THEORETICAL FOUNDATIONS OF FISH FARMING - I.M. Sherman - 2011

3. THEORETICAL FOUNDATIONS OF FISH BREEDING TECHNOLOGICAL PROCESSES

3.1 Mechanism of Formation of Interior and Exterior Traits in Fish Broodstock

Modern breeding and pedigree work in aquaculture encompasses a wide range of objectives aimed at achieving specific qualities in broodstock. However, the predominant focus remains on developing commercially valuable traits designed to yield a sufficient quantity and quality of offspring capable of ensuring efficient marketable fish production.

From a commercial perspective, quantity and quality combined with minimal yet rational expenditure are the decisive factors for funding such operations. Alongside this core principle, when aiming for the stable transmission of economically valuable traits to offspring, a key objective in shaping the interior and exterior parameters of broodstock is maximizing the yield of edible parts from individuals that have reached marketable body weight under specific technological conditions while simultaneously exhibiting high reproductive performance. In turn, high reproductive performance implies reaching sexual maturity at an optimal age for production, supported by high-quality Gametes from both females and males.

Given the above, it is clear that achieving high performance metrics in broodstock relies on genetic prerequisites and the potential for their realization under cultivation conditions.

Attaining superior interior and exterior traits in broodstock requires specific mechanisms grounded in targeted breeding practices during The formation of broodstock herds under cultivation conditions.

The Organization of breeding programs in Selection and commercial farms may vary depending on their goals, Methods, and management forms. The success of these efforts is largely determined by the initial material selected by specialists. As N.I. Vavilov pointed out, local material that has undergone natural selection during phylogeny and adapted to specific conditions is naturally of great value; it must be comprehensively utilized for breeding, and selection work should begin with it. This principle is particularly important in fish breeding, as their development and productivity depend far more heavily on specific regional conditions than is the case for other farm animals. Across different soil and climatic zones, producers and consumers of various trophic levels form a specific bioproductive potential in different Water bodies, which essentially serves as the food resource for fish. Depending on the COMPOSITION OF THE local ichthyofauna, this food resource can be transformed into a reliable food base. In this regard, specific regional zoning has been developed directly for efficient carp cultivation. From the foregoing, it is evident that, on the one hand, ecological and technological Background indicators provide control levels that serve as regulatory criteria. On the other hand, genetic prerequisites are of paramount importance in shaping the interior and exterior traits of broodstock. Therefore, it is useful to theoretically outline the possible genetic parameters, which requires assimilating relevant information.

The basic principles for establishing broodstock herds in reproduction and commercial farms are as follows: the initial broodstock should be stocked with fish of known origin, preferably from specialized farms. The Structure of the broodstock must enable industrial crossbreeding. To this end, farms should maintain two unrelated groups of fish, namely different breeds or lines. Each of these groups is bred "pure". The crossbred progeny obtained from them is then used for commercial grow-out.

In two-line breeding, it is advisable that the two cultured groups differ in some hereditarily fixed trait, such as scale cover, coloration, or biochemical markers. Such a trait serves as a tag that prevents the accidental mixing of fish from different groups, making the correct selection of breeding material exceptionally important.

A critical issue in fish breeding on commercial farms is the avoidance of Inbreeding. This principle is exceptionally important today because most fish farms are small-scale operations, which significantly complicates management. Broodstock herds on such farms typically consist of a small number of spawners or just a few pairs, inevitably leading to inbreeding. To prevent inbreeding during the establishment of broodstock and its subsequent reproduction, at least 20 pairs of spawners should be used. To obtain offspring for breeding purposes, group spawning is recommended, combining eggs and sperm from multiple spawners (polyspermic Fertilization). Inbreeding can also be avoided by exchanging spawners (males or females), gametes, or fertilized eggs between farms.

To correctly determine the size of the broodstock and replacement stock, one must take into account the farm's production capacity (the planned output of larvae, fingerlings, and yearlings) and the fecundity of females as shown in Table 3.1.

When calculating the size of the broodstock, a spawner reserve of usually 100% is factored in. The number of replacement young across various age groups is determined by the duration of spawner utilization and The rate of annual broodstock replenishment, which under normal conditions reaches 25–35% of the total broodstock population. The operational lifespan of spawners

is determined by their physical condition and ranges from 5 to 7 years. The best offspring are obtained from spawners utilized in reproduction 2 to 4 times.

