attributed to the layer of fat beneath the scales, which protects the individuals from low temperatures during the overwintering period. To address the issue of winter hardiness and enhance it, special breeding programs have been undertaken to develop hybrid forms of carp and Amur wild carp, as well as crossbreeds between Ukrainian, scaled, and Ropsha carps. As a result, winter-hardy carp breeds have been obtained, namely the Ropsha, Ukrainian, Nivka, and Sarboyan breeds. In commercial farms where creating near-optimal wintering conditions is difficult, it is advisable to use first-generation hybrids, which exhibit high survival rates during rearing and overwintering. The survival rate of juveniles during the winter period is significantly influenced by the quality of broodstock, their age, and origin. Close Inbreeding, as well as using excessively old or young males and females, is unacceptable; preference in artificial reproduction should be given to middle-aged groups. As noted above, overwintering results depend on the preparation of wintering ponds and the creation of appropriate environmental conditions for the wintering fish. In this regard, it is advisable to follow specific criteria: the depth of wintering ponds should be such that the non-freezing water layer is 1.2 m, and 1.5 - 1.75 m in northern regions. The optimal pond size ranges from 0.2 to 1.0 ha with a length-to-width ratio of 2:1. The bed of wintering ponds should be sloped toward the drainage outlet. Fish collection channels are constructed along the bottom, and fish collection pits (receivers) or fish catchers are built near the outlet. This accelerates the harvesting process, reduces fish trauma, and significantly shortens the duration of antiparasitic treatments. The outlets of wintering ponds are equipped with two rows of special boards, or stoplogs (flashboards). Water supply can come from various sources: a main pond, springs, a river, or an artesian well. The incoming water should contain no more than 0.8 mg/L of dissolved iron, have a total hardness of 5-8 mg-eq/L, an oxidizability of no higher than 10 mg O₂/L, a pH of 7.2 — 8.6, and a minimal content of sulfates and chlorides. The presence of dissolved harmful gases such as methane and hydrogen sulfide in the water is strictly unacceptable.

The oxygen content in the aquatic environment should be within 8 - 9 mg/L. The inflow of wastewater or harmful substances (phenols, herbicides, crude oil) into wintering ponds is strictly prohibited.

The wintering pond system is equipped with aeration units of various designs, which allow for increasing the dissolved oxygen content in the water when necessary. If the reaction of the water source is acidic, lime filters are installed on the water supply channel or trough. When supplying water, fish-guiding screens, filters, and trash catchers must be installed at the respective facilities to prevent fish from entering the ponds. Troughs, water supply channels, and pipes are insulated by covering them with boards and topping them with soil and manure.

Measures are also taken to protect wintering ponds from melting snow runoff by diverting it through a bypass canal. Special attention is paid to the timely repair of hydraulic structures to ensure an uninterrupted water supply in sufficient quantity and of good quality throughout the winter.

Wintering ponds are kept dry during the summer, which facilitates their disinfection and the Mineralization of organic matter. Following the spring draining, the ditches along the pond bed, as well as the water supply and drainage channels, are cleared of silt and vegetation, and the wintering ponds are mowed 2 - 3 times during the summer period. In late September or October, the ponds are disinfected at a rate of 2.0 - 2.5 tons of quicklime per 1 hectare, flushed, and filled with water 10 - 15 days before stocking the fish.

The timing of transferring fingerlings to wintering quarters is determined by temperature conditions (the cessation of feeding and growth), but it must be done before the onset of frosts, which can lead to increased trauma and freezing of the fish. However, if the water temperature is still relatively high (above 6-8 °C) and natural food is available in the rearing ponds, transferring them to wintering ponds may prolong the fingerlings' fasting period and increase mortality over the winter. At the same time, the transfer should not be delayed until hard freezes occur, as sub-zero temperatures can cause gill frostbite, resulting in fish mortality. If a farm has multiple wintering ponds, it is best to transfer fish from specific rearing ponds to designated wintering ponds, though a chronic shortage of wintering area makes this recommendation quite problematic.

During transfer, vats, live-fish transport trucks, and stretchers must not be overloaded to prevent injury to the fingerlings. Canvas sleeves and chutes are used to unload special containers, allowing fingerlings to be discharged directly into the pond along with the water. The stocking density of fingerlings in wintering ponds depends on the specific soil and climatic zone. The longer the wintering period, the lower the stocking density should be, and vice versa; taking this principle into account will ensure successful overwintering (see Table 3.16).

