THEORETICAL FOUNDATIONS OF FISH FARMING - I.M. Sherman - 2011
2. THEORETICAL FOUNDATIONS FOR THE FORMATION AND UTILIZATION OF THE PRODUCTIVE TRAITS OF FISH
2.1. Mechanism of action of environmental factors on the productive traits of fish and possibilities for their improvement
Thus, Temperature is one of the primary abiotic factors of the aquatic environment that significantly impacts the growth rate of aquatic animals and their internal metabolic processes. As noted above, fish are poikilothermic animals, meaning they have virtually no ability to regulate their body temperature. In fact, a fish's body temperature closely matches that of the surrounding environment, potentially exceeding it by only 0.5-1.0°C due to metabolic heat production. Consequently, Water temperature fluctuations critically influence all aspects of fish life processes, which can
withstand narrow (stenothermal species) or wide (eurythermal species) temperature ranges. Fish from tropical and subtropical latitudes are more stenothermal than those from temperate and high latitudes, where seasonal temperature distribution is much clearer. Marine species are more stenothermal than freshwater ones, which is attributed to the large water mass capable of ensuring thermal stability.
When examining the Impact of water temperature on fish life processes, It is important to emphasize its multifaceted nature, which is modulated and determined by age-related characteristics and vital Functions. Spawning in freshwater Fishes occurs in the spring-summer or autumn periods, and in some species, during winter. However, the onset of spawning requires specific, species-dependent water temperatures known as threshold temperatures. An optimal thermal regime, for both thermophilic and cold-water fish, is necessary not only for spawning itself, but also across various stages of embryonic and post-embryonic development and growth. In other words, if reproduction takes place at a certain water temperature during different seasons, all subsequent Stages of the reproductive cycle must also occur within strictly defined temperature limits that are most optimal for ontogenetic processes in a given species. Furthermore, in many fish species, water temperature changes act as a signaling mechanism—a natural stimulus determining the onset of spawning migration, spawning, wintering, and so forth. The exact timing of spawning in spring-spawning fish is determined not only by their ADAPTATION TO ENVIRONMENTAL conditions, but also by the physiological readiness of the Organism for reproduction. The correlation between spawning times and the thermal regime during the spring flood can be utilized in fish farms to predict spawning times based on temperature trends and dynamics in the pre-spawning period.
In this regard, information regarding METABOLISM/18.html">The Influence of water temperature on natural fish spawning is quite fascinating. Observations by individual researchers have established that in areas where water temperature was either below or above the threshold temperature range for Caspian roach (11.5-15.5°C) and wild carp/sazan (18-22°C), spawning of these species did not occur. A drop in water temperature on spawning grounds interrupts the reproductive process. Accordingly, specific reproductive patterns have been identified in wild carp, showing that spawning occurs exclusively during daylight hours, while nighttime spawning is generally absent because the nighttime water temperature on spawning grounds can drop below the lower optimal limit for this species. Therefore, based on the foregoing, water temperature plays a crucially important role in fish reproduction; it dictates The behavior of spawners during the spawning period, ensuring the most effective Fertilization of eggs, their dispersal, attachment to spawning substrates, and subsequent embryonic development under specific ecological conditions characteristic of phytophilic fish species.
It should be noted that for each fish species, the threshold spawning temperature has a certain variation range and a corresponding zone of thermal adaptation. Therefore, for the same fish species inhabiting water bodies across different latitudes, the spawning threshold temperature will differ slightly.
Water temperature plays a vital role not only in determining the timing of reproduction and germ Cell development, but specifically in the progression of intensive trophoplasmic growth of oocytes. However, it is noteworthy that reaching certain threshold temperatures for spawning in a body of water does not always immediately trigger the Reproduction of a given species. A certain time interval is required for gamete maturation and the transition of Gonads from stage IV to stage V of ripeness under optimal temperature conditions. In other words, reaching water temperatures close to the optimum for a given species stimulates both gamete maturation and the spawning behavior of spawners.
However, in some cases, even when appropriate threshold temperatures are reached, spawning may be somewhat delayed due to the fish not being fully prepared for egg release. In years with lower water temperatures, initiated spawning can be suspended and essentially occur in two stages. If the water temperature has reached a certain optimum for spawning to begin and the individuals are also fully ready to release reproductive products, but there are obstacles preventing access to spawning sites or a lack of spawning substrate, a mass resorption of the older generation of fish oocytes begins after some time, which negatively impacts the reproductive capacity of the fish in subsequent years.
Water temperature acts as a signaling factor—a unique "intermediary" between the spawner organism and the complex of ecological components necessary for the subsequent development of eggs during Embryogenesis, juveniles during early post-embryogenesis, and all subsequent stages of ontogenesis. Today, it is scientifically proven that the temperature range for egg incubation varies across fish species, is species-specific, and the optimal conditions for embryonic development lie within the thermal adaptation zone where the reproduction of the given species begins.
All processes of gamete development, especially during the trophoplasmic growth phase of oocytes, are linked to a thermal regime that promotes a targeted shift in metabolism, concentrating it on the accumulation of nutrients within the developing Germ Cells. Only upon reaching species-specific threshold temperatures does the release of eggs occur, followed by The Development of offspring within a temperature adaptation zone specific to each fish species.
Fish spawning occurs within certain temperature limits, the magnitude of which may vary slightly without exceeding the species' thermal adaptation zone. Typically, fish species that reproduce in early spring and autumn have a narrower temperature range than phytophilic fish species, whose spawning occurs in summer and whose eggs are deposited on vegetation.
Without diminishing Structure/19.html">The Importance of females and the egg cells they produce, it is appropriate to focus attention on The Role of males that produce spermatozoa, bearing in mind that life begins with The formation of a zygote, which requires high-quality Gametes from both females and males. Guided by this, we consider it pertinent to examine the EFFECT OF WATER temperature on sperm motility, which has a profound impact on the fertilization process. Significant water temperature fluctuations and prolonged drops below the threshold for reproduction not only delay spawning times directly, but also lead to interruptions in the spawning process, cause mass resorption of older oocyte generations in some species, and substantially affect reproduction rates, year-Class strength, and consequently the volume and stability of commercial catches.
One of the most crucial conditions for obtaining high-quality eggs and sperm in sturgeons is the thermal regime used when holding spawners. The highest percentage of egg fertilization after insemination and the lowest mortality rate during incubation are observed when spawners are kept at optimal average temperatures of 12-15°C for a relatively short period. During the maturation period of spawners, the optimal average temperature range for holding can be broader, spanning 12-18°C. However, prolonged holding of females outside these ranges sharply decreases egg fertilizability and embryo viability.
Low temperatures reversibly, and high temperatures irreversibly, block ovulation. Sublethal and suboptimal temperatures for oocyte maturation cause, to varying degrees, three MAIN TYPES OF thermal damage to oocytes: effects on activation that hinder fertilization, fertilization followed by teratogenic development, and loss of fertilizability without activation.
The impact of water temperature on embryonic development in poikilothermic animals in general, and fish in particular, acts as a powerful regulatory factor for developmental processes against the Background of other abiotic components. The Influence of the thermal factor is expressed not only in changes to the rate and duration of embryonic-larval development, but also in alterations to certain morphophysiological indicators of developing embryos.
Many years of dedicated research have practically proven that any fish species can live and develop normally only within a specific temperature range. This range, or thermal comfort zone for normal development, varies not only among different fish species, but also among distinct ecological forms inhabiting water bodies across various soil and climatic zones within their natural range. When conditions shift toward the upper limit of the temperature range normal for a given species' development, certain morphophysiological traits consistently emerge, forming so-called "warm-water" and "cold-water" forms.
The response of fish at the onset of development—during the embryo and early post-embryogenesis stages—consists of two interrelated yet distinct dependencies in character and orientation: thermolability (changes in developmental rate with temperature fluctuations within the optimal zone) and thermostability (the reaction of an organism at a specific developmental stage to environmental temperature changes across a broader amplitude than the optimal zone, up to sublethal and lethal levels).
Typically, the maximum swimming speed and average acceleration of spermatozoa increase as the water temperature rises. Within the optimal temperature range, The change in maximum sperm velocity relative to temperature exhibits a linear relationship. However, at temperatures exceeding the optimum, sperm swimming speed decreases. The shortening of the duration of progressive sperm movement, coupled with an increase in their maximum speed as water temperature rises up to a certain critical limit, is explained by the fact that elevated temperatures within a specific interval accelerate the biochemical reactions governing sperm motility, leading to more intensive consumption of the sperm's limited energy reserves. The fertilizing capacity of spermatozoa in this temperature range is high and fairly stable, but beyond the thermal optimum limit, it drops sharply. The decrease in maximum sperm speed alongside a shortened duration of movement is explained not only by the depletion of energy reserves, but primarily by the disruption of the Structural and functional integrity of the sperm's motility apparatus, including Protein Denaturation. The level of thermoresistance in fish spermatozoa is species-specific and correlates well with the ecological traits of the spawners: the higher the average optimal water temperatures on the natural spawning grounds of certain fish species, the greater their heat tolerance.
