How fish caviar appears. How fish breed: features and methods
Reproduction methods. Fish reproduce sexually. In rare cases, fish are found:
1. Parthenogenesis (development of eggs without fertilization), the development of eggs reaches only the stage of crushing (herring, sturgeon, salmon cyprinids) and only in exceptional cases to larvae that survive until the yolk sac is resorbed (burbot, herring). In most cases, such development does not lead to the production of viable juveniles, but in the Issyk-Kul grouse during parthenogenetic development of eggs, normal offspring are observed. In salmon, unfertilized eggs, having found themselves in a spawning hillock in a place with fertilized ones, often develop parthenogenetically. As a result, they do not rot and the entire egg laying does not die.
2. Gynogenesis (the birth of females), spermatozoa of related fish species penetrate the egg and stimulate its development, but fertilization does not occur. As a result of such reproduction, only females are observed in the offspring. In Central Asia, Western Siberia and Europe, there are populations of silver carp, in the reservoirs of Mexico - mollies (a detachment of carp-tooth-shaped), consisting of almost only females.
Fish, as a rule, are of the same sex, but among them there are also hermaphrodites. Among bony fish, hermaphrodites include rock perch, in which eggs and spermatozoa develop in the gonads, but their maturation usually occurs alternately, and red pagell, in which a change (reversion) of sex occurs during life: in young individuals, the gonads function like ovaries, in older ones - like testes. Occasionally, hermaphroditism occurs in herring, salmon, carp, perch fish.
In fish, fertilization occurs:
1) external (in most fish);
2) internal (in cartilaginous fish, in some bony fish - sea bass, eelpout; many carp-tooth-like - gambusia, guppies, swordtails, etc.).
In fish, there are:
1) oviparous, laying eggs in the external environment (most species);
2) ovoviviparous, giving birth to fry. Fertilized eggs linger in the posterior sections of the oviducts and develop there until the juveniles are hatched (most cartilaginous fish are the katran, white shark, fox shark, pilonos); in some species, for example, in the stingray, the walls of the posterior sections of the oviducts (“uterus”) even have special outgrowths, through which nutrient fluid enters the oral cavity of the embryos through the sprinklers;
3) viviparous - in fish in the posterior sections of the oviducts ("womb"), something similar to the placenta of mammals is formed, and the embryo receives nutrients from the mother's blood (blue shark, marten shark, etc.).
The adaptive significance of live birth and ovoviviparity of fish lies in the fact that during intrauterine development, a greater survival rate of juveniles is ensured.
Depending on the nature of reproduction, fish are divided into:
1) monocyclic - fish die after a single spawning (river eel, Pacific salmon, river lamprey, Baikal golomyanka);
2) polycyclic - fish breed several times during their life (most fish).
The age of puberty in fish varies considerably - from 1 - 2 months (gambusia) to 15 - 30 years (sturgeons). Fish with a short life cycle mature earlier (tulka, smelt and some gobies - at the age of 1 year), fish with a long life cycle become sexually mature much later (Atlantic cod - at 7-10 years, sea bass - at 12-15 years, etc.). .d.).
The age of puberty of fish depends on the species, the living conditions of the fish, primarily on the conditions of fattening. The onset of puberty in fish occurs when a certain length is reached. As a general rule, the better a fish is fed, the faster it grows and, therefore, the faster it matures. Males usually mature before females.
The rate of maturation is also influenced by climatic conditions. So, in a bream, puberty occurs in the Aral Sea at 3-4 years, in the Northern Caspian at 3-6, on the Middle Volga at 6-7, in Lake Ladoga at 8-9 years.
The age of sexual maturity is important in determining the size of the fish catch and the estimation of raw materials. In short-cycle fish maturing in the 2nd-3rd year of life (sprats, sprats, sardines, etc.), the allowable commercial removal from the population can be 40-60%; in long-lived fish, this removal should be significantly less (5-20% ).
Sexual dimorphism. In most fish, sexual dimorphism (secondary sexual characteristics) does not appear, females and males are outwardly difficult to distinguish. In some species, secondary sexual characteristics are pronounced: females are larger than males, males are characterized by a brighter color, elongated fins, etc. Males of the polar flounder have ctenoid scales, females - cycloid. Male cartilaginous fishes have copulatory organs (pterygopodia), while females do not; in tench males, unlike females, the first ray of the ventral fins is thickened, etc.
In some fish, during the pre-spawning period, under the influence of sex hormones, a nuptial attire appears, which disappears after spawning. Many cyprinids and whitefishes develop white horn formations on the head and body of males - “pearl rash”; round goby males become completely black by the time of spawning; the abdomen of the male stickleback changes from silvery to bright red. In Pacific and Atlantic salmon, during spawning, the silver color darkens, black, crimson spots appear on the body, significant morphological changes are observed (the jaws lengthen and bend, changes in the cranial skeleton are observed, and a hump grows in pink salmon).
Sex ratio. The sex ratio is an adaptive property of fish and is aimed at ensuring successful reproduction. In most fish it is close to 1:1.
The sex ratio in fish can change under the influence of various factors. In guppies, a significant development of saprolegnia is sometimes accompanied by the transformation of most of the surviving females into males. The effect of high temperature on the fry of the green swordtail leads to the predominance of males.
Hormonal drugs can also contribute to changing the normal sex ratios in fish. In fish farming, when breeding commercial fish, a directed sex change can occur by introducing steroid hormones into the feed (rainbow trout).
Among the fish are distinguished:
1) monogamous - one male (salmon) usually spawns with one female;
2) polygamous - for one female there are 3-4 or more males (carp) or one male ensures the fertilization of eggs of several females (stickleback).
Terms of reproduction and features of spawning. Depending on the timing of reproduction, fish are distinguished:
1) spring spawning (pike, perch, grayling);
2) summer spawners (carp, sturgeon, anchovy);
3) autumn-winter spawning (salmon, Pacific salmon, whitefish, burbot, navaga).
The timing of reproduction of each species, as well as the timing of hatching of larvae and development of juveniles, are related to the availability of their food. Thus, the pike breeds immediately after the ice melts - much earlier than cyprinids, which allows its juveniles to reach a length of 5-6 cm and completely switch to feeding on larvae of cyprinids.