Class="center">Table 3.16. Standards for fish maintenance in special wintering ponds

During the winter period, regular monitoring of the fish condition and water quality in wintering ponds is carried out. Wintering ponds must not be allowed to become covered with deep snow; this requires timely snow removal to ensure ice transparency, as well as constant control over water supply pipes, chutes, and spillways to clear ice from them.

A 15- to 30-day water exchange rate is established in wintering ponds. This means that a complete water replacement should occur over such a period. Increasing water flow without valid reasons is impractical, as it leads to increased fish mobility and, consequently, their exhaustion. It is necessary to monitor the water level, preventing any decrease or increase in its horizon to avoid ice destruction, damage to the spillway, or water overflow. To monitor the condition of the fish, several ice holes are made measuring 1 x 2.0 m or 1.5 x 2.5 m, sometimes L-shaped; they must be cleaned of ice daily and covered on top with shields or reed mats to prevent freezing.

The amount of water required to supply oxygen to wintering ponds is determined by the formula

where A is the number of fingerlings stocked into the pond, ind.; B is the average mass of fingerlings, g; P is the oxygen consumption per 1 kg of fingerling mass per day, ml; K, Ki are the oxygen content in the water at the inflow and outflow, mg/l.

At water temperatures up to 3°C, 250 ml of oxygen is consumed per 1 kg of fingerlings, while at 15 - 16°C consumption reaches up to 5000 ml, which confirms The Need for continuous monitoring of chemical and thermal regimes in wintering ponds.

Water temperature is a crucial indicator, as its compliance with species-specific requirements of fish during the winter period is the most important condition for a successful wintering process. Special studies have established that the optimal temperature is 1-2°C. A drop to 0.1 - 0.2°C causes fish diseases due to supercooling. At temperatures above 4°C, fish begin to move, expend energy reserves, become exhausted, and exhibit higher sensitivity to unfavorable environmental factors and various diseases, which causes numerous problems arising when wintering conditions fail to meet species requirements.

The gas regime affects the wintering results to no lesser extent than the temperature regime. Therefore, a complete hydrochemical analysis of water is carried out at least three times during the wintering season. After ice formation, water analyses for dissolved oxygen content should be performed every 2 - 5 days, and daily if the oxygen content drops. The sampling time is not critical during the winter period because ice significantly impedes light penetration into the pond when transparency is lost, thereby limiting the photosynthetic activity of Algae. Water samples in wintering ponds are taken at two points: at the water supply inlet (from a depth of 0.5 - 0.7 m, at a distance of 2 - 3 m from the falling water jet) and near the spillway (closer to the bottom). If the gas regime deteriorates or if There is a difference in oxygen content between the water supply and discharge (2.0 - 3.0 mg/l), samples are also taken in the center of the pond. The dissolved oxygen content is considered normal at 4 - 5 mg/l at the outflow, with carbon dioxide up to 20 mg/l at pH 7 - 8 and an oxidizability of 10 - 25 mg O₂/l.

When the oxygen content drops to 3 mg/l or less, the water must be aerated, pumped from ice hole to ice hole using various mechanisms and machines, or stepped drops and aerator tables should be arranged on water supply channels or chutes below the water intake Structure. Good aeration can be achieved using mobile and stationary air compressor units.

In cases of oxygen deficiency, diseases, and severe exhaustion of fingerlings, they begin to move around the pond and swim up to the control ice holes. To determine the cause of movement, fingerlings are caught and examined, their condition factor is determined, and physicochemical conditions are analyzed. Observations of the wintering fish and ponds are recorded in a special logbook. Sometimes, to monitor the condition of fish in a wintering pond, cages (measuring 1 x 0.5 x 0.5 m) are installed, into which 100 fingerlings are placed. By lifting the cages 2 - 3 times during the winter, the fish are inspected, measured, weighed, and their condition factor is determined, keeping in mind that mortality may occur if the condition factor drops below 2.0, fat content below 0.5, and protein below 7% for common carp.

Wintering of older age groups (replacement and brood stock) is carried out in winter-replacement and winter-brood ponds (Table 3.17).

Under normal conditions, older age groups overwinter with almost no mortality. The wintering of herbivorous fish spawners should be conducted separately from common carp, although their wintering environmental requirements are identical.

Table 3.17. Results of wintering of experimental fish

The stocking density of replacement and brood stock in wintering ponds is determined taking into account their total mass. According to current regulations, stocking density ranges from 10 to 20 t/ha. During the wintering of two-year-olds, replacement young, and spawners, thorough care of the fish and control over their living conditions are carried out. The results of observations on the wintering progress (data on hydrochemical, thermal, and hydrological regimes, characteristics of fish behavior, potential mortality and its causes) are recorded in a logbook.

