Features of reproduction and patterns of fertility in fish under aquaculture conditions

Managing fish reproduction in aquaculture directly determines the profitability of a farm. Unlike terrestrial animals, most fish have external fertilization, and eggs develop directly in the water. This leads to high offspring mortality at early stages, which requires the fish farmer to calculate the fecundity of the stock precisely and create optimal conditions for spawning.

Spawning Biology and Fecundity Patterns

Working fecundity depends on the biological characteristics of the species and the rearing conditions. The highest number of eggs is produced by fish that release them into the water column (pelagic). Species that attach eggs to plants or guard their nests are less fecund. There is also an inverse relationship between the diameter of the eggs and their number.

• Polar shark — a few units
• Sand lance — 200 million units
• Ocean sunfish — 300 million units

In natural conditions and under cultivation, special forms of reproduction may occur. In parthenogenesis (in herring, sturgeon, salmon, and carp families), unfertilized eggs begin to divide in water, but viable larvae are rarely obtained — usually, they survive only until the yolk sac is resorbed, as in Baltic herring and burbot. In gynogenesis (in silver crucian carp), the population consists of triploid females, and the sperm of another species only stimulates the development of the egg cell without nuclear fusion, resulting in the birth of only females.

Maturation of Broodstock and Spawning Management

The age of sexual maturity depends on the fish's life cycle. Species with a short cycle (gobies, Caspian tyulka, anchovy, smelt) mature quickly. Large, long-lived fish require prolonged rearing before the parent stock is formed, with males usually maturing a year earlier than females.

Water temperature and feed availability allow for the regulation of maturation timing. For example, carp in the temperate zone becomes sexually mature at 4–5 years, in southern regions — at 2 years, and when raised in Cuba — as early as 6–9 months. The sex ratio for most fish is close to 1:1, except for gynogenetic populations.

To plan the work of an incubation facility, it is important to consider the natural spawning periods:

• Spring spawners: rainbow trout, pike, perch, roach, orfe, herring.
• Summer spawners: carp, tench, rudd.
• Autumn-winter spawners: Pacific salmon, whitefish, burbot, saffron cod.

The daily rhythm of spawning is also individual. Salmon, burbot, and anchovy spawn at night, anchovy — in the evening, carp — primarily at dawn. At the same time, spawning periods vary geographically: carp in temperate climates spawn in May–June, while in the tropics (in Java and Cuba) — all year round. In aquaculture, light regime management allows for the artificial shifting of these periods to obtain market-sized fish (e.g., trout) year-round.

Biological Characteristics of Eggs and Adaptation to Habitat

In fish farming, the shape, size, and buoyancy of eggs directly determine the incubation technology. For example, fractional spawning, which is characteristic mainly for tropical and subtropical fish, helps juveniles survive during fluctuations in water level and increases overall fecundity. In temperate latitudes, there are fewer such species, and in the Arctic, there are practically none. Understanding these biological features helps in properly preparing incubation apparatuses for a specific fish species.

The shape of the released eggs in most species is spherical, although other variants are encountered in practice:

• oval (in anchovy);
• cigar-shaped (in gobies and rotan);
• teardrop-shaped and cylindrical (in some goby species);
• rectangular with projections at the corners (in sharks and skates);
• conical with a spirally twisted ridge, spherical, and pear-shaped.

The color of the eggs usually varies from yellowish to orange due to carotenoids, but in sturgeon, it is black, and in gobies — green. Interestingly, the size of the eggs does not depend on the dimensions of the fish itself. For example, in a giant tuna weighing up to 450 kg and measuring 3–3.5 m, the eggs are small (1.0–1.2 mm), while in the freshwater catfish, they are larger — 2–3 mm. The deep-sea coelacanth lays only a couple of dozen eggs the size of a medium orange (9–10 cm), and in sharks and skates, fecundity is low despite egg sizes of 4–10 cm (a whale shark egg measuring 65 × 40 cm has been recorded).

Conditions for egg development classify them as pelagic (floating) or demersal (sinking). The buoyancy of pelagic eggs is ensured by high yolk hydration (up to 97% compared to 60–76% in demersal species), an enlarged perivitelline space, the presence of oil droplets, or special shell outgrowths. It is important for fish farmers to distinguish between these types when setting up incubator flow systems.