The level of metabolism and the maintenance of structural-functional integrity and fertilizing capacity—not only in activated but also in unactivated spermatozoa—are directly dependent on temperature.
As water temperature rises from near-zero values to the upper limit of the optimal zone corresponding to spawning temperatures, the duration of development processes in fertilized eggs decreases sharply, meaning Cleavage rate accelerates. With any further increase in water temperature, The rate of embryonic development slows down.
Thus, in the middle zone of optimal temperatures, the duration of various developmental stages changes proportionally with uniform temperature shifts. However, outside the optimal zone at low and high suboptimal temperatures, this proportionality in morphological development is disrupted because the embryo reacts differently to deviations in optimal temperature conditions across various periods. At high suboptimal temperatures, cleavage anomalies occur; at low suboptimal temperatures, proportional disruptions in the timing of mitotic cycle phases during synchronous cleavage are observed. Desynchronization of development in the suboptimal temperature zone manifests not only during cleavage but across all subsequent developmental stages. In cases of significant desynchronization within the suboptimal temperature range, development proceeds abnormally, resulting in malformed fish whose survival is impossible. Sublethal temperatures lead to the thermal denaturation of protein molecules, suppress Protein Biosynthesis during Transcription and Translation by repressing individual genes and inhibiting specific Enzymes involved in this process.
Despite the absence of any formed thermoregulatory Organs in developing eggs, particularly during early embryogenesis, the eggs of spawning fish can develop not only in favorable conditions but also across a broader temperature range, extending from low negative to relatively high positive values.
Regularities regarding the influence of developmental temperature on specific growth characteristics and yolk utilization during early ontogenesis in teleost fish have been established. Temperature exerts an equally significant influence on the size formation of pre-larvae, larvae, and fry. During the first 2-3 days after hatching, linear growth in grass carp pre-larvae differs very little across various rearing temperatures. Later, with the transition to external feeding, a sharp divergence in size is observed: at lower temperatures, the linear dimensions of grass carp larvae during the transition from one stage to the next are larger than at higher temperatures, meaning that temperature has a relatively stronger impact on larval differentiation than on growth.
At the same time, the view that the size of fish embryos and larvae decreases with increasing temperature is quite widespread.
Scaled carp in ponds at the Cytology/cytology/16.html">Early stages of postembryonic development can withstand water temperature increases up to 36°С quite resiliently. As water temperature rises, the mortality rate of larvae depends on their age and developmental stage; at 38.8°С, free embryos die completely, while larvae at stages B–D die almost completely (from 46 to 84%). At stages E and F, with a fingerling length of 18–24 mm, their mortality upon a 5-hour exposure to this same temperature does not exceed 4–5%. Within the range of favorable temperatures from 16 to 30°С, the GROWTH AND DEVELOPMENT of larvae depend on their specific values: at 30°С, they proceed significantly faster than at 16–20°С. When transitioning from one stage to the next, the length of larvae cultivated at higher temperatures is always smaller than that of those grown at lower temperatures. In other words, under the influence of high temperature, larval growth accelerates to a lesser extent than morphogenesis does. At a high favorable temperature (34°С), the feed conversion ratios, condition factors, and fat content in carp fingerlings weighing from 0.25 to 2 g are maximal compared to fingerlings raised at other temperatures. The temperature range within which carp fingerlings and yearlings utilize feed most efficiently and grow well lies within the zone of thermal optimum.
Carp larvae and fingerlings can withstand short-term significant temperature drops: down to 0°С for free embryos (or pre-larvae) and fingerlings with fully developed scale cover; down to 2–4°С for fingerlings during the period prior to scale formation on the body; and down to 6–7°С for larvae at the stage of fin-ray differentiation in unpaired fins. Following a sharp or prolonged exposure to a temperature of 8–16°С, 3- to 4-day-old carp pre-larvae initially experience cold Shock, and most of them sink to the bottom. After some time, some pre-larvae rise from the bottom, while others perish at the bottom due to silt clogging their gill filaments. Transferring them back to the optimal temperature is accompanied by the mortality of a certain number of pre-larvae that were subjected to the low-temperature exposure.
In grass carp, pre-larvae are tolerant to lowered (13–17°С) and elevated (34–36°С) temperatures, but their development and growth are somewhat suppressed. Grass carp larvae can survive in a temperature range from 0 to 43°С, with their survival rate being highest in the temperature range of 24–32°С. As the temperature rises within this range, their growth and development accelerate. At temperatures below 20°С and above 36°С, larval mortality sharply increases—to 44–56.4% and 52.4–60.6%, respectively, compared to the control (which ranges from 5.4 to 26%).
It has been demonstrated for sevruga, certain salmonids, whitefish, and pike that the body length of embryos at hatching decreases with increasing temperature during the embryogenesis period.
Therefore, by modifying the incubation temperature regime of eggs to bring it closer to optimal values, one can regulate the efficiency of utilizing reserve substances stored in the yolk, influence the growth of the embryonic protein mass, and affect the organism's Energy Requirements.
Based on The Study of the mechanisms by which natural environmental factors affect the productive traits of fish and the possibilities of improving them, it is advisable to examine the influence of aquatic environmental factors on metabolism within the organism of aquatic organisms (hydrobionts), focusing on the Various Forms of metabolism.
Within the fish organism, assimilation and dissimilation processes occur differently during various periods of life. During spawning Migrations, when fish cease or almost completely stop feeding combined with significant locomotion, metabolism shifts toward dissimilation processes. A similar metabolic shift is observed during any intensive muscular work in fish. The metabolism of a fish performing significant muscular work is referred to as Energy Metabolism. During periods of accelerated growth or fattening, metabolism is directed toward the creation of new cells and structural neoformations. Under such conditions, an increase in living protoplasm mass or deposits takes place. This type of metabolism is termed plastic (or anabolic) metabolism. In this regard, it is appropriate to consider the established concept in connection with the existing aspects of modern fish farming.
Plastic metabolism includes: 1) the replacement or replenishment of certain constituent components of cellular protoplasm; 2) an increase in cell number (growth); 3) the deposition (primarily) of fat. Under experimental conditions and in some cases in the natural environment, a fish may be in a state of rest—meaning it not only performs no external work (locomotion), but intestinal activity is also suppressed to a large degree (mild starvation). In
such a state, the fish lives, maintains its Specificity, exhibits a certain level of metabolism, and this physiological state is considered to be the basal metabolism. Basal metabolism is the metabolism without which a fish cannot exist even for a short period of time. In sexually mature fish during the maturation of reproductive products, metabolism is significantly altered and differs from that of an immature specimen. This form can be distinguished as generative metabolism, which is associated with the formation of germ cells. Thus, in a normal environment, an adult fish carries out total metabolism, which consists of various metabolic forms that can be schematically and simplified represented as follows:
Total metabolism = basal + energy + plastic + generative metabolism
The ratio of various metabolic forms within total metabolism changes not only with the age of the fish, its physiological state, and the Developmental Stages of the gonads, but also depending on interconnected environmental factors. At the same time, undeniably, all forms of metabolism are closely interrelated, and theoretically all three considered forms of metabolism always take place within the fish organism, though quantitatively and qualitatively they are quite different, with the ratio of components changing across time and space. From a fisheries perspective, the entire metabolism of a fish can be viewed exclusively in two directions: metabolism directed toward maintaining vital life functions, and metabolism directed toward growth, ensuring the accumulation of ichthyomass.
To achieve optimal growth, it is necessary to know not only the PHYSIOLOGICAL AND BIOCHEMICAL characteristics, but also the general Biological features of commercial fish species against the background of corresponding external conditions, or environmental factors, under which the most complete conversion of nutritional substances into fish flesh is achieved.
Alongside the aforementioned, there is the so-called maintenance metabolism of fish, which merely ensures the preservation of essential vital functions. In this state, fish feed but do not grow, maintaining their body mass at a previous level, which is of exceptional biological importance for preserving the species within its habitat.
The magnitude of maintenance ration varies for different fish species and age groups, which, along with the aforementioned, depends on feed quality and habitat conditions.
With increasing temperature (from 10 to 20°С), maintenance metabolism increases approximately threefold, adequately reflecting the rise in the thermal regime.
Examining the relationship between fish age and the magnitude of maintenance metabolism, It is worth emphasizing that it is direct in nature. Younger age groups require a higher level of maintenance metabolism than older age groups. Ceteris paribus, it is approximately 3.4 to 5.9 times greater between fry and two-year-olds. At the same time, digestive processes, food search, and ingestion account for up to 70% of the total maintenance metabolism. About 30% goes toward internal work, specifically the work of The Heart, the gill apparatus, glands, and the functional systems of life support.