Wintering ponds are drained (harvested) depending on climatic and weather conditions from March to mid-May. After the ice melts on wintering ponds at a temperature of 3 - 5°C, The transfer of one-year-olds to fattening ponds begins, along with multi-age replacement groups to summer-replacement ponds, and spawners to pre-spawning or other specially adapted categories of ponds. A prolonged delay in fish transfer can lead to the death of a significant amount of stocking material. If, for any reason, transfer is impossible, feeding the fish directly in the wintering ponds is organized. This makes it possible to maintain the juveniles in a satisfactory physiological state and, moreover, to conduct a course of preventive or therapeutic feeding of one-year-olds and replacement stock of various age groups directly in the wintering ponds prior to stocking the fattening ponds.

During the harvesting of wintering ponds, water is drained, and fish are caught first with seines in the receding water, and then in the fish sump or fish collector. The overwintered fish are counted, their survival rate relative to the number of individuals stocked in the fall is determined, and their average mass, condition, and epizootic status are assessed (Table 3.18). Each batch is treated in anti-parasitic baths, in transport tanks, or directly in the ponds if appropriate conditions are available. The survival rate of one-year-olds after wintering should be 70 - 80%, and for older age groups, 95 - 100%.

Along with the classical wintering method in specialized ponds, nursery ponds are also used, especially in southern regions. Fish stocking density for wintering is calculated based on the deep-water part of the pond, in the area where the non-freezing water layer reaches 1.0 - 1.2 m. The survival rate of one-year-olds relative to the number of stocked fingerlings is 60 - 75%, and mass losses are small because the fish are provided with feed during the late autumn and early spring periods, as well as during prolonged winter warm spells, which frequently occur in farms in southern Ukraine.

Of considerable interest is The Effect of wintering on the hematological parameters of experimental fish (Table 3.19). Blood parameters of wintering fish exhibited certain fluctuations throughout the experiment.

The three studied fish species of the Far Eastern complex are characterized by an increase in the blood color index with a downward trend in blood Hemoglobin content (statistically significant only for silver carp) and a decrease in erythrocyte counts (statistically significant for silver and bighead carp). Changes in total blood serum protein content due to wintering were statistically insignificant. Hematological parameters of common carp showed minor fluctuations and were statistically insignificant.

The decrease in blood formed elements and hemoglobin content is attributed to the suppression of the overall metabolic rate of fish under winter conditions.

Table 3.18. Effect of wintering on linear dimensions (L, l), body mass (p), and condition of common carp and herbivorous fish (M±m)

Table 3.19. Effect of wintering on hematological parameters of common carp and herbivorous fish (M ± m)

The increase in the color index (hemoglobin content per erythrocyte) is caused by an enhancement of the blood's respiratory function, which compensates for the reduction in total hemoglobin and erythrocyte counts.

The conducted studies indicate that under identical conditions, each species exhibits a characteristic Specificity in the variation of hematological parameters.

It is appropriate to examine the effect of wintering on the Biochemical Composition of the fish body (Tables 3.20, 3.21). Material analysis indicates the presence of species-specific Variability. In common carp and grass carp, body composition before and after wintering remained virtually unchanged (except for an increase in moisture content in the body of the grass carp), which is characteristic and statistically significant for all three species of the Far Eastern complex.

In the bodies of silver and bighead carp, the mass fraction of fat decreased significantly and statistically reliably over the winter period (calculated on both dry and wet matter bases). In silver carp, the mass fraction of protein also decreased (calculated on a dry matter basis).

Studies have demonstrated that under the conditions of the "Prydunaisky Rybalka" fishery cooperative, large fingerlings of common carp and phytophagous fish can successfully overwinter at stocking densities higher than standard rates, while the apparent impact of wintering on exterior and interior traits is not critical for these species.

We acknowledge that the results obtained from cage wintering cannot be directly extrapolated to wintering ponds as a whole. However, the experimental work indicates the feasibility of expanding these studies in the future and verifying the obtained results under commercial production conditions.

Under appropriate conditions, the ethology of overwintering fish is characterized by relative immobility, quiescence, and the absence of significant movements across the wintering pond. Increased mobility, intensive movement within the wintering pond, and aggregation near water-supply structures indicate abnormal wintering conditions or compromised fish health.