The timing of reproduction of the same species may be different. Thus, capelin near Finmarken and western Murman spawns from March to May, near eastern Murman - in June-July, in the eastern part of the Barents Sea - in August-September. Inhabitants of middle latitudes usually spawn once a year, but some of them do not spawn annually, but at intervals of 2 to 6 years (sturgeons), many tropical fish breed several times during the year.
According to the duration of the spawning period, 2 groups of fish are distinguished:
1) with one-time spawning - all eggs ripen at the same time (pike, whitefish, salmon);
2) with batch spawning - caviar matures and spawns in portions for a long time (anchovy, bleak, Caspian herring, catfish, etc.).
In stickleback, the process of spawning is measured in several seconds, in roach and perch - in hours, in carp and bream - in days. Cod spawning 3-4 portions of caviar during the spawning season spends 1.5-2 months on the spawning ground, sultanka - 3 months.
Often, one and the same species in one reservoir has a one-time, and in another batch spawning. So, in the Aral Sea, bream spawning in portions is observed, in the northern reservoirs (Lake Onega, etc.) it spawns at a time.
Portion spawning is an adaptation of the species to the influence of environmental factors and contributes to an increase in fertility, greater survival of eggs and larvae, better nutrition of juveniles due to the uniform use of the food supply.
Scale, coefficient and maturity index. To assess the degree of maturity of reproductive products in fish, maturity scales are used, of which the six-point scale is the most common for polycyclic fish with simultaneous spawning.
I stage - juvenile (juvenales), immature fish. The sex cells of the ovaries are indistinguishable with the naked eye, and the sex is not visually determined. The ovaries and testicles look like thin transparent bands of yellowish or pinkish color.
Stage II - maturing individuals or individuals with developing reproductive products after spawning. The eggs are very small and only visible under a magnifying glass. The ovaries are transparent and colorless, a large blood vessel runs along them. The testicles increase in size, lose their transparency and look like rounded bands of a grayish or pale pink color.
Stage III - individuals in which the gonads are far from maturity, but relatively well developed. The ovaries fill from 1/3 to Y of the entire abdominal cavity, filled with opaque eggs, clearly visible to the naked eye. Seed plants are dense, elastic. When pressed, liquid milk cannot be released from the testicles. Their color is from pinkish-gray to yellowish-white.
Stage IV - individuals in which the gonads have almost reached full development. The ovaries and testes are the largest and fill up to 2/3 of the entire abdominal cavity. The eggs are rounded, transparent and flow out when pressed. The testicles are soft, white, filled with liquid milk, flow out when pressed.
Stage V - current individuals. Caviar and milk are so mature that they flow freely with light pressure on the abdomen.
Stage VI - spawned individuals (stocking). Sexual products swept out completely. Gonads in the form of falling bags. In the ovaries, the remaining eggs may be observed, in the testes - the remnants of sperm. The ovaries and testicles are inflamed, dark red. Some time after reproduction, the ovaries and testes pass into stage II of maturity.
In fish with batch spawning, the stage of maturity is determined by the state of the portion that is most developed and will be spawned before anyone else. After the first portion is laid, the ovaries do not pass into stage VI, as in fish with simultaneous spawning, but into stage IV or III, and these stages of maturity are designated VI-IV or VI-III. Then, after the completion of the entire spawning period, the state of the ovary is assessed as being in stage VI, and then in stage II. If the remaining oocytes (next year's reserve) begin to grow already at stage VI, then the ovary from stage VI passes into stage III and is designated VI-III.
When assessing the degree of maturity of the gonads of fish, the coefficient and index of maturity are used.
Maturity coefficient - the ratio of the mass of the gonads to the mass of the body of the fish (in%). In fish with spring-summer spawning, the maturity coefficient is highest in spring, decreases in summer, and begins to increase again in autumn (carp, zander, vobla, etc.). Fish with autumn-winter spawning have the highest maturity coefficient in autumn (salmon). Maturity index - the percentage ratio of the gonadal maturity coefficient, calculated in certain periods of gonadal maturation, to the maximum maturity coefficient.
The structure of the sex products. Fish eggs differ in shape, size, color, presence of fat droplets, shell structure. In fish, the eggs are usually spherical in shape, but other shapes are also found. Representatives of the garfish have a spherical egg with filamentous outgrowths; in gobies, pear-shaped eggs at the lower end are equipped with a rosette of threads for attaching to the substrate; anchovies have elliptical eggs, etc.
The size of the eggs, like other morphological characters, is a stable feature of the species. Large fish lay eggs of a larger diameter. The size of the eggs depends on the content of the nutrient (yolk) in them and fluctuate significantly. Among bony fish, the smallest eggs are found in the limand flounder, the largest in salmon (chum). Cartilaginous fish have the largest eggs, so in different-toothed sharks 1.5 m long, the length of the egg capsule is about 10 cm. The development of embryos in some of them lasts a very long time - 18-22 months (katran).
The color of eggs is specific for each species. Caviar that develops under less favorable oxygen conditions is usually more intensely colored. In vendace, the eggs are yellow, in salmon - orange, in pike - dark gray, in carp - greenish, in greenlings - emerald green, blue, pink and purple. The yellowish and reddish color of caviar is due to the presence of respiratory pigments (carotenoids). Pelagic eggs that develop with sufficient oxygen content are poorly pigmented.
The eggs of many fish contain one or more fat droplets that provide buoyancy to the eggs.
The eggs are covered with shells on the outside:
1. Primary - yolk (radiant) membrane, formed by the egg itself, penetrated by numerous pores through which nutrients enter the egg during its development in the ovary. In some species, this shell is two-layered (sturgeons).
2. Secondary - gelatinous, sticky (develops over the primary shell), with a variety of outgrowths for attaching eggs to the substrate.
At the animal pole of both membranes there is a special channel - the micropyle, through which, during fertilization, the sperm enters the egg. Teleosts have one canal, while sturgeons can have several.
3. Tertiary - horny (in cartilaginous fish and hagfish) and protein (only in cartilaginous).
In lampreys, as in bony fish, the eggs are small, in myxines they are ellipsoidal in shape, 2-3 cm long. On the cornea of the myxin there are hook-shaped processes, with the help of which the eggs are attached to each other and to underwater objects. The cornea of cartilaginous fish is much larger than the egg itself, often horny filaments depart from it, with the help of which the egg is attached to aquatic plants.