Class="center">Table 3.1. Estimated productivity of female carp under hatchery-based reproduction

Selecting individuals for breeding is best conducted among fingerlings, yearlings, and selected two-year-olds that have reached sexual maturity. Typically, about 50% of the total number of male fish among yearlings and two-year-olds are selected. For other groups of replacement young, a corrective culling is performed, meaning about 5% of fish that are stunted, sick, injured, or exhibit exterior defects are discarded.

When transferring fish into the breeding broodstock, 50–75% of females and an equal proportion of males are retained. The estimated number of carp replacement young across different age groups for hatchery reproduction is given in Table 3.2.

Table 3.2. Number of replacement young of various carp age groups (for selection) based on an annual rearing of 100 pairs of spawners

* Males of this age are transferred to the broodstock herd.

Determining the timing and establishing evaluation criteria for broodstock based on offspring quality remains one of the least explored areas in aquaculture and requires additional dedicated research. From a practical standpoint, shifting evaluation and selection to earlier stages is of considerable interest, which is especially important when working with salmonids and tilapias. It has been established that METABOLISM/18.html">The Influence of spawners is clearly pronounced in juveniles with a body mass of 1 g, while maximum phenotypic diversity for this trait is observed at 1.5 g, making effective body-mass selection among young fish feasible with high intensity.

Important Factors Determining the success of rearing breeding stock and broodstock are stocking density and feeding regime, as these directly dictate the availability of natural food organisms for the fish.

For the summer maintenance of broodstock and replacement young, drainable ponds with independent water supply and well-designed productivity profiles—categorized as summer-replacement and summer-broodstock ponds—are required. It is preferable to house replacement young of all age groups, as well as sexually mature males and females, separately, since co-stocking leads to a decline in productive performance.

Carp breeding stock is predominantly cultivated in monoculture. In southern regions, polyculture of carp and phytophagous fish is practiced, as the latter act as effective biological ameliorators, improving environmental conditions for the carp.

Fish weight gain over the vegetative period is one of the key indicators characterizing, on the one hand, the foraging conditions of fish and, on the other hand, their breeding quality. Estimated body weights of replacement juveniles across various age groups of certain pond fish species are presented in Table 3.3.

The annual weight gain of carp and herbivorous fish spawners should be at least 1 – 1.5 kg. Naturally, achieving such body weight is possible only under appropriate stocking densities and with a sufficient supply of complete, natural feeds.

Table 3.3. Estimated average body weight of broodstock, g

Stocking density standards required to achieve the target fish body weight gain are given in Table 3.4.

Table 3.4. Fish stocking density, ind./ha

It should be noted that specialists still lack a consensus regarding the optimal stocking density for younger age groups of fish, particularly fingerlings and two-year-olds. In our view, this depends on the development level of the natural food base in specific ponds and the actual possibilities for its management.

At the same time, it is clear that fish stocking density in ponds is determined by the rearing technology and feeding regime. Feeding is of particular importance when fish are practically deprived of natural food, which is typical for cage and tank culture, as well as

under high stocking densities in ponds. Feeding fish with diets unbalanced in essential nutrients negatively affects their physiological condition and significantly impacts the quality of the broodstock produced. Fish must be fed taking into account the planned weight gain, while also accounting for weight losses during wintering and pre-spawning maintenance. For females, feed allowances are planned not only considering the expected weight gain but also to replenish body mass lost during the spawning season; therefore, the actual weight gain for females should be 2.0 – 2.5 kg.

The daily ration of broodstock depends on A number of factors, primarily the Temperature regime and their body weight (Table 3.5).

Table 3.5. Daily feeding rates for carp replacement stock and broodstock at a water temperature of 20 °C

As water temperature and dissolved oxygen content decrease, The amount of feed should be reduced. It is advisable to use various feeder designs during feeding, guided by the Specific features of the fish farms, which significantly increases feed utilization efficiency.