Table 3.20. Biochemical analysis of experimental fish stocked for wintering, % (M±m)

Table 3.21. Biochemical analysis of experimental fish after wintering, % (M±m)

Providing the aforementioned wintering conditions based on an appropriate thermal foundation ensures normal survival rates after wintering and guarantees a sound physiological and biochemical status.

To date, modern fish farming has practically no typical breed groups or breeds analogous to animal husbandry (with the exception of common carp and partially trout). The domestication process in aquaculture is in its early Selection/3.html">Stages of development, and when directly engaged in fish breeding, the fish culturist essentially works with the original wild forms of specific fish species. Therefore, fish breeding requires a sufficient body of knowledge regarding fish reproduction under natural conditions. In other words, it is necessary to study the phylogenesis of a species in relation to its reproductive ecology. According to G.V. Nikolsky, reproduction is viewed as a link in fish ontogenesis that, in interaction with other links, ensures population reproduction and species preservation.

Unlike warm-blooded domestic animals and birds, fish breeding is quite specific due to their species diversity combined with the exceptionally high significance of abiotic, biotic, and anthropogenic factors. The First and Second groups can affect fish directly, whereas the third group—anthropic or anthropogenic factors—can act both directly and indirectly through Changes in the quantitative and, consequently, qualitative CHARACTERISTICS OF THE habitat and reproductive conditions. Unfortunately, objective reality shows that the intensity of anthropogenic factors tends to increase due to the expansion of human economic activity on a global scale. The hydrological regime and the physicochemical parameters of continental and marine waters are changing, negatively affecting the species composition, Abundance, and biomass of hydrobionts, particularly fish.

At the same time, it must be taken into account that Materials characterizing the ecological plasticity of a particular fish species must be differentiated in relation to their life cycles. With this approach, it becomes evident that the fish reproduction process is characterized by a rather narrow ecological valence and is exceptionally conservative. This biological feature is reflected in the dynamics of fish population numbers when reproductive ecology is disrupted, resulting in weak year-classes appearing in certain years. Thus, the disappearance of a certain fish species from commercial fisheries occurs over a rather long period, followed by a series of low-yield generations of juveniles, with each successive generation having lower yields than the previous one. If this temporal trend persists, it leads to a complete cessation of reproduction and the disappearance of the species not only from commercial catches, but also from the ichthyofauna of the given water body. Nevertheless, the water body may retain its fishery value. Even with the complete cessation of reproduction of major commercial fish species in a water body due to disrupted reproductive ecology, it can still be utilized as a feeding ground and operated on THE PRINCIPLE OF pasture aquaculture, which has found application in world fish farming. However, this practice of preserving the fishery value of water bodies has become a reality only because THEORETICAL FOUNDATIONS OF artificial fish propagation and rearing of valuable fish species to hardy stages have been developed, which are used as introductions and translocations for the targeted formation of ichthyofauna in Artificial and natural water bodies.

Returning to the Specific features of reproduction in each species, this process must be examined from an adaptive perspective. The specificity, or species-specific features, of this process is nothing more than an adaptation to specific conditions of reproduction and juvenile development, ensuring cyclic replenishment and the maintenance necessary to preserve the species and sustain population numbers within its range. In turn, the abundance of recruitment and its quality depend on the quantity and quality of the spawning population, as well as the conditions of Embryogenesis and early post-embryogenesis.

Fish reproduction has specific features characteristic of aquatic animals and determined by Life in water. Unlike warm-blooded terrestrial animals living in water, in the absolute majority of fish, Fertilization of egg Cells occurs outside the maternal organism, in the external environment—water. Before fertilization, eggs and milt (or an egg and a spermatozoon) remain in the water outside the brood individuals for some time, where the spermatozoon penetrates the egg Cell and a zygote is formed, which indicates that fertilization has taken place.

When considering fish reproduction and forming a Concept of the subject, it is necessary to outline the range of issues that define and constitute this exceptionally important process in The life cycle of all living things, and fish in particular. According to A.P. Ivanov, this includes the development and formation of Gonads, spawning, fertilization, and embryonic and post-embryonic development. Regarding the latter point, it would be appropriate to clarify that it refers to early post-embryogenesis. The Regulation of the Reproductive System in fish, as in other classes of vertebrate animals, is controlled by the endocrine gland system, which in turn is under the control of the Central Nervous system. Despite the well-known autonomy of the interaction between the reproductive, endocrine, and nervous systems, this complex is constantly influenced by various environmental factors that also exert a targeted effect on The Development of various Links of the reproductive system at all stages of ontogenesis. The regulation of reproductive system function has a multi-level cascading character. The interaction of different regulatory levels within the epiphysis-pituitary-gonad system is ensured by positive and negative feedback systems.