Based on the foregoing, it is advisable to consider the General characteristics of metabolic processes in the fish organism under conditions of changing water temperature, which occurs both in natural environments and under cultivation. Given that fish are poikilothermic animals, water temperature is indisputably established as one of the primary factors exerting a substantial influence on metabolism. It is known that for each fish species There is a specific temperature tolerance range, which allows for the determination of their thermal comfort zones. Typically, fish are divided into stenothermal—those adapted to a narrow amplitude of temperature fluctuations—and eurythermal—those capable of living within a broad temperature gradient. When deviations occur from optimal temperatures or the zone defining thermal comfort, both overall general metabolism and functional, plastic, and generative metabolism in particular undergo changes in fish.
Fish typically select temperatures close to the mean values of their adaptive range. It is precisely these temperatures that are optimal for ensuring the vital activities of fish as a whole and metabolic processes in particular.
However, many fish species during their biological cycle in nature are repeatedly exposed to critical temperatures: in summer when temperatures rise to maximums, and in winter when they drop to minimums. Under such conditions, the synchronization of physiological processes in fish is disrupted. During these periods, more energy is expended than is supplied through food, and its distribution within the organism is disrupted, which in turn leads to a slowdown in mass accumulation rates and an acceleration of maturation rates. At minimum temperatures, feeding and Digestion cease, and the organism's energy reserves are expended for life support without external replenishment. Maximum high temperatures are likewise unfavorable for fish life activities, as they suppress virtually all physiological processes and can be regarded as sublethal.
It is well established as a general principle that water temperature is one of the primary aquatic environmental factors determining nutritional requirements and participating in the Regulation of the intensity and direction of metabolism in fish and other poikilothermic animals.
Special studies conducted on numerous fish species have established that with an increase in water temperature within certain limits, the intensity of food consumption increases and, consequently, growth rate rises, which is a regular pattern. Experimental conditions have shown that raising water temperature within optimal values is accompanied by an increase in metabolic and growth intensity in both embryos and definitive individuals, i.e., adult fish. When water temperature increases in an arithmetic progression, the developmental and hatching speed of free embryos or pre-larvae increases in a logarithmic progression. As temperature rises, the overall duration of
embryonic development accelerates, and in live-bearing fish, the embryo gestation period is shortened.
The intensity of food digestion and nutrient assimilation is nearly three times higher in fish during the summer period than in winter, and gastric secretion increases with rising water temperature. Under such conditions, direct nutrient absorption from water also increases, which holds true for all fish species inhabiting temperate and high latitudes.
An increase in water temperature that exceeds the optimal limits, even under conditions of unlimited feeding, is accompanied by a slowdown in fish growth. This is explained by a sharp decrease in food consumption by the fish. Moreover, the smaller amount of food consumed by fish at elevated temperatures is utilized to a greater extent for functional metabolism rather than for growth.
Water temperatures exceeding the optimal range lead to wasteful feed consumption without any increase in growth rate, as well as to enhanced fat accumulation accompanied by impaired Liver function and a subsequent decline in marketable product quality.
Research by numerous scientists has established a distinct pattern: the thermal limits are shifted to the left in cold-water fish species and to the right in warm-water species. This means that cold-water fish are characterized by a narrower optimal temperature range compared to warm-water fish. For rainbow trout juveniles, the temperature range fluctuates between 2 and 20°С (in some cases up to 22°С), whereas for the more cold-loving whitefish, it ranges from 10 to 16°С. Depending on ecological conditions, this parameter can vary even within the same fish species across its natural habitat. Meanwhile, the High Metabolic Rate of fish living at elevated temperatures can lead to a shortened life cycle compared to cold-adapted species.
From the foregoing, it is evident that water temperature determines, regulates, and controls the nutritional requirements and metabolic rates of fish, which holds true for all poikilothermic animals. For this reason, the impact of the aquatic environment's temperature on the indicators of various forms of metabolism in fish is of exceptional importance.
In this regard, studies on numerous fish species have demonstrated that as the temperature of the aquatic environment rises within certain limits, the intensity of food consumption and, consequently, the growth rate increase. At the same time, studies on salmonids have established that at water temperatures above 19-20°С, feed consumption by fish drops sharply. Under conditions of unlimited feeding, this phenomenon was observed in char juveniles, where increasing the temperature beyond the optimum resulted in a marked deceleration of growth. Furthermore, the feed consumed by fish in smaller quantities at elevated temperatures is utilized to a greater extent for functional metabolism rather than growth.
The thermal regime of specific water bodies has its own distinctive features, which directly affect their chemical composition. In this context, considerable attention is paid to Biochemical changes in the bodies of fish that have developed under varying temperature conditions. Specifically, it has been established that at low winter water temperatures, the triacylglycerol content in carp fingerlings decreases, while The amount of Phospholipids increases. Yearlings that have successfully survived the winter show an increased docosahexaenoic acid content compared to weakened fish.
Despite the substantial body of empirical data regarding the impact of water temperature on various metabolic processes across different fish species, coupled with the identification of certain temperature adaptation mechanisms, until recently very little attention has been paid to studying Changes in the quantitative ratios of metabolic elements under varying temperature conditions. Meanwhile, the quantitative ratios of different metabolic processes hold both great theoretical and practical significance. These indicators can be employed as bioassays, and changes in their values can be used to assess the intensity and direction of metabolic processes within the organism under the influence of the temperature factor.
Elevated water temperatures stimulate the intensity of utilizing assimilated food energy for somatic metabolism (growth) in various fish species. However, an increase in water temperature beyond optimal limits is accompanied by the suppression of fish growth and a decrease in the somatic growth coefficient. Nevertheless, it should be noted that its decline with rising water temperatures occurs more gradually than along the pre-optimal segment, which may be attributed to the quite broad intervals of favorable temperatures for fish.
The general nature of changes in the assimilation coefficient (A/C) and the somatic growth coefficient (P/A) with temperature rising up to a certain limit may indicate its stimulating effect on the assimilation of food energy and matter for somatic metabolism. Within the zone of favorable temperatures, food resources are transformed into the food base more efficiently, which is reflected in a substantial increase in fish growth. When the water temperature deviates in either direction from the optimum, the relative indices of utilizing assimilated food energy for functional metabolism increase, while those for somatic metabolism decrease accordingly. Such a phenomenon may be the result of a high degree of balance among the physiological and biochemical processes in fish organisms within the thermal comfort zone. A minimal amount of energy is expended to produce each unit of production within this temperature range. Changes in the ratios of somatic metabolism to food assimilation (P/A) and functional metabolism to assimilated food (R/A) under optimal temperature conditions occur on The basis of a high level of substance and energy assimilation, indicating that the thermal conditions of fish organism development not only ensure but also regulate a high degree of balance among nutritional requirements, functional metabolism, and somatic metabolism. Under optimal thermal development conditions, fish meet their nutritional requirements and somatic metabolism rate at a maximum level while exhibiting minimal functional costs.
Important evaluative indicators characterizing the potential impact of abiotic environmental factors on the state of ichthyofauna can include quantitative indices of various metabolic processes in fish, as well as the ratios of their absolute values. In this regard, it is appropriate to outline the mechanism by which water temperature affects the indicators of various forms of metabolism in fish. Specifically, such indicators may include quantitative values of food matter and energy transformation processes and their ratios. Of paramount importance in this aspect is the establishment of regularities in the changing quantitative ratios between the consumption and assimilation of food energy, and the indices of its utilization for somatic and functional, somatic and generative, as well as functional and active metabolism. Within this complex of considered indicators, the primary place is occupied by the Nutritional Requirements of fish and their quantitative ratios with assimilated nutrients, somatic and energy metabolism, and their constituent elements. At the same time, it must be taken into account that a significant portion of somatic metabolism is usually represented by generative metabolism involving gametes shed during life. In functional metabolism, a substantial share belongs to active metabolism, which ensures fish locomotion. Furthermore, during organism development, a portion of the matter and energy of somatic metabolism directly associated with the utilization of the protein fraction of the feed causes profound alterations in Protein metabolism.
Water temperature has little direct effect on the Digestive System regarding protein assimilation, but at the same time exerts a powerful influence on the biological direction in which assimilated dietary Proteins are utilized. As the temperature drops, the relative fraction of assimilated protein used for various vital needs—rather than for mass gain—increases. Research has established that fish (carp) expend 31.3% of assimilated nitrogen on growth, whereas with a drop in temperature, only 7.4% goes to growth and 92.6% to vital maintenance costs. An increase or decrease in water temperature does not merely raise or lower Gas Exchange in fish; it also disrupts the coordination of metabolic processes and the balance between individual metabolic pathways, which under certain conditions can reach a critical level and lead to a lethal state.