Broodstock are mainly fed a feed mixture containing 26 - 30 % protein; for females, it is preferable to use feeds with a higher content of

CARBOHYDRATES, whereas for males, a higher protein content is recommended. During the period when the water temperature in summer replacement and summer spawning ponds drops to 8 - 12 °C, fish feeding should be continued despite the lack of weight gain. This is the so-called maintenance feeding, which helps preserve the weight, condition factor, and good physiological state of the fish going into wintering.

Wintering is a critical stage in the management of replacement juveniles and broodstock. Most warm-water fish species, including carp, practically do not feed during this period, expending significant body nutrient reserves accumulated over the summer to sustain life. Wintering ponds for carp and herbivorous fish broodstock and replacement juveniles should be relatively small (0.1 — 0.2 ha) and sufficiently deep. Females and males, as well as distinct age groups of replacement juveniles, should be kept separately at a stocking density not exceeding 10 t/ha. As the water temperature rises to 12 - 13 °C, carps should be supplemented with compound feed at a rate of 0.5 - 1 % of their body weight.

After wintering, broodstock are transferred to pre-spawning ponds with an area of 0.2 - 0.3 ha. Stocking density should not exceed 200 - 400 ind./ha for females and 300 — 500 ind./ha for males, i.e., 50 - 60 fish per pond. Higher stocking densities deteriorate conditions and often cause premature spawning, precluding the targeted use of males.

Immediately after transferring the broodstock to pre-spawning ponds, they must be fed. Feed mixtures with an enriched content of animal Proteins and added vitamin complexes are used for this purpose. RGM-5V and RGM-8V pelleted trout feeds can be used for pre-spawning feeding of carp broodstock, feeding them to satiation. The average feed rate ranges from 1 to 3 % of fish body weight, depending on the water temperature. It is advisable to Supplement herbivorous fish broodstock with silkworm pupae during the pre-spawning period. For silver carp, pupae should be ground into a meal-like fraction, whereas grass carp can be fed whole pupae along with green mass. Buffalo and channel catfish are not fed during the pre-spawning period. When carp and grass carp broodstock are kept together in pre-spawning ponds, the latter are supplemented with soft terrestrial vegetation. Feeding macrophytes leads to The production of a large biomass of excreta, which creates an organic fertilization effect, stimulates relevant links in the trophic chain, and improves feeding conditions for carp and, under certain conditions, other fish species as well.

Rainbow trout broodstock are transferred to pre-spawning tanks with a water temperature of 6 — 12 °C 1.5 — 2 months prior to spawning, maintaining a stocking density of no more than 25 ind./m². During this period, broodstock are intensively fed pelleted or paste-like feeds, and 15-20 days before spawning begins, the ration is reduced to 0.5 — 1.5 % of fish body weight.

As stated above, on the one hand, ecological and technological background indicators provide control levels that serve as regulatory criteria. On the other hand, genetic prerequisites play an exceptional role in shaping the exterior and interior performance traits of broodstock. In this regard, it is advisable to understand theoretically possible genetic parameters, which requires mastering relevant information.

Given that the Department of Pond Fish Culture at the Timiryazev Agricultural Academy (TSGA) served as the methodological center for breeding and selection work in the former USSR for a long time, the Materials in this section are based on the core principles described by Yu.O. Pryvezentsev, which remain relevant today.

Special fish genetics is an integral part of breeding work, and an essential characteristic of a species in this regard is the set of chromosomal features known as the karyotype. The chromosome number in fish varies widely, which can be observed by comparing their individual species in Table 3.6.

In breeding, karyotype analysis is necessary when performing distant Hybridization and developing special genetic selection methods.

Cytogenetic control of developing embryos is used when obtaining progeny via artificial (industrial) propagation methods. Currently, the genetics of carp has been studied most thoroughly, and to a lesser extent, that of rainbow trout and peled. Special genetics of other fish species cultivated in commercial farms remains insufficiently studied.

Table 3.6. Chromosome numbers in various fish species

Among qualitative traits in common carp, the inheritance pattern of scale cover has been studied most thoroughly. Based on the degree of scale development, four carp types are distinguished: scaled, scattered, linear, and nude (leather). Scaled carp have a continuous scale cover, with scales forming regular rows along the body. The other three types exhibit a reduction in scale cover, which is most pronounced in nude carp, being almost entirely devoid of scales. Scattered and linear carp are highly variable in the number and distribution of scales. The type of scale cover is determined by two unlinked autosomal genes, each represented by two alleles (dominant and recessive: Ss and Nn). The combination of alleles at both loci determines the carp scale cover type as follows: ssnn, Ssnn — scaled, ssnn — scattered, SsNn — linear, and ssNn — nude.