In the Pituitary Gland—the central link of neurohumoral regulation—two main Hormones differing in physicochemical properties and biological action actively function to regulate gonad development at all stages of ontogenesis: LH- and FSH-like gonadotropic hormones (GtH), which exert their effects through steroidogenesis and the corresponding development of generative and somatic elements of the gonads. The relative development and functional interrelationships of these gonad components determine all subsequent gonadogenesis.

The basis of physiological regulation for the development and maintenance of a defined sex in fish of various systematic groups is the sex specificity of pituitary GtHs. Disturbance or alteration of this Structural and functional specificity of GtHs alters The Nature of steroidogenesis in the gonads in a specific way, leading to various DISORDERS OF SEXUAL function, which is predominantly observed as gonad sterilization, sex reversal, and premature physiological Aging of the reproductive systems.

The absolute majority of commercial fish species, as well as cultured and farmed species, are dioecious. The gonads of females are Ovaries, in which mature germ cells—eggs (ova)—develop. The gonads of males are Testes, which produce mature Male Germ Cells—spermatozoa. The testes of teleost fish possess their own sperm ducts that open into the urogenital opening. During spermiation, mature spermatozoa are released from the testicular tissue and expelled through the sperm ducts into the external environment.

Ovulation and spermiation are complex processes governed by neurohumoral control. Knowledge of the mechanisms underlying these processes is of great importance for artificial fish propagation, and therefore, sufficient information in this field is required. Ovulation and spermiation are characterized by specific, successively alternating cytological processes. From a physiological perspective, these processes are essentially a functional metamorphosis of tissue under The Influence of hormones. The follicular tissue disintegrates, extrudes oocytes and sperm, and ovarian and spermatic fluids are formed. Ovulation occurs either immediately throughout the entire Ovary, in which case the fish is capable of single spawning, or it affects only a portion of the ripening oocytes, making the fish capable of multiple fractional spawning. Due to changes in reproductive ecology, a transition from simultaneous to fractional spawning is possible in A number of fish species. However, this phenomenon should be considered an exception. Spermiation generally occurs on a smaller scale; mature spermatozoa remain in the testes for a long time, and males usually participate in spawning multiple times, which is widely utilized in artificial reproduction.

The fecundity of females in individual fish species varies significantly, which is also characteristic of the egg sizes of specific species. Elasmobranchs have very low fecundity, but most offspring survive due to their development inside the maternal organism or within a tough egg case. The yolk of skate and shark eggs is similar in shape and composition to avian yolk and is surrounded by a true "albumen". Elasmobranch eggs develop for a considerable time—sometimes up to two years. In contrast, the small eggs of fish characterized by high fecundity perish en masse. However, in general terms, the fecundity of teleost Fishes varies within extremely wide ranges, which is dictated by objective survival conditions. In species with large eggs—such as salmonids and notothens—there is one egg per gram of body mass; in livebearers, there are several dozen eggs per gram; in cyprinids, several hundred; and in some marine fish with very small eggs, there is a thousand eggs per gram of body weight.

The quantity of milt and its sperm concentration vary significantly among different fish species. The total volume of milt produced by a fish during the reproductive period may exceed the mass of its testes. This circumstance largely explains the realistic possibility of repeated male participation in spawning. At the same time, testicular Tissues continue to produce sperm. In rainbow trout spawners, up to 77 ml of milt (20 ejaculates) can be obtained over 40 days of the spawning season; in pike, considerably less—up to 6 ml (8 ejaculates); in large carp spawners, up to 25 ml of milt can be obtained at once; and in sturgeons, up to 1 liter. The volume fraction of spermatozoa in milt (spermatoctrit) also varies widely. In salmonids, spermatozoa comprise 25% of the milt; in common carp, 45%; in annular seabream, 11%; and in some flounders, even 97%. Spermatozoa are immotile within the testes. Their activation occurs only upon dilution with the secretion of the Seminal Vesicle, and for certain fish, specific activation conditions are required. It has been established that trout spermatozoa are not activated in an acidic environment, and potassium ions, which are abundant in the seminal fluid, act as motility inhibitors for the spermatozoa of some species.

In the scientific literature on the reproductive system of fish, along with testes and ovaries, the term "gonads" is frequently used to refer to the reproductive glands of both females and males. Expressions such as "female gonads" or "male gonads" are also common. Functionally, fish gonads are producers of eggs (ova) and spermatozoa.