Theoretical premises and numerous experimental studies indicate that, depending on the temperature of the aquatic environment, the indices of food energy assimilation and its utilization for somatic and functional metabolism change significantly. It has been established that with rising water temperatures within the range of 3 to 11–15°С, the A/C coefficient (the ratio of assimilated food to total consumption) in larvae of freshwater salmon, rainbow trout, and Sevan trout increases systematically; however, with any further increase in water temperature, the A/C coefficient begins to decline. This established relationship indicates that as water temperature rises up to a certain interval, salmonid juveniles exhibit enhanced feed utilization efficiency through increased assimilation.
As a rule, maximum values of the A/C coefficient can indicate the optimality of temperature conditions for the growth and development of juveniles of various fish species, while a water temperature deviation within 10–15% may signify its favorability.
In various fish species, the ratio of assimilated food to total consumption (A/C) varies differently depending on water temperature. In cold-loving fish species with autumn-winter spawning, the curves are shifted toward the lower part of the temperature interval, and the maximum favorable temperatures for their larvae lie within 1–13°С.
In more heat-loving fish species that spawn in the spring, both the curves themselves and the favorable temperature interval are shifted to the right (13–15°С). It should be noted that such shifts in the favorable temperature interval among different fish species, considering their genesis, must be genetically fixed.
Along with species-specific adaptations of fish to altered thermal regimes, analogous changes are also clearly manifested from an age perspective. Special studies have established that in salmonid larvae, the favorable temperature interval—manifested in the maximum increase of the A/C coefficient—is shifted slightly to the left, whereas in fry, conversely, it is shifted to the right. Evidently, this implies that the range of temperature adaptation capabilities is narrower in larvae than in fry.
Such a phenomenon may be caused by varying degrees of development in the energy supply mechanisms for fish thermal acclimation, including the intensity of the so-called thyroid mechanism. Under these conditions, the ratio of glycolytic to oxidative processes may also play a specific role. As is well known, the intensity of oxygen consumption (oxidative processes) increases with rising water temperatures up to a certain limit.
Elevated water temperatures stimulate the intensity of utilizing assimilated food for somatic metabolism (growth) across various species and age groups of fish. In salmonid embryos, the P/A curve rises sharply within the temperature range from 0.2 to 2.5–8.5°С, depending on the fish species. Further increases in water temperature suppress embryonic growth, and the P/A coefficient declines (where P stands for somatic metabolism and A for assimilated food). For larvae, the peak of the curve falls within the temperature interval of 12–16°С. Approximately within this same interval lie the maximum values of the A/C coefficient, which represents food energy assimilation.
When the temperature deviates from optimal values, the relative utilization of dietary matter and energy for somatic metabolism decreases, while their use for functional metabolism increases. Such a phenomenon can only be the consequence of the highest degree of balance among the physiological and biochemical processes in fish organisms under optimal thermal conditions. A minimal amount of functional metabolism Energy is required to produce each unit of production within this temperature range.
It should be noted that changes in the P/A and R/A indices under optimal conditions occur on the basis of high dietary matter and energy assimilation, meaning that the thermal conditions of fish organism development not only ensure but also regulate a high degree of balance among nutritional
requirements, functional metabolism, and somatic metabolism. Under optimal thermal development conditions, the fish organism meets its nutritional requirements and somatic metabolism rate at a maximum level while incurring minimal functional costs.
These regularities make it possible to confidently conceptualize The Use of highly sensitive quantitative ratios of various metabolic processes (A/C, P/A, and R/A) as bioassays for aquatic environment quality. On the other hand, it becomes possible to manage metabolic processes in fish by regulating the thermal regime of their development. Optimal natural water quality can be assessed by maximum values of the A/C and R/A coefficients and minimum values of R/A. When these indicators deviate from the optimum by ± 10–15%, the aquatic environment can be considered favorable for fish from a thermal standpoint. Meanwhile, taking into account the deviations of A/C, P/A, and R/A from optimal values provides an opportunity to quantitatively assess the degree of water quality impairment.
Thus, in order to rear fish more rapidly with the most rational use of feed, it is necessary to create thermal conditions under which the largest proportion of assimilated proteins is directed toward somatic metabolism—a matter of exceptional importance when fish are farmed in thermal and heated waters.
The established regularities provide a reliable basis for concluding that highly sensitive quantitative ratios of various metabolic processes (A/C, P/A, and R/A) can be utilized as bioassays for the water quality of fisheries water bodies. On the other hand, this establishes a theoretical foundation, opening up the possibility of managing metabolic processes in fish by regulating the thermal regime, which is feasible under the conditions of appropriate aquaculture facilities.
The Challenge of achieving high levels of fish productivity with minimal expenditure of corresponding resources requires a thorough Study of the impact of water temperature on protein, lipid, and Carbohydrate Metabolism.
CARBOHYDRATES, fats, proteins, and other chemical compounds serve as potential energy carriers in fish; the transformation of these compounds releases energy, which is then converted into ATP and heat. However, under extreme conditions, the energy supply mechanisms for adaptive processes within the organism undergo substantial restructuring. During the acclimation of fish to emerging extreme conditions, the energy supply for adaptive processes is primarily met through The breakdown of carbohydrates. Following the near-complete exhaustion of these reserves, energy provision proceeds via The oxidation of Lipids, predominantly neutral fats. It has been established that as water temperature rises from 15 to 30°С, the Glycogen content in fish organs and Tissues decreases. Conversely, the reverse mechanism operates when water temperature drops from 20–25°С to 10°С, which is accompanied by an enhancement of glycogen biosynthesis in the carp liver.
Along with this, it has been established that changes in water temperature significantly affect not only glycogen content but also Blood glucose levels. When the water temperature sharply rose from 12 to 21 °C in rainbow trout, the Blood Plasma glucose concentration increased within just two hours under experimental conditions, whereas cold stress caused by a sharp drop in temperature to 6–8 °C elevated Muscle glycogen levels. As tissue glycogen decreases in fish during the winter period, its replenishment is accomplished through Gluconeogenesis from precursor substrates represented by amino acid lactose. Special studies have proven that fish extensively utilize Glycolysis under cold-adaptation conditions, and carp can survive for prolonged periods relying on the ENERGY OF GLYCOLYSIS and The conversion of carbohydrates into fats. At the same time, during water temperature drops, the acclimation process in rainbow trout involves engaging glycolytic pathways, whereby glucose is diverted from ATP synthesis and instead channeled through the Hexose monophosphate pathway, which accumulates energy in the form of triacylglycerols. The observed differences in metabolic pathways during low-temperature acclimation between warm-water representatives (such as carp) and cold-water representatives (such as trout) may be attributed to species-Specific features of metabolic processes within their organisms.
As noted above, thermal acclimation significantly alters The activity of The Tricarboxylic Acid Cycle enzymes, which serve as the primary supplier of substrates for the synthesis of Fatty acids and, ultimately, lipids. This indicates that water temperature fluctuations can inherently alter the Qualitative and quantitative characteristics of these energy-yielding substances in the organism of fish.
During the growth of juvenile carp, regardless of temperature (26–34 °C) and other equalizing factors, a relative increase in fat content is observed. However, the intensity of this process varies in fish raised at different temperatures. The efficiency of feed utilization for fat deposition in juveniles reared at 30 °C is lower than in those reared at lower temperatures. The conducted research suggests that higher water temperatures shift metabolism toward fat accumulation. The fat content in fish raised at 34 °C becomes significantly higher, and the efficiency of feed utilization for lipid storage increases. Consequently, when water temperatures exceed the optimum for a given fish species, enhanced fat accumulation occurs in organs and tissues, which diminishes the commercial quality of the fish product. It must be emphasized that this lipid accumulation comes at the cost of additional feed consumption.
Meanwhile, The Significance of reserve fat for juvenile fish is exceptionally high, as it ensures successful overwintering and, at abnormally high water temperatures exceeding the optimum, AIDS in thermal insulation of the organism. It should also be noted that in fish ontogeny, higher resistance to low water temperatures is directly linked to increased fat deposition in tissues. At water temperatures exceeding the optimum, incoming nutrients are primarily channeled into fat biosynthesis rather than somatic growth, and this shift in metabolic direction leads in turn to a relative deceleration of linear growth in fish. As in other animals, the primary role in plastic metabolism belongs to proteins. Proteins utilized for body growth enter the organism as part of food and are not synthesized from other nutrients. It has been established that the most productive Protein Synthesis in fish (such as carp) occurs at water temperatures not exceeding 29 °C. Experiments have shown that the protein content in the liver of snakeheads kept for 20 days in water at 25 °C was 1.5 times higher than in fish maintained at 15 °C. Similar changes were observed in Muscle tissue.