Spermatozoa vary greatly among fish species. The spermatozoon is divided into a head, a middle part and a tail. The shape of the head is different: spherical (in most bony fish), rod-shaped (in sturgeons and some teleosts), spear-shaped (in lungfish), cylindrical (in sharks, crossopterans). The core is placed in the head. Acrosome is located in front of the nucleus in sharks, sturgeons and some other fish. Bones don't have it. The sperm secreted by the male consists of spermatozoa immersed in a spermatic fluid similar in composition to saline. In the spermatic fluid, spermatozoa are immobile. In contact with water, their activity increases dramatically. Having met the eggs, they penetrate them through the micropyle, after which fertilization occurs. The duration of sperm activity depends on the salinity and temperature of the water. In salt water, it is much longer - up to several days (Pacific herring), in fresh water - no more than 1-3 minutes (in most fish - carp, salmon, perch).
In the same male, spermatozoa are not qualitatively the same and differ in size; during centrifugation, they are isolated: small (light), medium (intermediate) and large (heavy). Among large spermatozoa, X-gametes are found in large numbers, among small spermatozoa, Y-gametes. Thus, mainly females are born from eggs fertilized by large spermatozoa, and males are born from small ones. This is of great importance in the artificial breeding of valuable fish species.
Laying eggs. The start of fish spawning depends on various factors (readiness of sexual products, water temperature and salinity, availability of spawning substrate, etc.). Water temperature is an important abiotic factor. Each species during the breeding season is characterized by optimal and limiting water temperatures. Polar cod, navaga, arctic and antarctic fish breed at sub-zero temperatures. The minimum water temperature at which spawning is possible is -2.3°C for saffron cod, +3.6°C for cod, +4.5°C for Atlantic herring, and 13°C for carp. In many cyprinids, the most intensive spawning is observed at a temperature of + 18-20 ° C and above.
Fish lay eggs in different conditions, some species spawn in the tidal zone (pinagor), others in the oceanic pelagial at depths of more than 1000 m (eel). The vast majority of marine fish spawn in relatively warm areas of the coastal zone at depths of less than 500 m, where a high concentration of food organisms and larvae are provided with food. If there are no conditions for reproduction and spawning does not occur, or the eggs are not completely spawned, they are absorbed.
Caviar is distinguished:
Pelagic (floating);
Bottom (demersal), deposited on the ground and bottom vegetation.
Depending on the place of spawning, fish are divided into the following groups:
1) lithophiles - lay eggs on stony-pebble soil (sturgeon, salmon, kutum, shemaya, chub, podust);
2) phytophils - lay eggs on plants and algae (vobla, bream, carp, crucian carp, perch, Pacific herring);
3) psammophiles - lay eggs on the sand (gudgeon);
4) pelagophiles - lay floating eggs in the water column (sprat, anchovy, Atlantic cod, sabrefish, grass carp, silver carp);
5) ostracophiles - lay eggs in the shells of bivalve mollusks (mustards).
Caring for offspring. Most fish do not care about their offspring. However, there are a number of species that build various nests, protect eggs and larvae.
Pacific and Atlantic salmon dig nests up to 2-3 m long and 1.5-2 m wide in the ground with their tails, lay their eggs in them, fertilize them and cover them with gravel. The male stickleback builds a muff-shaped nest from plant remains and guards the eggs. The male pikeperch clears a place at the bottom for future laying of eggs, then guards it, cleans it from silt, washing it off with strong movements of the pectoral fins. If the clutch is left without a guard male, then another one continues to guard. Labyrinth fish build a nest of air bubbles, enveloping them with sticky secretions from their mouths. The male lumpfish guards the laying of eggs laid in the littoral zone and, when dried, waters the eggs from his mouth with water.
Some fish bear fertilized eggs, for example, the female tilapia keeps it in her mouth. The most perfect form of care for offspring can be considered live birth in fish.
Fertility and reproductive ability of fish. In fish, absolute (individual), relative and working fecundity are distinguished.
Absolute (individual) fecundity - the amount of eggs laid by the female during one spawning period.
The fecundity of fish is an adaptive property of the species and varies considerably. Cartilaginous fish have the lowest fecundity. Manta ray gives birth to one cub. In sharks, fertility ranges from 2 to 100 eggs or fry, and only the polar shark spawns about 500 large eggs 8 cm long (without cornea). In bony fish, the fish that spawn pelagic eggs have the highest fecundity (moon fish - up to 300 million eggs, molva - about 60 million, cod - up to 10 million eggs).
For fish showing concern for offspring, less fecundity is characteristic. So, the viviparous eelpout spawns from 10 to 400 larvae, the stickleback lays 60-550 eggs.
The quantity and quality of caviar depend on body weight, age, fat content and environmental factors. With the growth of the fish and the increase in its body weight, the absolute fecundity increases.
Fish are able to regulate fertility depending on changing environmental conditions. Greater fecundity is developed in species under conditions of more intense mortality. Changes in absolute fecundity are regulated through changes in food supply. Improving feeding conditions leads to an acceleration in the growth rate and, consequently, to a higher fecundity of the same-sized fish. In this regard, the fecundity of one species in different water bodies is different, reflects the conditions for the existence of fish and is aimed at ensuring a certain amount of recruitment.
Relative fecundity is the number of eggs per 1 g of female body weight.
Working fecundity is the number of eggs obtained from one female for fish breeding purposes. In peled, it is about 70% of the absolute (individual) fecundity.
In some cases, species absolute and population fecundity are calculated.
The reproductive ability of fish is strongly influenced by their age, since the quality of reproductive products varies throughout life. In most species, the highest quality offspring are obtained from middle-aged fish. Young and very old individuals produce less viable offspring.
The duration of the incubation period, the survival of eggs and larvae. In fish, the duration of the incubation period ranges from several hours (zebrafish) to 22 months (spiny shark). For the incubation of eggs, a certain amount of heat is required, expressed in degree days. This value varies depending on the temperature of the water. With an increase in water temperature (within the limits characteristic of this species), the development of eggs proceeds faster. In carp fish, eggs develop within 3-6 days, in navaga - 3-4 months, in salmon - up to 5-6 months
The population size largely depends on the survival rate of embryos and food availability of larvae at the stage of transition to active feeding. These periods account for the highest mortality in comparison with all other periods of the fish's life. The main factors determining the survival of embryos and prelarvae are water temperature, salinity, gas regime, wind, and waves. The high fecundity of some fish cannot indicate their high abundance, since the survival rate of eggs and larvae is very low.