Eggs and spermatozoa originate from primordial germ cells, the differentiation of which takes place as early as the embryonic development stage, measuring 9-20 µm. The length of spermatozoa capable of fertilizing eggs varies among different fish species from 30 to 60 µm, while eggs vary widely in size—from a fraction of a millimeter to several centimeters.

Different fish species reach sexual maturity at varying ages; even within the same species across its natural habitat, the age of maturation can fluctuate broadly, with some variations observed even within a single population. This phenomenon should be understood as a species adaptation in the course of phylogenesis to environmental conditions aimed at optimizing reproduction and species survival. Consequently, the age at which sexual maturity is attained is of clear interest in certain fish species.

Earlier maturation is characteristic of short-lived species, whereas fish with more extended life cycles mature later. Brook and rainbow trout mature at the age of 3-4 years, peled in their 4th-5th year of life, Lake Chud whitefish at age 2, and pike reach sexual maturity in their 3rd-4th year of life. Silver carp typically mature at three to five years of age under appropriate conditions, grass carp at 6-7 years (or at five years in southern Ukraine). Tench, common carp, and Prussian carp become sexually mature in their 3rd-4th year of life, catfish reach maturity at 4-5 years, and pikeperch at 3-4 years. Female perch participate in spawning starting from 2 years of age and older, largemouth bass reach sexual maturity at 4-5 years, snakehead matures in its third year of life, flathead mullet in its sixth to eighth year, and leaping mullet in its third year of life.

At the same time, it should be noted that the majority of cyprinids, percids, and salmonids reach sexual maturity between the ages of 2 and 6 years. Conversely, sturgeons become sexually mature at 6-12 years of age, while certain species within these orders inhabiting specific ranges mature as late as 18 years of age. Furthermore, as data from various authors indicate—consistent with fish-farming practices—males reach sexual maturity 1-2 years earlier than females, and some mature individuals of certain fish species do not spawn every year.

Abiotic and biotic factors, which are often closely interrelated or even driven by anthropogenic environmental impacts, can under certain conditions play a decisive role. All else being equal, the thermal regime and adequate food supply play an exceptionally significant role. Depending on environmental factors, the development of germ cells can either accelerate or decelerate, and in some cases even halt entirely.

Examining the age of sexual maturity within a species' range reveals three distinct groups of fish, which are driven by general biological regularities rather than species affiliation. Southern populations mature earliest; those occupying intermediate geographic positions mature later, while the latest maturation is characteristic of northern populations. Meanwhile, variations among individual specimens within a range can reach 2-3 years, a factor worth considering when organizing artificial propagation. It is also noteworthy that shifts in the timing of sexual maturity induced by environmental factors can occur not only among individuals of the same species from different populations, but even within the same populations. For instance, the largest representative of the sturgeon order, the beluga, reaches sexual maturity anywhere from 10 to 18 years of age, which serves as a guiding baseline for broodstock formation during domestication.

Due to the dynamics of reproductive gland development, of primary interest in our view is the information provided by A. P. Ivanov, where the stages of gonad ripeness in females and males are combined with histological data that accurately reflect the underlying biological processes.

The dynamics of oocyte development (oogenesis) and spermatozoa development (Spermatogenesis) in female and male gonads is a prolonged and complex process, the normal progression of which depends on a whole range of external and internal factors.

During maturation, each germ cell must go through several successive stages. It is useful to distinguish two main periods: the first spans from the appearance of primordial germ cells up to the formation of mature Gametes prior to the attainment of sexual maturity; the second involves the periodic maturation of a specific portion of gametes during the inter-spawning period. The first case examines the formation of mature eggs and spermatozoa in immature females and males, progressing from the juvenile to the sexually mature stage. The second considers an analogous process in mature individuals that have already spawned and produced offspring. The first period is quite extended, depending on the age of sexual maturation for specific fish species and prevailing ecological conditions. The second period—characterized by the regular maturation of sexually mature individuals at specific intervals—is considerably shorter, though its duration varies among fish species. Specifically, bream, common carp, pikeperch, and many other species reproduce annually, sturgeons sometimes every 3-5 years, and certain species only once every 9 years. Exceptions include semelparous species that spawn only once in their lifetime and then die, such as Pacific salmon (chum and pink salmon).

Long-term research into the dynamics of the fish reproductive system has been conducted using species belonging to various systematic groups, leading to certain differences tied to species specificity. Thus, despite common approaches proposed by several authors, the maturity scale for fish lacks universality due to species-specific traits, which complicates or even precludes its Practical Application.