Interestingly, the enhancement of protein synthesis can occur not only with rising water temperatures but also with their decline. This pattern has also been confirmed by field studies on carp cultivation under varying temperature conditions. It was revealed that within the temperature range of 23–28 °C, a significantly larger amount of protein is synthesized in the glandular and muscular tissues of fish than at lower temperatures. In the white Muscles of carp raised in cages using heated effluent waters, the protein quantity was more than twice as high as that characteristic of carp of the same age raised in colder ponds. Water temperature exerts a substantial impact on Protein Metabolism in the fish organism as well. As is known, the conformational structure of proteins is maintained by various chemical bonds. A sharp change in the aquatic environment's temperature can rupture these bonds, leading to impaired protein metabolism. Within the range we consider physiological, water temperature fluctuations are generally such that no disruptions occur. An optimal increase in water temperature promotes Protein Biosynthesis and, consequently, plastic Metabolism as a whole.
Therefore, the Role of water temperature in biological cycles, including the reproduction of various fish species as poikilothermic animals, is quite significant. It exerts a profound influence not only on the intensity and direction of metabolic processes and the rate of individual
development, but also serves as one of the decisive factors governing individual Links of the reproductive cycle.
Consequently, of paramount importance is The Effect of water temperature on the energy metabolism, morphological, physiological, and biochemical parameters of developing embryos, prelarvae, larvae, and fry of poikilothermic animals, which are highly sensitive to thermal factors. One of the main approaches to identifying the temperature optimum for development is determining survival rates across various incubation temperatures, with the optimum defined as the temperature zone where embryonic survival is maximal.
Studies on the Effect of temperature on respiration rate and the duration of individual embryogenesis stages have shown that the highest respiration rates are observed at low and high temperatures, whereas in the intermediate temperature zone, this parameter reaches minimum values. Thus, within the temperature range where oxygen consumption is lowest, the minimum amount of energy is expended to sustain development. Therefore, the zone of minimal oxygen consumption is generally considered the thermal optimum for development. At slightly higher temperatures that do not cause developmental abnormalities, the rate of embryogenesis is higher than within the optimal temperature zone. However, under these conditions, energy costs for sustaining development will be greater, which may adversely affect the Embryonic period and, ultimately, embryo survival.
Accordingly, as embryonic development progresses, the zone of thermal optimum shifts toward higher temperatures. This shift in the temperature optimum during embryonic development corresponds to natural conditions, where this environmental factor fluctuates throughout the embryogenesis of many fish species.
Physiological studies indicate that nutrients entering the fish organism with food are transformed in the gastrointestinal tract under the action of various enzyme systems. The major portion—reaching about 80%—of the transformed food is assimilated through the intestinal walls and transported to organs, tissues, and cells to build their structures and support functional activity. A much smaller fraction, amounting to 20%, is excreted from the organism as feces. The assimilated substances and energy are utilized within the organism for both plastic and functional metabolism. At certain Stages of fish development, plastic metabolism can account for up to 50% of the assimilated food, whereas under unfavorable conditions, such as starvation, it may assume a negative value. In turn, the magnitude of functional metabolism relative to assimilated food can reach 95%, and under adverse conditions, at the expense of plastic substances, it may even exceed this figure.
It should be noted that the magnitudes of any metabolic component can reciprocally change through the redistribution of matter (energy) from the assimilated portion of food, depending on age, sex, physiological state of the organisms, as well as the quality and quantity of consumed food, and especially changes in developmental conditions, which may be related to water temperature, dissolved oxygen concentration, and aquatic pollution by anthropogenic substances.
To quantitatively evaluate the diverse processes of food transformation and its utilization for plastic metabolism, coefficients such as P/C and P/A are calculated; for functional metabolism—R/C and R/A; for generative metabolism—G/C and G/A; for standard metabolism—Rs/C and Rs/A; for body mass growth—Pi/C and Pi/A; for reserve substance accumulation—Pr/C and Pr/A; for active metabolism—Ra/C and Ra/A; and for digestive functions—Rd/C and Rd/A, where C represents the energy of the food substance and A represents the assimilated portion of the consumed food's energy (matter).
Furthermore, quantitative ratios can be determined between the absolute indicators of body mass accumulation and the total level of plastic metabolism (P1/P), between functional and active metabolism (Ra/R), and between plastic and functional metabolism (Pa/R). As is well known, among abiotic factors, temperature exerts The most significant influence on metabolism in fish. A temperature drop leads to a substantial decrease in plastic metabolism, resulting in increased feed expenditure per unit of production.
Basal and energy metabolism also decline, but their reduction occurs with lesser intensity than the decrease in plastic metabolism.
Comparing basal and energy metabolism, it can be stated that basal metabolism is the most stable, being less susceptible to water temperature fluctuations compared to energy and plastic metabolism.
The effect of water temperature on the activity of enzyme systems in the fish organism. As is well known, the activity of digestive enzymes is largely determined by the ambient temperature. One of the Fundamental properties of enzymes is their thermolability, meaning that they lose activity upon heating, and an optimal temperature exists for their efficient action. Temperature sensitivity is also a characteristic property of enzymes, explained by their protein nature, and proteins, as is known, denature upon heating.
The optimal action range for most animal-derived enzymes is 40–50 °C. It should be noted that the activity of digestive enzymes at low temperatures in cold-blooded animals is significantly higher than in warm-blooded ones, meaning that the thermal activity optima for digestive enzymes in warm-blooded animals are substantially higher than in cold-blooded ones. If thermal acclimation affects the activity of enzymes and their systems, this must also be reflected in the Quantitative Aspects of substrates utilized by these enzymes during metabolism. During thermal acclimation, the content of energy substrates changes not only in tissues but also within cellular structures. In fish, as poikilothermic animals, digestive enzyme activity, metabolic intensity, and overall physiological state depend heavily on water temperature.
At extremely low and high temperatures, fish generally cease feeding even in the presence of food. The temperature range at which feeding occurs varies among different fish species. At the same time, differences in feeding temperature limits can be observed even among Representatives of the same species inhabiting the same water body but experiencing different thermal regimes.
It has also been noted that fish with low body condition generally feed even at water temperatures 3-5 °C lower, whereas well-conditioned fish do not feed at such temperatures. Unlike warm-water species, arctic fish species are cold-tolerant and feed at lower temperatures than tropical ones. This concept is based on the fact that high and low latitudes experience sharp seasonal water temperature fluctuations. Special studies investigating the temperature effect on the activity of Proteolytic Enzymes in the digestive tract of burbot, pike, catfish, pike-perch, bream, carp, and crucian carp inhabiting reservoirs have established:
- The Nature of the temperature dependence of intestinal mucosal proteinases in various fish species belonging to different ecological feeding groups and inhabiting diverse reservoirs is quite similar and in most cases depends on the type of substrate, which may be casein, Blood Plasma Proteins, or native muscle.
- During the summer period, the temperature optimum for intestinal mucosal protein in various fish species corresponds to 60 °C, while in the low-temperature zone (0-5 °C) it is 5-10%, and in the physiological temperature zone—10-20% of the minimum activity.
- In the winter period under the same experimental conditions (with an incubation time of 10 min, pH 7.4), the temperature function of protein in cyprinid fish is close to optimal. In pike, depending on the substrate type, the temperature optimum is 60 °C for blood plasma proteins and native muscle, or 50 °C for casein.
The thermal characteristics of homologous enzymes reflect the ecological conditions of the water bodies inhabited by fish and the nature of their feeding.
In studying the adaptation of the digestive system to the feeding habits of fish from various ecological groups, differences in enzyme activity optima were established between herbivorous (peaceful) and predatory fish. For peaceful fish—carp, crucian carp, bream, white-eye bream, roach—maximum starch Hydrolysis activity under the action of L-amylase is observed at 40 °C, whereas in typical and facultative predators (including burbot, pike, wild carp, perch, ruffe, and sabrecarp), it is in most cases observed at 30 °C.
During fish acclimation to water temperature changes, certain enzymes play a significant role, with their activity also capable of varying across a fairly wide range. Specifically, this applies to enzymes responsible for cellular energy generation, including those of the tricarboxylic acid cycle and the Respiratory Chain. Identical enzymes functioning at different temperatures exhibit distinct structural differences. One such mechanism of acclimation of the enzymatic apparatus responsible for fish tissue Respiration is the selective synthesis of Isoenzymes best adapted to their optimal temperature range.
During fish adaptation to low temperatures, the activation of virtually all Components of the Mitochondrial Electron Transport system is observed in their glandular tissues, meeting the organism's increased energy demands. Acclimation to lowered temperatures is primarily exhibited by enzymes involved in glycolysis energy production, electron transport, and Fatty acid oxidation.
In addition, enzymes responsible for utilizing stored cellular energy—specifically, Mg2+-ATPase and Na+K+-ATPase—play a major role in the mechanisms of fish adaptation to water temperature fluctuations. One of the Mechanisms for the acclimation of the enzymatic machinery responsible for fish tissue respiration is the selective synthesis of isoenzymes that are most suited to their optimal temperature range.
Thus, water temperature primarily affects enzyme systems associated with energy transformation processes, namely the enzymes of glycolysis and aerobic respiration.