Metamorphosis. In some fish, the development of larvae takes place with metamorphosis (flounder, river eel, moonfish, etc.). The flounder has symmetrical larvae that swim in the upper layers of water with their backs up, over time they gradually sink into deeper layers of water and lose their bilateral symmetry, one eye goes to the other side of the body, and after metamorphosis is completed, the young flounder begins to lead a bottom lifestyle . The larvae of river eel (leptocephalus), hatching from eggs in the Sargasso Sea, have a leaf-like shape. Within 2-3 years, they drift with the Gulf Stream, turn into transparent eel-like fish that enter the rivers of Europe, where they grow, lose transparency and turn into adult eels.
It is well known that fish lay eggs during the breeding season. However, not all fish hatch from eggs, there are also viviparous ones, but there are a minority of them, and, moreover, they are mainly inhabitants of great depths - some marine fish, but in our case we are talking about inhabitants of fresh waters *. And they usually spawn.
* (Some freshwater fish from tropical waters used for breeding in aquariums are also viviparous. - Approx. ed.)
The reproduction of fish is distinguished by features that depend on the conditions of their existence - on life in the water. First of all, in most fish, fertilization takes place outside the body. And not only eggs, but also sperm, before fertilization occurs, remain in the water for some time.
Each type of fish has its own characteristics of spawning and the caviar itself. They do not have the same number of eggs, their appearance, size, color, weight. Even in the same species in different places, and consequently under different conditions, there are differences in this respect.
The number of eggs in fish varies enormously - from a few tens of pieces to millions and even hundreds of millions. Fish are generally much more prolific than land vertebrates. Most spawners are those fish that lay pelagic eggs; they are followed by spawners in more reliable places - on plants or various underwater objects. There are still significantly fewer eggs in fish that hide their eggs or protect them.
The amount of caviar also fluctuates over the years. In some fish, it is more in years rich in food, and less in more hungry. In fish that spawn not all at once, but in portions, most often the largest portions are spawned at a time when the juveniles that have emerged from it are best protected from harmful influences and unfavorable conditions.
Most freshwater fish lay their eggs in the warm season - usually in the spring. A number of fish spawn at lower temperatures in autumn and winter (salmon, whitefish, burbot). In general, the timing of spawning is largely influenced by air and water temperature.
The duration of spawning itself is also not the same: for some it fits in a month or even less, for others it stretches for a whole summer.
The duration of the spawning process in individual fish is also different. Some fish spawn all eggs within an hour or several hours, while others do this for several days.
Most freshwater fish spawn on aquatic vegetation, branches in the water, on solid objects, on the bottom. Their caviar is usually heavy, sinking in water. In a number of fish, it swells in water, and a sticky substance is released from it, attaching it to the substrate (carp, pike perch, roach, etc.).
The ability to spawn, as well as to fertilize eggs, in different fish occurs at different ages, and here too there are very significant fluctuations: some fish become mature in the first year of life (for example, smelt), others only after having lived about a quarter century (beluga).
With poorer nutrition and low temperatures, the fish mature later.
When breeding time approaches, the fish begin to show a special mobility. Living constantly in a particular reservoir, the so-called residential, fish spawn there, while others travel to the spawning site - spawning migrations. These migrations are one of the protective devices that provide more reliable and safe conditions for eggs and juveniles, that is, for the preservation of the species as a whole.
The timing of spawning migrations, as well as the time of spawning, vary greatly among fish. Even in the same species, two varieties are sometimes noted, or, as they are called, races - winter and spring, depending on the timing of their entry for spawning.
Almost always, the movement of fish for spawning occurs just before spawning. But in some salmon, cyprinids and sturgeons, spawning migration coincides with wintering: fish appear in the area where they should spawn and stay here for the winter, and spawning itself takes place right there, but after the end of wintering.
There is a very significant difference not only in the time of migration for spawning in different fish, but also in its range. Some fish leave the sea for rivers for only a few kilometers, others for thousands of kilometers. And again, differences in this respect are observed even in the same species.
At present, in connection with the construction of reservoirs and the construction of dams, the path of a number of fish to spawning places is blocked, and therefore for some species the very route of their travel has changed.
Traveling fish to spawn, not without reason, the writer Prishvin called "selfless wandering." Indeed, many fish (salmon, trout) on their way to spawning grounds have to face difficult obstacles to overcome - shallow rifts, waterfalls, etc. And the journey itself against the current of water is associated with a very large expenditure of energy, especially with a fast current. Even more difficult is the journey of fish during the floods, when they overcome a particularly strong current.
During spawning migration, fish usually feed much less or even not feed at all. Meanwhile, they have to spend a lot of energy at this time. Therefore, they get to spawning grounds very exhausted. For example, in a white fish, the amount of fat decreases from 21 to 2 percent. In some fish, the fat on the internal organs disappears altogether.
In general, by the time of spawning, significant changes occur in the body of many fish. So, in connection with the cessation or weakening of nutrition, the intestinal walls become thin. The action of enzymes secreted by the glands also changes. In anadromous fish, the blood pressure in the vessels changes, which is associated with a change in the salinity of the water. The content of vitamins in their body also changes: instead of some vitamins, others are produced.
In some fish, the appearance also changes markedly, for example, in eels, the eyes are greatly enlarged (3-4 times), the head is pointed.
Many fish appear in the so-called mating attire, which manifests itself in a change in color, body proportions, the appearance of various formations on the body and head, etc. As a rule, males (salmon, many cyprinids) look the most “dressy” during the spawning period.
In freshwater fish, spawning migrations are of a different nature than in anadromous ones: they migrate within fresh waters - from lakes to rivers or to flooded meadows. Freshwater fish during the migration to spawning usually do not stop feeding, although it becomes less intense for them.
Spawning takes place differently: in some fish it is quiet, imperceptible (zander), in others it is violent, accompanied by splashes, movement of water, noise (carp, catfish).
Some fish are monogamous, others are polygamous, that is, in some fish, one female and only one male take part in spawning, while others have several males. In catfish, salmon and some other fish, there is a fight between males over females.
There are fish that, before spawning, arrange nests and then guard the eggs and juveniles; guards are carried either by the male together with the female, or by only one male (perch, stickleback).