According to data cited by A. P. Ivanov, maturity scales for cyprinids and percids have been proposed by other leading scientists for major aquaculture species. Each of these scales is of exceptional scientific interest and clear practical significance, but their application range is limited, as they can only be used for specific fish groups. Without calling into question the merits of these scales, O. F. Sakun and N. A. Butskaya developed two universal scales covering the vast majority of commercial fish groups. The first scale for females reflects the overall progression of gamete maturation, while the second pertains to males. These optimized and adapted scales make it possible to determine the maturity stages of ovaries and testes based on their external appearance and histological structure. Building upon these two scales, a universal gonad maturity scale for both females and males has been developed, accompanied by a brief description of oogenesis and spermatogenesis.

As A. P. Ivanov points out, the described scale of gonad maturity stages can be applied when working with fish characterized by simultaneous spawning. However, a number of fish species exhibit fractional (batch) spawning. In these fish, oocytes develop asynchronously, and this asynchrony manifests during the trophoplasmic growth phase (Stage III of ovarian maturity). When ovaries transition to Stage VI maturity, not all oocytes complete trophoplasmic growth simultaneously.

Consequently, after a female deposits her first batch of eggs, the ovary transitions not to Stage VI, but back to Stage III maturity, designated as $\text{Z}_g$ or $6-3_g$. At this stage, ruptured follicles and oocytes undergoing protoplasmic growth are present in the ovaries. Following the release of the second batch of eggs, the ovary again reverts to Stage III maturity. Such cycling of ovarian maturity stages in batch-spawning fish continues until the female releases her final batch of eggs, which serves as the prerequisite for the ovaries to transition to Stage VI maturity.

For males exhibiting batch spawning, the process is somewhat prolonged, with spermatogenesis continuing throughout the entire spawning season, causing the maturity Stages of the testes to shift accordingly.

The final stages of egg development are characterized by intensive yolk accumulation and envelope formation, whereas spermatozoa develop a flagellum, a motility apparatus, and mechanisms for penetrating the egg during this period. A distinctive feature of gonad mass dynamics is that as spawning approaches, their weight increases dramatically, and the substances accumulated within them represent a substantial portion of the organism's overall metabolic balance.

Changes in the relative mass of testes and ovaries in individuals of the same species occur in parallel. The relative mass of mature gonads is significantly greater in females than in males. Furthermore, in certain fish species, the mass of the gonads can exceed 30% of the total body weight in pre-spawning individuals.

Tropical fish species that spawn multiple times throughout the year produce a total egg mass that significantly exceeds their own body weight. Fish gonads are elongated and, in most species, represented by paired Organs suspended within the body cavity by mesenteries. During ovulation, oocytes exit the follicles through ruptures and enter the ovarian cavity or directly into the body cavity, from which they are expelled outward into the water. In cyclostomes, eggs and sperm are released from the gonad walls directly into the body cavity. Subsequently, gametes pass through pores into the urogenital sinus or urinary duct and exit via the cloaca or urogenital papilla. Sturgeons and cartilaginous fish release eggs through Müllerian ducts derived from the pronephric urinary ducts, whereas sperm is transported via renal tubules, which in sharks form a specialized organ known as The Organ of Leydig.

In the absolute majority of teleost fish, each ovary possesses its own oviduct terminating in a genital pore. In certain salmonid species featuring large egg diameters, the reproductive glands rupture during maturation, releasing eggs into the body cavity, from where they are discharged into the external environment via a wide funnel-shaped opening of the oviduct. The testes of teleost fish feature dedicated sperm ducts that open into the Urinary Bladder. During spermiation, ripe spermatozoa are released from the testicular tissue and conveyed to the external environment through the sperm ducts.

Female reproductive glands not only produce ova; the ovaries also synthesize Female Sex Hormones, represented by estrogens and progesterone. In lower vertebrates, estrogens are secreted in the form of $17\beta$-estradiol or estrone. In the bloodstream, they circulate bound to Proteins, with estradiol being the most active. Estrogens can also be produced within the testes (by interstitial cells) as well as by adrenal cortical tissues.

At low concentrations, estrogens stimulate follicular differentiation, whereas high doses suppress this process by reducing the secretion of follicle-stimulating hormone (FSH). They stimulate Protein Synthesis, facilitate the retention of sodium, calcium, phosphates, and water within the body, and exert a positive, stimulating effect on The Emergence of secondary female sex characteristics that govern Sexual Behavior. Progesterone stimulates The production of follicle-stimulating hormone.