The mechanisms of fish adaptation to changes in water temperature are quite specific. As is known, each species of aquatic animal, particularly fish, has its own temperature optimum. For salmonids in the summer, it ranges between 20-21°C, and for whitefishes, 22-23°C. The highest feeding activity and assimilation in carp are observed at temperatures of 20-27°C. The most favorable temperature for herring juveniles is 12°C, and for grass carp, 18.3-29.4°C. Many researchers have recorded the highest growth intensity of these fish species precisely within these temperature ranges, which is directly related to feeding efficiency.
However, throughout their life cycle, especially across different seasons, fish are exposed to a wide range of water temperature fluctuations. Typically, rapid changes in water temperature, even within the adaptive range, can lead to fish mortality. However, this does not occur under conditions of gradual temperature increase or decrease. Furthermore, gradual fish acclimation significantly broadens their temperature tolerance range. Water temperatures extending beyond optimal limits result not only in a substantial decrease in food consumption and a slowdown in growth rates, but also in the disruption of physiological systems and biochemical processes.
Seasonal water temperature fluctuations typical of temperate latitudes, even when remaining within the species-specific temperature range, require fish to undergo a significant metabolic restructuring, as well as to employ evolutionary-derived adaptation mechanisms aimed at adjusting the organism to water temperature changes and normalizing physiological and biochemical systems.
The adaptive mechanisms developed by fish during phylogeny enable them to withstand water temperature fluctuations across a fairly wide range without significant negative consequences for the functioning of various physiological systems of the organism.
A clear correlation exists between water temperature and the course of physiological and biochemical processes in aquatic animals, which fully applies to fish.
Since changes in water temperature induce alterations in the metabolic rate of fish, internal mechanisms exist that aim to stabilize this level and minimize environmental influences.
The intensity and direction of metabolic processes in fish are closely linked to changes in water temperature. Fish of various taxonomic groups under specific physiological states establish their own temperature adaptation zone at a given temperature (within the physiological norm), characterized by the most stable metabolism—essentially a form of stabilization that significantly reduces temperature dependence.
In our view, metabolism is the key process in the mechanisms of physiological adjustment or adaptation in fish. What poses a danger to their organism is not A change in the metabolic rate itself, but disruptions in the ratios of individual metabolic pathways, its dysharmony, and the impairment of its qualitative parameters. Overall, the physiological well-being of an organism, or its ADAPTATION TO A given temperature, is determined by its ability to establish a personal temperature adaptation zone.
One of the integral indicators reflecting the response of fish to water temperature changes is their oxygen consumption rate. Thus, a drop in water temperature is accompanied by a decrease in oxygen consumption by fish, while an increase leads to a significant rise in this parameter. With rising water temperatures, not only do the oxygen requirements of fish increase, but the oxygen capacity of the blood also rises due to an elevation in erythrocyte count and Hemoglobin content. Supplying fish with the necessary amount of oxygen under such temperature conditions is achieved by increasing the volume of water passing through the glandular apparatus of the gills per unit time, along with enhanced oxygen extraction from it—a characteristic pattern typical of thermophilic fish.
Simultaneously, it has been established that during cold adaptation in certain cold-water fish species, the rate of oxygen consumption may instead increase 1.5–2 times, whereas with rising water temperatures, oxygen consumption by fish may decrease.
The differing responses of thermophilic and cold-water fish regarding oxygen consumption under changing water temperature conditions are explained by species-specific differences rooted in the Ecological and Physiological features of their metabolic processes.
It should be noted that fish acclimation to rising water temperatures occurs more rapidly compared to cold adaptation, the duration of which may reach up to 20 days in some cases. The adaptation of fish to low and high temperatures occurs not only via different pathways of metabolic alteration but also differs in the timeframes of these processes. As a rule, fish adaptation to low temperatures proceeds more slowly than to high temperatures. Overall, 5–10 days are sufficient to stabilize metabolic processes and, consequently, for fish to adapt to new temperature conditions.
Fish acclimation to lower water temperatures is accompanied by the inhibition of mitochondrial energy generation processes, which affects ATP levels. The energy supply for fish cold acclimation is maintained not through aerobic respiration, but via alternative energy-generating mechanisms. In particular, during cold acclimation, the role of the Pentose Phosphate Pathway of Glucose Catabolism may increase. Concurrently, glycolytic activity diminishes against the backdrop of enhanced gluconeogenesis, accelerated tricarboxylic acid cycle reactions, and increased activity of the associated enzymes.
The Influence of Environmental factors, especially water temperature, and the General Theoretical Foundations of this phenomenon constitute the basis for cultivating various ecological groups of fish under specific farming conditions, creating opportunities to develop optimal fish production technologies across various aquaculture enterprises.
GENERAL PATTERNS OF metabolic processes. Ecological, physiological, and Biochemical characteristics of individual periods of fish ontogeny. The study of this section should begin with An Overview of S. G. Kryzhanovsky's Classification of individual fish development, which he divided into the following periods: embryonic, larval, juvenile, sexual maturation, adult sexual state, and Aging.
Before proceeding to a detailed study of the physiological and biochemical processes occurring during various periods of fish ontogeny, it is advisable to familiarize oneself with the morphological and biochemical regularities of early fish development. As known, a specific developmental interval in fish is the stage—the time during which growth and gradual qualitative and quantitative changes take place, creating the prerequisites for transitioning to subsequent developmental stages. Depending on their biology, different fish species may have varying numbers of stages. Each stage is divided into substages, providing a specific characterization of a particular morphological and physiological state of the organism. Accordingly, the transitional stage is viewed as the concluding phase of each stage, which, through complex morphophysiological transformations, ensures the transition to new developmental phases.
Most fish species exhibit periodic fluctuations in physiological and biochemical processes during ontogeny. The Essence of periodic changes in oxygen consumption intensity during the early ontogeny of freshwater fish lies in a significant increase in its values during transitional stages. Furthermore, it has been established that each developmental stage is characterized by specific limits and average levels of gas exchange.
Each qualitative stage of development begins with a marked surge in growth rate, which subsequently slows down somewhat.
A general regularity in the development of freshwater fish during early ontogeny is the increase in absolute indicators of plastic (anabolic) and functional metabolism. Minimum values of plastic metabolism are typically observed during periods of intensified morphogenesis accompanied by high functional metabolism throughout the respective stage.
Upon completing the transition to a new developmental stage, the intensity of functional metabolism decreases while plastic metabolism increases; that is, a redistribution of total food energy occurs among various metabolic processes, each having specific significance for the organism's development and growth at any given moment. Throughout each developmental stage, plastic metabolism dominates over functional metabolism, the latter of which declines toward the end of the stage—a phenomenon that may be associated with the exhaustion of the organism's morphological and functional capacities at that specific stage. Alongside this, studying processes occurring in ontogeny, particularly during embryonic development, requires familiarization with the physiological and biochemical processes during vitellogenesis, or oocyte maturation. After all, it is during this period that yolk formation takes place, the degree of accumulation of which determines the supply of plastic and energy metabolism substrates necessary for embryo development, growth, and the transition to exogenous feeding.
It should be understood that even during stages preceding the onset of vitellogenesis, significant metabolic and structural changes occur in oocytes, manifested by enhanced oocyte synthetic activity, accumulation of Ribosomes in the Cytoplasm, and intensive development of The Endoplasmic reticulum membranes and Golgi apparatus. All these metabolic and ultrastructural changes in oocytes represent preparations for synthesizing yolk components. It is worth noting that during this period, significant structural changes occur in the fish liver, where a portion of the yolk components is synthesized, while another portion is synthesized by the oocyte itself. Endogenous yolk, in turn, is a product of the Endoplasmic reticulum and Golgi apparatus activity. The ratio of these two pathways of yolk formation varies among different fish species, although the bulk of it is evidently of exogenous origin.
The physiological and biochemical CHARACTERISTICS OF THE embryonic period in fish serve as a foundation for understanding the processes and phenomena occurring during embryogenesis. The embryonic period encompasses the development of eggs and larvae up to their complete transition to exogenous feeding. Following ovulation and the release of mature ova, the development of eggs and embryos within the egg membranes relies on nutrient reserves accumulated during the period of trophoplasmic oocyte growth. Throughout this phase, the transformation of stored substances occurs alongside the biosythesis of various new compounds, primarily enzymes, Vitamins, and DNA. The organism's core metabolic functions are sustained by these yolk-derived nutrients, supported by specialized structures that emerge during development.
A distinctive feature of the embryonic period in fish is that, during incubation and larval development, the absolute percentage of organic matter—comprising proteins, lipids, and carbohydrates—decreases, while both the absolute and relative water content increases.
It should also be noted that changes in nucleic acid biosynthesis form the basis of growth in any multicellular organism during early ontogeny. Experiments have established that within two weeks post-hatch, intensive nucleic acid synthesis occurs in sturgeon tissues, accompanied by a 3- to 4-fold decrease in protein content. In the absence of external feeding, this indicates that the synthesis of Nucleic Acids during this period draws primarily upon the energy and breakdown products of Reserve Proteins, as evidenced by a concurrent increase in free Amino Acids.