Dominant alleles in the homozygous state exhibit a lethal effect, which manifests during late embryonic development stages and the hatching period. Crossbreeding carps carrying dominant genes results in 25% non-viable homozygotes in the offspring. Information on the Genetic control of scale cover in carp makes it possible to predict the outcomes of any crossing. The theoretical results of all possible crosses between carp with different scale cover types are presented in Table 3.7.

Table 3.7. Inheritance of scale cover in common carp

Differences in survival rates between carps carrying the N Gene and those lacking it are sharply amplified under unfavorable maintenance and rearing conditions.

The genes controlling scale cover significantly influence many other traits, generally causing major differences among carps with varying types of scale cover. There are about 29 such differences, encompassing morphological traits, biochemical and physiological characteristics, and productivity indices.

A number of morphological traits, specifically the number of soft rays in the fins, are used as additional diagnostic features. Linear and nude carps are characterized by underdevelopment and a reduced number of soft rays in the anal and dorsal fins. Differences in productivity indices also exist: scaled and scattered carps are the most productive when grown in ponds, whereas nude and linear carps exhibit a 15–20% higher growth rate, though lagging behind in overall productivity.

Variability in body coloration is also observed in carp. Common carp typically possess a silvery-gray body color with a greenish sheen. However, individual specimens with altered coloration occasionally occur: blue, black, golden, or gray. Hereditary color variations are also known in other fish species. Several color types have been identified in rainbow trout: albinism, golden, dark yellow, and metallic. Golden fish, much like albinos, are less active than normal rainbow trout; they avoid light, grow more poorly, and are more frequently preyed upon by fish-eating birds. Albinos have also been identified among channel catfish, characterized by poor survival rates, low growth rates, and lower fecundity, which highlights The Importance of taking this fact into account in breeding work.

In recent years, considerable attention has been paid to The Study of biochemical polymorphism in fish. Among cultivated species, carp, trout, and peled have been studied most thoroughly, while herbivorous fish, buffalo, and tilapias have been studied to a lesser extent.

In carp, out of 43 studied protein-coding loci, 21 turned out to be polymorphic. High polymorphism was recorded for transferrin. Nine hereditary transferrin types have been described, determined by nine alleles of the transferrin locus. They are usually designated by capital letters of the Latin alphabet in order of increasing electrophoretic mobility. Transferrins A, B, and C are most frequently found in carp, while transferrin D, characteristic of carps carrying Amur wild carp heredity, is rarer.

Data on the genetics of qualitative traits are widely applied in breeding practice, as evidenced by work with carp, where varying winter hardiness has been established for different transferrin types. Increased Resistance to Oxygen deficiency has been noted in carps heterozygous for the serum esterase gene and in carps with transferrin A. When working with rainbow trout, the best results are obtained by crossing females and males with identical homozygous composition for transferrins and heterozygous for albumins.

In breeding programs, analysis of polymorphic genes makes it possible to determine the degree of genetic differences between various breeding groups. Differences in qualitative traits are also utilized for genetic marking. Scale cover type and biochemical markers are employed as tags when working with carp. Testing fish across a range of protein systems allows for the genetic passportization of individual broodstock and the identification of offspring during co-rearing.

The category of Quantitative Traits includes the majority of economically useful features, including all primary productivity indicators (body weight, survival rate, fecundity, Disease resistance), exterior traits, and physiological-biochemical indices.

Unlike clearly visible qualitative traits, quantitative traits depend on many genes. Traits such as body weight and length, dimensions of individual Organs, and others are characterized by continuous variability. A crucial feature of quantitative traits is the significant Influence of Environmental factors on their magnitude. These features necessitate the application of special biostatistical methods to estimate the proportion of genetic variability within the total phenotypic variability of a trait. To characterize variability, the square of the standard deviation is frequently used, expressed as a percentage of the arithmetic mean (coefficient of variation):

where C_V is the coefficient of variation; σ is the standard deviation; x is the arithmetic mean.