Fish often arrive at the spawning site in a certain order: the larger females appear first.
There are other differences between the sexes, mainly in the structure and size of the fins, which are larger in males, and sometimes there are swellings between them. The larger size of the fins in males contributes to the fertilization of eggs (the swirl of water near it, due to which the spermatozoa more easily fall on the eggs).
In order to improve the conditions for spawning and the development of juveniles of semi-anadromous and river fish, reclamation measures are being taken on natural spawning grounds.
* (Reclamation means improvement.)
The regime of spawning grounds in different basins differs in its own characteristics, in connection with which the nature of reclamation measures is also not the same. For example, in the deltas of the Volga, the Urals, and the Danube, semi-anadromous fish spawn on ilmens and fields. These places are constantly clogged with sediment that comes with flood waters. In dry years, fish cannot spawn here.
In the delta of the Kuban and a number of other rivers, fresh and slightly saline estuaries serve as spawning and feeding grounds for commercial juveniles (perch, ram, carp, bream). They warm up well, are rich in food organisms, and are bordered by vegetation. Therefore, estuaries are excellent spawning grounds for semi-anadromous fish. But often the estuaries become silted up and overgrown (the river fills the channels with silt); they are poorly connected with the sea, as the mouths are clogged with drifts; they have insufficient depths, weak flow, high salinity. There are a lot of weed and predatory fish in the estuaries.
In view of the systematic deterioration of the conditions for spawning of semi-anadromous and river fish, it is necessary to improve the natural spawning grounds of valuable commercial fish.
It is necessary to clear and deepen the channels and erics on the ilmens and hollows in the deltas of the Volga, the Urals, and the Danube. In order to combat the swamping of water bodies, it is necessary to systematically mow tough vegetation and clear the rifts in the channels. To regulate the water regime of the hollows during the flood period, sections of the delta and channels are collapsed, a system of locks is arranged.
Land reclamation should aim to desalinate the estuaries, protect them from overgrowing with hard vegetation, and ensure a normal connection between the estuaries and the sea. The lack of connection with the river leads to salinization of estuary spawning grounds, and the loss of connection with the sea leads to complete drying, which is often accompanied by salt precipitation.
To improve the conditions in spawning estuaries, it is also necessary to build powerful desalination systems, consisting of a whole network of canals.
The marriage attire serves to recognize the sexes, stimulate maturation and return of the reproductive products. As a rule, eggs are laid by fish in the evening or early in the morning. The duration of spawning varies from 30 minutes to several hours and even days with rest intervals - brachidanios, atherine, Princess Burundi, etc. An increase in productivity can be achieved with separate keeping and proper feeding of producers for 10-15 days before spawning. Key stimuli that stimulate can be individuals of the opposite sex, composition (pH, dH, EH), movement, change, water level, temperature changes, duration and intensity of lighting (lunar phases in Indian cyprinids, catfish), substrate (soil, vegetation , and with a certain leaf configuration and bushiness, etc.), the composition of microorganisms, the presence of satellite fish, water-soluble, species-specific metabolites responsible for (copulin), etc. Thus, exposing aquariums to direct sunlight stimulates the coloration, growth and reproduction of young females. The spawning cycle also depends on the length of daylight hours. The addition of calf thymus gland to feed leads to an increase in the number and size of offspring, fry feeding thyroid gland accelerates metamorphosis, but slows down growth.
Before breeding fish, you should check the availability of food for future offspring. The best food for juveniles will be "live dust", so all breeding is planned for the period from April to September (in the absence of cultivated organisms). Monogamous (paired family) include four-toothed: cichlids, randidae, snakeheads, etc., polygamous (schooling family) - iris, goby, eleothre, stickleback, etc. School spawning (among cyprinids, characids, etc.) is not always justified, especially if the fish destroy their eggs. In such cases, the amount of substrate should be increased, and spawning grids should be laid on the bottom. Fish with high intraspecific aggressiveness (distichodes, leporines, tropheuses, etc.) should be gradually accustomed to each other or the spawning area and the number of shelters should be increased. The volume of breeding tanks can vary greatly. For nanostomes, 200 ml is enough, and for a pair of discus, at least 100 liters. Fish that take care of their offspring (Loricaria catfish, anabas, cichlids, etc.) are able to raise young in a common aquarium. But usually, the development of caviar occurs either in adjustable containers, or in special incubators, modifications of the Weiss apparatus, etc. For disinfection, methylene blue, rivanol (1-2 mg/l), malachite green, violet K, bright green oxalate (0.5-2 mg/l) are added to the water.
During the development of caviar, as a rule, 6 phases are distinguished:
1. Dead, unfertilized eggs with no signs of crushing (soonest removal after spawning).
2. Not yet developed, but fertilized (for opaque caviar, division occurs at the sharp end).
3. Germinal disc.
4. Transparent, young embryo.
5. Pigmented embryo.
6. Embryo with pigmented eyes.
Over-ripening of eggs does not reduce its ability to fertilize, but greatly increases the percentage of waste during development. The moment of pigmentation of the eyes of the embryo (eye stage) signals that the critical periods of development are left behind. In this state, eggs can be subjected to transportation and other manipulations without significant harm to the embryo (minor death is also observed in the early stages, immediately after spawning). The formation of eggs and embryos is accelerated by temperature, an increased content of oxygen in water, iron (up to 1 mg / l), vitamin B 2, and sometimes an increase in salinity (0.5 - 3% o). The hatching process is regulated by a special enzyme - chorionase. Mass hatching can be stimulated by adding water from an aquarium where the fry have just hatched or freshly shed egg shells to the eggs. The sum of temperatures multiplied by the number of hours or days of incubation is a roughly constant value called degree days or degree hours. For trout, development lasts 205 days (410 degree days) at 2°C, 82 days (410) at 5°C, and 41 days (410 degree days) at 10°C. After hatching of the embryos, the doses of the drug are gradually reduced to zero (by water changes, activated carbon filtration), and the salinity of the water (for soft water fish) is slowly raised. A free embryo has a yolk sac with a supply of energy substances that feed it in the first hours or days of life. At this time, he leads a passive existence, lies on the ground, suspended from plants, snags and stones with the help of a cement organ or sticky threads. In polyfins, the attachment organ arises due to a protrusion of the intestinal wall, in African and American bilungs it is a transverse groove in the pharynx, in most bony fish it is formed on the upper side of the snout or in the brain part of the head. Gill covers and pseudobrancia in larvae supply oxygen to the brain and eyes, and capillary networks of blood vessels, yolk bladder, pectoral, dorsal, anal fins and external gills supply the rest of the body. Resorption of the bag is a signal for the immediate feeding of the ear to the actively moving larval embryo. Petrochromes, Cyphotilapias, etc. to stimulate the resorption of the bag and the correct differentiation of the gastrointestinal tract, plankton microdoses are given 3-5 days earlier than expected. The larva becomes a small fish only by acquiring a scaly cover and the structure of an adult fish.