Alongside proteins and nucleic acids, the composition of fatty acids and lipids undergoes significant changes during fish embryogenesis. Against the backdrop of a general increase in the relative content of Unsaturated fatty acids—corresponding to the preferential utilization of triglycerides from the system—the content of polyunsaturated fatty acids simultaneously rises while certain other fatty acid fractions decline.
Following fertilization, throughout the cleavage stages, germ layer formation, and up to the completion of Gastrulation, there is a sharp decline in lipid content, particularly structural fractions such as phospholipids and Cholesterol. Concurrently, over a 30-hour developmental window, the depletion of triglycerides and glycogen—the primary energy reserves of the ovum—is also observed in fish.
Attention should be drawn to the specific features of energy supply in the metabolic processes of early fish ontogeny. Glycogen serves as the primary substrate of Carbohydrate Metabolism During the early embryonic development of Echinoderms, teleost fish, and amphibians. However, during the phase of intensive oocyte growth, the principal carbohydrate substrate is not glycogen, but glucose. It is precisely at the onset of this intensive growth phase that the respiration rate of oocytes, which serves as an integral indicator of energy metabolism, reaches its peak. As embryos, larvae, and fry develop, the mechanisms of oxygen consumption undergo a shift. Organismal growth and differentiation are accompanied by an increase in body mass and energetic demands, necessitating a progressively higher oxygen intake. The primary driver behind the elevation of respiration rates during early individual development is the disproportionately rapid growth in the oxygen consumption rate of embryos and larvae relative to their increase in mass. At this stage, rapid growth and Differentiation of the Respiratory system take place, reflected in the Increasing complexity of oxygen uptake mechanisms. As embryos and larvae mature, The regulatory mechanisms of respiration become more sophisticated; specifically, with the formation of gills, additional control mechanisms emerge at the level of oxygen absorption by the Circulatory system and its transport to various organs and tissues. Hormonal and neural mechanisms regulating respiration are already operational at these developmental stages. It is important to note that early developmental stages are characterized by a very narrow range of environmental tolerance, as homeostatic mechanisms remain immature during this period. Consequently, mass mortality of larvae is frequently observed during this phase of ontogeny.
The specific patterns of metabolic processes during the larval period in fish, alongside theoretical principles, inform the optimization of larval aquaculture technologies. As is well known, this period entails the growth and development of larvae fueled by exogenous sources—namely, their transition to external feeding. In fish, this period commences shortly before the final resorption of the yolk sac. If this process is delayed, the organism begins to consume its own body tissues, leading to starvation. Following the switch to external feeding, subsequent growth and development rely on assimilated food. From this juncture onward, a systematic decline in the efficiency of converting ingested organic substances into body tissues begins. The direction of metabolism shifts, marking the gradual accumulation of lipids, enhanced synthesis of certain fatty acids, and increased carbohydrate expenditure. As juveniles grow, the water, protein, and ash content decrease, while the caloric value of the fish increases.
Simultaneously, the larval period in fish is characterized by an expansion in the range of environmental tolerance due to the formation of structures that secure vital functions: respiration, excretion, feeding, locomotion, and others, which is accompanied by an increase in body size and mass.
Metabolic processes in fish vary across time and space. Consequently, it is essential to understand their nature during the juvenile period. In this regard, G. V. Nikolsky identifies the pre-pubertal phase in fish, which encompasses two sub-periods:
the fry (or juvenile) stage and the sexual maturation stage. The juvenile period is characterized by the transition from larval to juvenile Morphology, which varies in duration among different species—ranging from a few days in clupeids and gadids, to several months in flounders, and up to several years in eels. A series of critical processes occur during this timeframe, notably the inflation of the swim bladder with air and the final establishment of vital functions related to respiration, feeding, and excretion. However, it has been established that natural mortality rates increase in some fish species during this phase of life.
Alongside this, the fat content and caloric value of fish gradually increase throughout the juvenile period. Evidently, following metamorphosis, the growth of individual organs and the organism as a whole is driven primarily by cellular hypertrophy and the expansion of Extracellular matrix mass, rather than by cell proliferation.
The juvenile period of ontogeny features intensive linear growth, accompanied by an increase in protein content relative to dry matter. Concurrently, carbohydrate and lipid levels rise progressively, while ash content declines. Toward the end of the first feeding season, the Rate of protein synthesis in young-of-the-year fish slows down, whereas lipid accumulation intensifies, accompanied by shifts in lipid fraction and fatty acid composition. In some fish species, the proportion of triglycerides within the fat fraction increases during the first year of life, while the content of cholesterol—which plays an active role in differentiation processes—decreases.
During the Juvenile Stage, not only do the proportions of protein and lipids in the dry matter shift, but the relative carbohydrate content also rises. While carbohydrates are preferentially utilized during endogenous feeding, followed by proteins and lipids, juveniles in the larval and juvenile stages accumulate carbohydrates at a faster rate than proteins and lipids. Conversely, when fingerlings experience starvation, fats are depleted first, followed by carbohydrates and proteins.
The period leading up to sexual maturation is associated with the rapid development and growth of germ cells. During this phase, the deposition of nutrients begins in oocytes, Spermatogenesis accelerates, and the gonads of both males and females rapidly increase in size. Generative metabolism demands a substantial share of the organism's resources, leading to a suppression of somatic growth. Whereas metabolic processes in the preceding ontogenetic phase were directed toward optimizing linear growth and mass accumulation, the onset of sexual maturation initiates a new metabolic paradigm—generative metabolism. Here, priority is given to establishing optimal metabolic conditions for gamete maturation and effective spawning. Consequently, in the year sexual maturity is attained, a significant portion of assimilated energy is directed not toward protein accretion, but toward increasing the lipid content of the organism.
It should be noted that the age at sexual maturity in most fish species is determined by reaching specific size thresholds rather than a chronological age. It is also noteworthy that in fish with accelerated maturation, both absolute and relative fecundity, the relative dry matter and lipid content of the gonads, definitive egg size and mass, and the absolute amounts of dry matter, protein, and lipid in a single mature ovum are lower than in individuals of the same cohort that mature a year later.
Metabolism during sexual maturation differs between males and females. The energetic costs of generative metabolism are lower in males than in females; however, males expend more energy during the spawning process. Furthermore, the lower expenditure of structural and energetic resources on generative metabolism accounts for the earlier attainment of sexual maturity in males across many fish species. Sexual maturation in fish is also linked to the accumulation of a threshold level of BIOLOGICALLY ACTIVE SUBSTANCES within the organism, among which vitamins, enzymes, amino acids, and Fatty acids are of paramount importance. In cohort members that fail to reach the requisite threshold of reserve substances, the transition of oocytes into the phase of trophoplasmic growth is delayed, meaning this growth phase is extended by one or several years.
In relatively early-maturing individuals characterized by rapid growth and intensive fat accumulation, the period of trophoplasmic oocyte growth is accelerated, and vitellogenesis initiates at smaller initial oocyte sizes. Subsequently, during vitellogenesis, the dimensions and mass of developing oocytes increase. This is evidently tied both to the size achieved prior to trophoplasmic growth and to the amount of structural and energetic resources the organism can allocate toward reproductive products. In early-maturing fish, this organic matter content, both in absolute and relative terms, is lower than in later-maturing individuals of the same cohort. The production of smaller eggs in these fish is dictated by the smaller initial size of proto-plasmic growth oocytes and a more limited supply of structural and energetic resources allocated to generative metabolism.
Consequently, different segments of a cohort reach sexual maturity in varying physiological states, as convincingly demonstrated by the physiological and biochemical profile of the maturation period. The onset of this phase is tied to the individuals' participation in population reproduction, alongside the rapid development and growth of germ cells. Throughout the growth and development of germ cells—starting as early as maturity stage III—the protein, lipid, and glycogen content in fish gonads increases significantly. Testes accumulate 1.5 to 3 times less protein and lipids than Ovaries, yet they store considerably higher amounts of glycogen.
As ovaries and testes develop, their caloric value increases, alongside the accumulation of vitamins and various other biologically active substances. The rapid growth and maturation of gonads from stage III onward occurs against a backdrop of substantial consumption of reserve compounds, supported by the creation of optimal metabolic conditions within the body for generative metabolism.
In many autumn-spawning fish species in boreal regions and batch-spawning species in subtropical and tropical regions, gonad constituents are formed largely from exogenous sources. Nearly all batch-spawning fish feed intensively between the release of successive egg batches. Certain fish species inhabiting high latitudes lack opportunities for pre-spawning fattening; the structural and energetic reserves accumulated during a single vegetative season prove insufficient for complete gonad maturation. Under such conditions, regular spawning skips are observed, and in some cases, a portion of the oocytes undergoes resorption during vitellogenesis, which simultaneously facilitates the maturation of remaining egg batches.