Although the indicator of overall variability is of certain significance for breeding practice, by itself it is still insufficient to determine critical genetic parameters. Therefore, the heritability coefficient — h² — is used to characterize the proportion of phenotypic variability (conditioned by heredity) for various economically useful traits of a specific fish group grown under defined conditions.

The magnitude of heritability depends on many factors and is determined by The Nature of the trait itself. Morphological traits are transmitted more fully, whereas productive qualities are transmitted much more weakly. At the same time, one must account for the significant influence of rearing conditions, which act directly on the realization of an individual's potential capabilities. It is known that the heritability of the very same trait varies considerably across different fish stocks. This indicator is calculated using various methods, predominantly based on biometric Analysis of the degree of dependence of these traits in relatives of varying degrees of kinship, such as through parent-offspring regression. In this case, it is necessary to calculate the regression coefficient b, i.e., to determine by how much a trait Changes in the offspring when it changes by one unit in the parents. For this, the linear regression equation can be used:

where x and y are the mean values of the trait in parents and offspring, respectively; a is a constant representing general differences in trait expression between generations; b is the regression coefficient.

Heritability is also determined by the magnitude of correlation between trait values in close relatives. In fish farming, the correlation between parents and offspring is more frequently applied to calculate heritability coefficients.

Heritability is calculated using the formula H = r and is denoted either as a fractional number from zero to one or as a percentage from zero to one hundred.

The heritability coefficient is the most important population-genetic indicator, as the success of breeding work depends directly on it. If the heritability of a trait is low (H < 0.3), indirect methods of genotype evaluation prove to be ineffective. For traits with high heritability, mass selection based on phenotype is effective. When utilizing heritability coefficients, one must remember that they characterize only those stocks for which their calculation was performed. When initiating selection for a particular trait in a specific stock, the heritability coefficient must first be determined.

To estimate the heritability index, the repeatability coefficient of a trait is calculated, which determines the extent to which the level of productivity (or other quantitative traits) in the current season will repeat in subsequent years. Repeatability indices for a particular trait are most often computed by calculating correlation coefficients between the studied traits over two adjacent years or other time periods; low repeatability indicates low heritability. In stocks with a low repeatability coefficient, breeding work has low efficiency. The accuracy of trait estimation may be reduced due to certain circumstances, specifically drastic changes in housing and feeding conditions, incorrect calculations, or errors in data recording.

The heritability of breeding and certain morphological traits has been most thoroughly studied in carp, rainbow trout, peled, and channel catfish (Table 3.8).

A crucial factor determining breeding efficiency is the interval between generations (generation interval). If the selected trait has a high degree of heritability and high repeatability, shortening the generation interval significantly accelerates the breeding process.

Shortening the generation interval can be achieved by intensifying ontogenesis and accelerating sexual maturation, for example, by rearing fish in warm waters.

Table 3.8. Heritability of some breeding and morphological traits in fish

To plan breeding work and predict the selection response, the selection differential is determined, which is the difference between the mean value of a trait in selected individuals and that of the entire breeding stock prior to selection. The selection differential is measured in the same units as the selected trait, and to compare the selection intensity for different properties, it is expressed in standard deviations.

All the listed indicators are used to determine a more important metric — the selection effect, or selection response:

where E_b is the selection effect; Sd is the selection differential; h is the heritability coefficient; i is the generation interval.

Therefore, breeding and pedigree work requires appropriate qualifications and a material-technical base, which today is practically feasible exclusively under the conditions of

specialized enterprises with the active participation and under the control of relevant research institutions.

The application of the discussed mechanism makes it possible to obtain broodstock with appropriate linear parameters, body mass, and age, as well as to calculate condition factor indices. At the same time, it is clear that obtaining broodstock is a relatively lengthy process, and the necessary conditions must be provided starting from the replacement of young stock.

Under the conditions of the discussed formation mechanism and the application of the proposed process principles, broodstock will possess not only appropriate internal and external (interior-exterior) parameters but also optimal rates of PHYSIOLOGICAL AND BIOCHEMICAL processes, which form The basis of overall health status.

Guided by the information provided, it is clear that optimal combinations of the genetic capabilities of individuals and the ecological-technological conditions under which the interior and exterior traits of fish broodstock are formed constitute the basis of the mechanism under consideration.