Well-grown, even in size (undergrowths and overgrowths are culled), healthy juveniles with desirable traits (brightness and purity of color, proportions of the body and fins, mobility, etc.) are selected for the tribe, from which spawning pairs or groups are subsequently formed. This is the so-called mass selection. With purposeful work and keeping diaries, internal, no less important features can be taken into account: growth rate, disease resistance, maturation speed, etc. Here, individual selection comes into force, which consists in evaluating each specific producer according to the quality of the offspring or in comparing quality indicators several families (family selection). A skillful combination of both selection methods will provide the best end results.
Free crossing in nature of all individuals (more often at the subspecies level) is called panmixia. Numerous experiments on sexual selection have shown that the female selects the most brightly colored male during copulation. Thus, the golden principle in the final pairing will be - best for the best.
Without knowledge of the basics of genetics, today it is impossible to competently deal with the issues of reproduction of any animals, and in particular fish. Fish breeding is based mainly on selection for quantitative traits due to changes in both internal (genotype) and external (phenotype) factors. Hereditary inclinations (genes) of germ cells (gametes) are closely related to the behavior of fibrous, spirally twisted structures - chromosomes enclosed in cell nuclei. Genes are segments of a continuous molecular chain of DNA (deoxyribonucleic acid). The commands transmitted by chromosomal DNA using transfer RNA to numerous ribosome protein synthesizers scattered throughout the cell are based on the genetic code that determines the order and ratio of amino acids (their number is 20) in newly formed protein molecules. Transformations of any part of the DNA molecule lead to a change in the protein composition of the organism and ensure the adaptation of the species through natural selection to new living conditions. Each species usually has a constant number of pairs of chromosomes from 16 to 240. When a new organism develops, any pair of its homologous (outwardly similar) chromosomes (diploid set) includes a chromosome (haploid set) from each parent. The chromosomes of each pair are thus different from the chromosomes of all other pairs. With two pairs of chromosomes, as a result of divisions of maturation, four types of gametes are formed (in females and males). The addition of one pair of chromosomes doubles the number of chromosome combinations each time. As a result, the number of gamete types is equal to In, and the number of different zygotes (after fertilization) is equal to 4n, where n is the number of pairs of chromosomes. With 24 pairs of chromosomes - the number characteristic of many fish species, the number of individual gametes will exceed 16 million, and zygotes (in one crossing) - 250 trillion.
The problem of closely related breeding (inbreeding) and its extreme manifestation - inbreeding (mating of producers with their offspring) is that the offspring inherit from their parents the same properties (harmful and beneficial), which, with further breeding, increase in themselves (homogeneity). The genetic material obtained from non-identical parents ensures the diversity (heterogeneity) of future generations, i.e., the effect of heterosis - hybrid force - will constantly manifest itself. As can be seen from previous mathematical calculations, the danger of inbreeding for fish is greatly exaggerated. However, outbreeding (periodic, once every 3-5 years, crossing with unrelated producers) and optimal conditions for keeping and breeding allow it to be finally removed.
Dominant (strong) traits always prevail over recessive (subordinate). When crossing in the first generation, all individuals will have a dominant type. Only with further breeding do we get 25% purebred dominants - 50% crossbreds, but with a dominant trait, and 25% purebred recessive individuals, which visually looks like a 3:1 split. Selection on two grounds gives a distribution of 9:3:3:1.
The culling of individuals deviating from the desired breeding type must be rigid. The fry, which have the worst exterior compared to their parents, are removed from the aquarium. A hybrid is the end product of crossing at least different species, while a crossbreed is the result of a fusion of breeds. In practice, the following forms of crossing are used:
1. Industrial (mass) - ensures the growth of diversity, but does not consolidate the breed.
2. Synthetic crossing, which allows you to combine the desired characteristics of the original breeds and at the same time increases heterogeneity.
3. Introductory crossing - contributes to the improvement of the breed by strengthening ce on a new improving genetic material.
4. Absorption - after the initial crossing of two breeds, the combination of crossbreeds with individuals of the improver breed is carried out.
5. Alternative crossing - alternate mating of crossbreeds after the first crossing with individuals of 1, two original breeds, in the fourth or fifth generations is replaced by reproductive crossing, stabilizing the desired traits as a result of breeding in itself.
An additional and very significant source of variability in fish are mutations - changes in chromosomes and genes. Mutant genes that cause steel coloration and albinism reduce the viability of pure lines. Influencing hard radiation and chemical compounds (nitrosmethylurea - 0.97 - 9.7 mm (millimol), dimethyl sulfate - 0.11-0.13 mm, etc.), artificial mutations can be caused. They are classified into point (gene), chromosomal rearrangements (inversions, translocations, etc.) and polyploidy (presence of one or more additional gene sets). The peciliops (P. turrubarensis), for example, has a triploid set of chromosomes. The multiploidy of larvae is caused by cold and exposure to cytochalasin.
When determining the sex of fish, it turned out that guppies, pecilia sphenops, medaka, and others belong to the XX-XY type (male heterogamety). A Xiphophorus maculatus have both male (XY) and female heterogametic (WY) and even three types of sex chromosomes (WY, WX, XX).
Different lines of Mozambican tilapia have heterogametic females and males. Crossing these lines led to the formation of only males in the offspring. In green swordsmen (X. helleri) and black macropods (M. orercularis concolor), sex differentiation depends on male and female hereditary factors located in autosomes (there are no sex chromosomes).
It has long been noted that female fish under the influence of certain conditions (environment, hormonal preparations, etc.) turn into males (gambusia, melanochromis, etc.). The female sex hormone (estrol) and the male sex hormone (methylgestosterone), when added to water or food, redefine sex. Japanese geneticist Yamamoto turned female goldfish into males. With further breeding, all the offspring turned out to be female. In guppies, females with male fins are sterile.