Monocyclic fish lack the homeostatic mechanisms required to ensure parental survival post-spawning. The expenditure of physiological and biological resources dedicated to reproduction upon reaching sexual maturity reaches an irreversible threshold in these species. The high post-spawning mortality in monocyclic species frees up food resources for progeny or adjacent cohorts. Unlike monocyclic fish, polycyclic fish engage in repeated spawning events, with each successive spawning occurring against the background of a distinct physiological state.
Thus, during the development of germ cells, optimal metabolic conditions for their growth and maturation are established within the organism. At the same time, maintaining systemic Homeostasis presents challenges, which are mitigated through various strategies: skipping spawning seasons, utilizing exogenous nutrient sources, and resorbing a portion of the oocytes. In extreme cases, processes of exhaustion and regulatory system failure reach an irreversible tipping point, culminating in the death of the organism or the entire spawning population—a typical fate for monocyclic species. Concurrently, it is a well-established fact that biosynthetic activity declines with age across animal taxa.
Throughout fish ontogeny, the rate of protein synthesis experiences a steady decline, associated with a qualitative shift in the organism's physiological state. As the mature adult phase progresses, the share of generative metabolism within total metabolism continuously rises, while the efficiency of converting assimilated food into somatic cell growth diminishes. This demands ever-increasing expenditures of structural and energetic compounds. The accumulation of lipids over the course of ontogeny provides the energy required for metabolic and generative processes. During the mature phase, the amplitude of seasonal fluctuations in total fat content increases, while protein synthesis gradually slows down. This period also witnesses a marked decrease in the activity of several enzymes, notably Nucleases. With advancing age, the role of proteins in oxidative metabolism wanes while the significance of fats grows; glycogen levels increase in The Liver and muscles, whereas total body water content declines. Driven by the age-related reduction in protein synthesis compounded by mounting generative costs, the degree of post-spawning exhaustion intensifies with each successive reproductive cycle.
It has been established that the fractional composition of proteins and lipids also shifts during ontogeny. Specifically, within the lipid fraction, the proportion of phospholipids decreases, the content of triglycerides—the primary reserve lipid fraction—rises, and the level of highly unsaturated fatty acids, which serve as key energy Donors during early developmental stages, declines. Conversely, the relative content of monoenoic fatty acids, glucose, and glycogen increases.
A hallmark of sexually mature fish is the continuous increase in body mass with age. Many species reach sexual maturity when their body mass is approximately 1/5 to 1/10 of their ultimate size. The increase in individual absolute fecundity in fish is linked by a linear relationship to body mass. Thus, rapid somatic growth directly drives an enhancement in the reproductive potential of the spawning stock.
There are certain specific features in the course of metabolic processes in fish during the spawning period. This period is quite specific, which is why attention should be drawn to the most drastic changes in body chemical composition observed in fish that, having almost reached
sexual maturity, undertake long migrations to spawning grounds. To overcome significant distances against the current of river systems and ensure the maturation of sexual products, migrating fish must store large reserves of high-energy substances in their bodies. The main feature of metabolism in such fish during the migration period is high energy metabolism under conditions of fasting.
The intensification of oxidation processes in the migrant's body requires a large amount of energy substrate, primarily fat. However, fat does not oxidize on its own within the body; it Burns only with the mediation of protein, which imparts a specific biochemical direction to the oxidation processes. Consequently, the loss of fat by a migrating fish is also accompanied by the loss of protein, and these losses during reproduction are so substantial that the fish is often unable to survive and dies.
The percentage content of total and protein nitrogen gradually decreases during migration by approximately 1/3, with 2/3 of these losses occurring before spawning and 1/3 after spawning. It should be noted that significant fat losses are observed in fatter fish, and following substantial fat loss, profound protein alterations begin, with muscle tissue undergoing particularly drastic qualitative changes. For instance, chum salmon during the fasting period can lose up to 80% of its protein, which is typically replaced by water.
Individuals of different sexes exhibit certain differences and specific features in changes of overall metabolism. Profound changes in general metabolism and its individual pathways related to reproduction have been studied using perch and sterlet as Examples. During the maturation of sexual products, males and females show unequal oxygen consumption and release of carbon dioxide and ammonia as End products of Metabolism. During the growth phase of oocytes (stages III and IV) and spermatozoa, a general increase in oxidation processes occurs within the body. This increase is more pronounced in males than in females. In addition to this general similarity, there are quite profound differences between the rate and nature of metabolism in females and males. While in females the increase in oxygen consumption occurs quite gradually as the eggs mature from stage II to stage IV inclusively, in males oxygen consumption drops sharply immediately after spawning, only to rise again at stages III and IV. The respiratory quotient at stages III and IV is very close to 1, accompanied by protein breakdown down to the final product, although the process proceeds very slowly.
Thus, metabolism in each sex proceeds differently from both a quantitative and a qualitative standpoint. In general, testicular tissue respires more intensively than ovarian tissue, and males consume more oxygen per 1 gram of live weight than females.
It should be noted that the period of sexual product maturation and spawning in spawners is characterized by significant changes in physiological parameters: the hemoglobin content in the blood of females drops sharply during and shortly after spawning, increasing again as autumn approaches. It is known that hemoglobin levels characterize, to some extent, the intensity of oxidation processes in the body. The second parameter characterizing the rate of oxidation processes is the oxidation-reduction potential (Eh), or the so-called electronic tension. The level of electronic tension in the blood is crucial for the level of electronic tension in the gonad, which is continuously supplied with various substances by the blood. As the gonads mature, electronic tension increases, indicating an elevation in oxidation processes during this period. Thus, the primary condition for the normal maturation of fish sexual products at The final stage of their development is a high level of oxidation-reduction metabolism in the gonad, and consequently, an appropriate physiological state of the organism.
It should be noted that the onset of sexual maturity in fish is associated with achieving a certain level of body fat content. Based on the foregoing, we can highlight the key determinants in the process under consideration:
- in younger fish, the size, mass, and dry matter and fat content of eggs are lower than in older, though not the oldest, individuals;
- in fish that mature early, grow rapidly, and accumulate fat, the period of protoplasmic oocyte growth is shortened, and vitellogenesis begins at smaller oocyte sizes;
- in fish with an elevated body fat content, the absolute and relative quantity of produced eggs increases;
- fish that mature at an earlier age differ from late-maturing individuals of the same generations by a reduced individual reproductive capacity, smaller size and mass of mature eggs, and a lower content of dry matter and fat in them.
There is a certain correlation between the quality of feed supplied to fish spawners and the quality of their offspring. For instance, in carp, the best growth is achieved with feeds having an amino acid ratio close to that of muscle protein, and spawners with superior growth produce better-quality offspring. An increase in muscle protein is accompanied by an increase in its content in the eggs, and the viability of larvae depends on the protein content of the eggs. An increase in egg amino acid content is linked to higher dietary protein levels, and the quality of fish embryos and larvae largely depends on The amino acid content of the eggs.
It is interesting to examine the typical, general signs that characterize the aging processes in fish. Aging is understood as a physiological state in which the normal course of metabolism—characteristic of previous stages of ontogenesis—is disrupted, and the bulk of feed is directed toward maintaining life rather than production processes. In older individuals, the quality and quantity of produced offspring decline, and the periodicity of reproduction is frequently disrupted. From a certain age, the relative mass of produced sexual products decreases, the enlargement of mature sex cells pauses, and then their size begins to decline. Prior to the appearance of these signs, a decrease in Lipid Metabolism intensity and an increase in water content within organs and tissues are observed. In older individuals, glycogen reserves in the body drop significantly; the disruption of glycogen synthesis may be caused by weakened activity of enzymes catalyzing glucose phosphorylation.
The contradictions between reproduction and the maintenance of individual homeostasis manifest most acutely during the spawning and post-spawning periods, when exhaustion reaches its peak. It is known that the proportion of generative metabolism in total metabolism remains constant with age in fish, while the efficiency of utilizing consumed food for the restoration and growth of somatic tissues drops faster than for generative metabolism and spawning processes; therefore, during this period, the organism expends not only nutrient reserves but also structural components. Organismal exhaustion occurs, which, with an increasing number of successive spawnings, reaches irreversible levels, resulting in fish mortality.
Thus, during aging, the synthetic Functions of the organism decline—first the efficiency of protein synthesis, later lipid synthesis, and subsequently reproductive efficiency.
It should be understood that fast-growing, early-maturing fish spawn fewer times and drop out of the spawning stock earlier. Conversely, slow-growing, late-maturing fish remain part of the spawning stock for a longer duration. Therefore, fast-growing fish age more quickly and exhaust the physiological and biochemical resources of their organism required for reproduction.
The information provided serves as a theoretical foundation, the mastery of which can broaden professional understanding of the phenomena and processes that govern the formation and technological management of productive traits in cultured fish.
Last update: 08/08/2026
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