Male germ cells produce specific male hormones—androgens, of which testosterone is the most active. Androgens are synthesized by interstitial Leydig cells. The action of androgens largely dictates male body conformation, the development of nuptial coloration, the formation of gonopodia in certain livebearing fish, and associated behavioral traits; it also promotes tissue growth and protein synthesis, enhances erythropoiesis, and increases BLOOD FLOW IN tissues. Low hormone concentrations stimulate spermatogenesis, whereas elevated levels inhibit it. Androgens may also be produced by the ovaries and the adrenal cortex.

Data shedding light on the Sex Determination of fish are of clear theoretical interest and practical significance. Fish lack a uniform system of primary hereditary structures determining an individual's sex. In some fish species, sex is determined—as in humans—by an XY chromosomal system. For example, in female riffle minnow, the diploid set reveals two identical Chromosomes (XX), whereas males possess two different chromosomes (XY). In Japanese sculpins, however, the presence of heteromorphic chromosomes (XY) indicates a female, while a pair of identical chromosomes (XX) is characteristic of males.

In certain species, sex determination depends on the presence or absence of specific sex chromosomes within the diploid set. For instance, female killifish possess two sex chromosomes (XX) while males lack them altogether, whereas in sparkling gouramies, sex chromosomes are present exclusively in males.

In the overwhelming majority of cases, sex in fish is determined by a polygenic trait structure, with genetic determinants governing sex distributed across multiple chromosomes (as seen in carp, salmonids, and sturgeons). In some instances, sex-determination systems vary even among closely related species, such as eels, or even among different populations of the same species.

External sexual characteristics of male or female development manifest primarily through the formation of gonads—either testes or ovaries. At Cytology/cytology/16.html">Early stages of ontogenesis, the external appearance and internal anatomy of a fish typically do not allow for gender identification, and such individuals are referred to as juvenile. At the same time, the early Developmental Stages of the fish reproductive system are rather complex and multifaceted. A distinct feature of primordial germ cells in fish is their ability to develop along either male or female pathways.

In some fish species, this characteristic has led to juvenile Hermaphroditism, where both male and Female Germ Cells develop simultaneously, with one type eventually regressing without subsequently affecting the individual's sex. Furthermore, phenomena such as protandry and protogyny are quite common, characterized by the fact that at a certain developmental stage, all germ cells develop exclusively into either male or the female type. Hormones play a critically important role here; the actual ratio of male and female sex hormones within the fish's body dictates the developmental pathway of the primordial germ cells.

In addition to nutritive and supportive Functions, interstitial Cells of the gonads also perform an endocrine role, as gonadal secretions exert a profound influence not only on the reproductive system, but on the entire organism as well.

Alongside the typical functioning of the reproductive system, The phenomenon of hermaphroditism is also known in fish, which is functionally differentiated into several types. This phenomenon is quite widespread among perciforms, lanternfishes, and cyprinodonts. Some fish species function as females at an early age and later become males, whereas others exhibit the reverse pattern. This feature is based on the simultaneous development of both male and female germ cells within the gonads of the same individual, which displays either male or certain female behavioral patterns. Certain species of sea bass are characterized by the simultaneous development of oocytes and spermatozoa within their gonads, while their mating behavior during the reproductive cycle can shift within minutes, accompanied by changes in nuptial coloration.

Of great interest, both theoretically and practically, is the phenomenon of sex reversal. Sex hormones play a vital role in sex determination and differentiation in fish. The administration of sex hormones makes it possible to manipulate sex, converting males into females and vice versa. In other words, a genuine capability has been established to induce sex reversal under experimental and commercial conditions, which holds practical significance in numerous cases. In practice, sex hormones and their analogues are employed for sex reversal. Substances with effects analogous to male hormones that induce masculinization (conversion into males) are termed androgens, while substances that cause feminization (conversion into females) are called estrogens. Optimal reversal results have been achieved by treating individuals during early postembryogenesis at the larval stage. Virtually complete sex reversal of females into males has been observed when feeding diets supplemented with approximately 50 mg/g of methyltestosterone. In experiments, this period ranged from 20 to 120 days. Research has established that The conversion of males into females occurs when feeding diets containing various estrogens (ethinylestradiol, estrone, estradiol) at concentrations of 20–60 mg/kg for 20–140 days.

In summary, it should be noted that the reproductive system of various fish species shares considerable anatomical, histological, and physiological commonalities; however, this does not preclude a number of species-specific traits that emerge during phylogeny and are stably maintained throughout ontogenesis.