Aquarists have bred about 200 species of aquarium fish (angelfish, goldfish, cockerels, viviparous fish, barbs, etc.). Often, very spectacular offspring are obtained by hybridization of spawning toothed carps, cichlids, gambusias, cyprinids, etc. As a result of "reprehensible" inbreeding (constant crossing of one-litter fry obtained from a single pair of breeders), in the 4th-6th generations, many albino appear (labeo, swordtails, minor, neon, pristella, aripirangsky nanno-stoma, platies, gambian barbs, oligolepis and sumatranus, speckled catfish, plecostom, guppies, pseudo-tropheus zebra, etc.), veiled (heteromorph parsing, hasemania, blackthorn, cardinal, zebrafish, fire barbus, angelfish) and chromic forms (cichlazomas, barbus- "mutant", parrot cichlid, etroplus, tilapia aurea, labeotropheus, etc.).
It is possible to noticeably accelerate the growth rate of fish with the help of intramuscular injections (mg / kg): bovine growth hormone or bovine insulin (10), 4-chlorotestosterone acetate (0.5), thyroidin (10), testosterone propionate (10), methidandrostenediol (4 mg / kg every 4 days); or when added to feed (mg/kg): ethylestrol (2.5), 17-methyl-testosterone (1-2), dimethazine (5), 17-ethynyltestosterone (2.5-3.5), methylandrosterone (15), androstedione (500), testosterone propionate (560), androsterone (580), dehydroepiandrosterone (3200), testosterone (10), oxymetholone (10), 11-ketotestosterone (10), stanazol (833), thyroidin (60) , 1-dehydrosterone acetate (15), dry thyroid (6), crezacin (10-20), and triiodothyronine (20 mg/kg). 100% of males can be obtained by adding methyltestosterone and ethinyltestosterone to fish feed (from 50 to 100 mg/kg), and 100% of females grow when fry are treated with ethinyl estradiol (50 mg/kg) and estradiol (20 mg/kg). With the help of supersolvents (dimethyl sulfoxide), any hormones, adaptogen drugs, vitamins, etc. can be administered to fish. without injections, direct transport of substances through the integument.
Most fish do not care for eggs, laying it in excess so that at least part of the offspring can survive. But in a limited volume of the aquarium, all caviar can become food for other inhabitants. Some species give birth to live fry, which also become a delicacy. But there are many who actively take care of both caviar and fry. These include, for example, cichlids. However, their strict observance of their territory often turns into a tragedy for other fish. Courtship of some species can also lead to injury. That is why, if you seriously decide to start breeding fish, do not let this process take its course. And for a breeding pair, as a rule, a separate aquarium is needed.
Depending on the type of reproduction, fish are divided into spawning, ovoviviparous, viviparous.
spawning- the main group of fish that spawn into the water column, where it is fertilized.
ovoviviparous- fertilization is internal, the embryo develops in the body of the female in special extensions of the oviducts, but it feeds on the nutrients of the yolk sac, and the mother's body serves only as a defense against external factors.
viviparous- in these fish, the connection of the egg and sperm occurs in the female genital tract, the placenta is being formed, which ensures the connection of the mother's organism with the embryo and supplies nutrients.
Live birth is a rare phenomenon, typical for aquarium fish (guppies, swordtails), sharks. There is no larval stage, an embryo develops in the oviducts of the female, and an already formed fry is born, which is capable of independent existence.
Features of fish breeding
Fish are dioecious animals. Females form eggs - eggs that develop in the ovaries have a thin, translucent membrane for quick and easy fertilization. Moving along the oviducts, they exit through the external opening located near the anus.
Males form spermatozoa, in paired testes - milk, they are a system of tubules that flow into the excretory duct. There is an extended part in the seed tube - this is seminal vesicle. Spawning and the release of seminal fluid occur almost simultaneously.
Exception - rock perch, has the sex glands of two sexes, but they do not mature at the same time, which prevents self-fertilization.
Fish only reproduce sexually., by fusion of male and female germ cells.
The process of laying eggs by females and fertilizing them with male spermatozoa is called spawning. For the spawning period, fish are looking for favorable conditions for the development of offspring, so they often leave their usual habitats. Some move from the seas to the mouths of the rivers flowing into them, others, on the contrary, rush to the seas.
If, due to unfavorable conditions, the fish did not manage to spawn, they are characterized by resorption of eggs and milk (gradual resorption of the sexual material).
Fertilization in most cases is external, the larva develops outside the body of the female (live birth is rare).
Fish lay eggs in huge quantities (from 100 thousand to millions of eggs). Such fertility ensures the preservation of the genus, because not all eggs will be fertilized, and some will die altogether.
When the eggs are released, spermatozoa can enter the egg through a special hole - micropyle. After the fusion of germ cells, the egg membrane becomes more permeable (adsorbs water) and durable.
After completion of fertilization, the eggs form zygote, in which there is a multiple division with the formation of a multicellular germ. In the abdominal region, the remains of the yolk sac remain, which provides nutrition for the larva in the first days.
Larval stage begins with the rupture of the egg shells, when the formed individual comes out and begins to feed on its own (single-celled, crustaceans, algae). Body shape elongated, large eyes, no fins.
The larva in the first days hangs motionless, attached to some kind of substrate, after the depletion of nutrient reserves, it begins to actively move in search of food. During this period, scales begin to form. Small fish have temporary organs that are needed to survive in a new environment:
- Fin fold;
- additional external gills;
- blood vessels.
This stage is also called critical, if the larvae cannot find food, then their mass death will happen.
For fry stage characteristic is the reduction of temporary organs and the formation of the structure as in adults. From this stage, the fish looks like all representatives of the species, only smaller. The body is completely covered with scales, fins of all kinds are formed.
adult fish has fully formed systems and organs, is covered with mucus and scales, has glands, sensory organs. When they reach sexual maturity, they soon begin to multiply.
What is the development of fish: direct or indirect?
Indirect development occurs in larvae that do not look like adults when they emerge from the egg. Such organisms develop gradually, acquiring the traits of their parents through a series of successive stages that differ in the way they eat and their way of life.
After the eggs ripen, a larva emerges from it, with undeveloped fins, scales, and in appearance is not similar to an adult. Therefore, such fish belong to organisms with an indirect type of development (mainly bony fish).
When babies are born that look like adults only with smaller sizes and incompletely formed organs, such development is called direct. So, fish that are inherent in live birth (for example, sharks) develop in a direct way.
Caring for offspring
Egg spawning in large quantities is due to the fact that fish do not care about their offspring. The left eggs die from enemies, drying out, adverse conditions, only a relatively small part can survive to the stage of a mature individual.
Some parenting fish choose crevices to spawn, build nests for protection, or carry eggs in their mouths. So, the female salmon with her caudal fin clears a place for laying eggs, making a depression on the sandy bottom, then throws the eggs with sand (protecting from predators and freezing).
Parents provide a constant supply of oxygen for offspring, with the help of fins aerate the water. To avoid drying out of the eggs, the male waters them with water from his mouth. The manifestations of care in fish are on an instinctive level, when the larvae are able to independently obtain food, can swim well, their parents leave them.
- Read more: Fish breeding: procreation
Peculiarities of fish breeding.
The main mode of fish reproduction is sexual. The vast majority of fish species reproduce sexually. At the same time, parthenogenesis and gynogenesis occur in fish, although rarely.
Parthenogenesis is a method of reproduction when the development of eggs occurs without fertilization by males. At the same time, the development of eggs usually reaches only the stage of crushing, as, for example, in herring, sturgeon, salmon and carp. It has been noted that unfertilized eggs in salmon, being in the spawning mound along with fertilized eggs, often develop parthenogenetically. Thanks to this, they do not immediately rot, which is why the entire clutch of eggs does not die, and fry hatch from fertilized eggs. In burbot and herring, the development of eggs can, in exceptional cases, go to the stage of the larva, but the latter survive only until the yolk sac is resorbed. Thus, in most cases, parthenogenetic development does not result in the production of viable fish juveniles. The exception is the Issyk-Kul chebachok, which, with the parthenogenetic development of eggs, produces normal offspring.
Gynogenesis is the birth of only females from eggs that are not fertilized by males of this species. At the same time, the development of eggs is stimulated by the spermatozoa of males of related fish species, which are able to penetrate the eggs, however, their real fertilization does not occur. As a result of the gynogenesis pathway of reproduction, only females are observed in the offspring. Thus, in the reservoirs of Central Asia, Western Siberia and Europe, populations of silver carp are often found, in which there are practically no males. Here, the development of crucian caviar is stimulated by the spermatozoa of carp, roach, and some other closely related species. In the reservoirs of Mexico, there are populations of mollies belonging to the order of carp-tooth-shaped, also consisting of almost the same females.
Most species of fish, as a rule, are unisexual, although true hermaphrodites are also found among them. Among the hermaphrodite species among bony fish is the stone perch, in which both caviar and spermatozoa develop in the genital gonads. However, the maturation of the gametes usually occurs alternately, due to which the eggs do not self-fertilize. In the red pagella, sex reversion occurs during life, i.e. gender reversal. If in young individuals of the red pagell the gonads function as ovaries, then in individuals of older ages they function as testes. In addition, hermaphroditism is occasionally found in some representatives of herring, salmon, carp, perch fish.
As for the methods of fertilization of eggs in fish, external and internal fertilization of eggs occurs. External fertilization occurs in most fish species and occurs in the external aquatic environment. Internal fertilization is more common in cartilaginous fish, although it is also characteristic of some bony fish, in particular, sea bass, eelpout; many cyprinids (mosquitoes, guppies, swordtails, etc.).
According to the place of development of caviar in fish, three groups can be conditionally distinguished: oviparous, ovoviviparous and viviparous. Most fish species are oviparous - they lay eggs (eggs) directly into the external aquatic environment. In fish from the group of ovoviviparous, fertilized eggs are retained in the posterior sections of the oviducts and develop there until the juveniles are born. Those. almost at the moment of the birth of the eggs, they break and immediately the fry are born. Ovoviviparous include most cartilaginous fish, such as katran, white shark, fox shark, pylon and others. In species such as, for example, the stingray, the wall of the posterior sections of the oviducts, which acts as a "womb", even has special outgrowths through which nutrient fluid enters the oral cavity of the embryos through special sprinklers.
In viviparous fish, in the posterior sections of the oviducts (“womb”), a structure is formed that is somewhat similar to the placenta of mammals, thanks to which the embryo receives nutrients along with the mother’s blood. Live birth is characteristic of many species of sharks, especially deep-sea sharks, and in particular for blue, mustelid, cat, and other sharks.
The adaptive significance of viviparity and ovoviviparity of fish lies in the fact that during intrauterine development, a greater survival rate of juveniles is ensured.
Depending on the nature (multiplicity) of fish reproduction, they are divided into monocyclic and polycyclic. Monocyclic fish breed only once in their life and after a single spawning they all die. Monocyclic fish include river eel, Pacific salmon, river lamprey, Baikal golomyanka and some others. Most fish species are polycyclic, i.e. they reproduce many times during their life, and the number of reproduction cycles depends mainly on the duration of their life.
The age of onset of puberty in different fish species can vary considerably - from 1 - 2 months for small species, such as mosquito fish, and up to 15 - 30 years for sturgeons. Usually, fish with a short life cycle mature faster: at the age of about 1 year, sprat, smelt and some gobies ripen. Fish with a long life cycle become sexually mature much later. Thus, Atlantic cod matures at 7-10 years, sea bass - at 12-15 years, etc.
At the same time, the age of puberty of fish depends not only on the species, but also on the living conditions of the fish, and primarily on the conditions of fattening. Therefore, the onset of puberty in fish occurs, as a rule, when a certain length is reached. It follows that the better the fish is fed, the faster it grows, and therefore, the faster it matures. Males in most fish species usually mature before females.
Climatic conditions also have an important influence on the rate of maturation. For example, in the Aral Sea, puberty in a bream occurs at the age of 3–4 years, in the Northern Caspian - at 3–6 years, while in the Middle Volga at 6–7 years, and in Lake Ladoga only at 8–9 years.
To determine the size of the withdrawal (catch) of fish from a particular reservoir and the assessment of raw materials, the age of puberty is important. Therefore, for short-cycle fish that mature at the 2–3rd year of life, the allowable commercial removal from the population can be 40–60% (sprats, sprats, sardines, etc.), while in long-lived fish, the annual removal should not exceed 5–20%.