Demersal eggs are laid on a substrate — plants, stones, or driftwood. To anchor themselves, they are equipped with an adhesive coating or special filaments, as seen in sturgeons, carp, and crucian carp. Perch, for example, spawn eggs in the form of long gelatinous ribbons up to 2–3 meters in length. At the same time, the eggs of salmonids and burbot, although demersal, do not attach to a substrate.

Chemical composition of eggs and specific features of sperm activation

Chemical composition of the eggs determines the energy reserve available to the embryo. Protein content in raw mass ranges from 12 to 30%, and fat — from 1 to 22%, with the protein fraction always predominating. Proteins are the main energy source, covering up to 70% of the developing embryo's requirements, whereas the role of fats is lower here than in birds. There are negligible carbohydrates in the eggs, represented by rare inclusions of glycogen.

The quality of milt and the activity of spermatozoa are the second most important factor for successful artificial fertilization. A mature spermatozoon consists of a head, a mid-piece, and a tail. In the fish's testes, they remain in an immobile state within the seminal fluid, which resembles a saline solution. Their instantaneous activation occurs only upon contact with an aqueous environment during spawning.

• Oxygen uptake — increases more than 2-fold
• ATP content — increases 3-fold
• Sperm movement speed — 164–330 μm/s

Influence of broodstock age on offspring quality

The age of the broodstock directly determines the quality of gametes and the viability of the offspring. The best reproductive capacity is demonstrated by males and females of medium age. In females, egg size and the ratio of their size groups change with age, while young fish provide unstable results with a high percentage of loss. As an example, let us consider the spawning performance of carp of different ages depending on the reproductive cycle:

Offspring from medium-age broodstock develop faster, transition to external feeding earlier, and show high growth rates in the first and second summer of life. Conversely, larvae from first-time and aging fish grow slowly. Loss of such juveniles during further rearing remains consistently high, and in carp at the age of 13–15 years, the ability to reproduce fades completely.

Sex regulation and sperm viability

The success of incubation depends on the environmental conditions in which the gametes meet. Spermatozoa penetrate the egg through openings in the shell called the micropyle; however, the time of their active movement in water is extremely limited. The energy resource is quickly depleted, forward motion is replaced by oscillatory motion, after which the spermatozoa die. Their lifespan depends directly on the temperature and salinity of the water.

• Sperm activity in fresh water — 1–3 min
• Herring sperm activity in salt water — up to several days
• Age of sex reversal in rainbow trout — 135–160 days
• Maximum breeding age of carp — 13–15 years

Most fish species are dioecious, but exceptions exist in aquaculture. Natural hermaphroditism is normal for sea bass and sea bream, while in chum salmon and mullet, male and female areas can alternate in the gonads. In freshwater aquaculture, hermaphroditism is extremely rare (for example, in carp, these are isolated cases). However, fish farmers can utilize the phenomenon of sex reversal, which sometimes occurs spontaneously in nature.

Targeted sex manipulation is economically beneficial when rearing commercial fish. In salmon farming, it is advisable to obtain herds dominated by females, as they are larger. In tilapia farming, on the contrary, it is more profitable to raise males, since female tilapia grow slowly and spawn too frequently. To achieve this, juveniles are treated with steroid hormones via feed, which allows for the redirection of gonad development and the production of full-fledged broodstock of the desired sex.

Depending on the substrate and conditions required for spawning, fish are divided into several ecological groups.

• Lithophils – spawn on rocky soil (in rivers with current or on the bottom of oligotrophic lakes or coastal sea areas) in places rich in oxygen. These include sturgeon, salmon, nase, etc.
• Phytophils – spawn among vegetation, laying eggs in standing or slow-moving water on dead or living plants. Oxygen conditions in this case can vary. This group includes pike, carp, bream, roach, perch, etc.
• Psammophils – lay eggs on sand, sometimes attaching them to plant roots. Often, egg membranes are encrusted with sand. They usually develop in places rich in oxygen. This group includes gudgeons, some char, etc.
• Pelagophils – release eggs into the water column. Eggs and free embryos develop while floating freely in the water column, usually in conditions favorable for respiration. This group includes almost all species of herring, cod, flounder, and some cyprinids (blue bream, silver carp, grass carp, etc.).
• Ostracophils – lay eggs inside the mantle cavity of mollusks and sometimes under the shells of crabs and other animals. Eggs can develop even without sufficient oxygen. These include some gudgeons, bitterling, etc.

This classification does not cover all fish; there are intermediate forms: the Vimba bream can spawn on vegetation and on stones, i.e., simultaneously as a phytophilic and lithophilic fish.

Most fish do not care for their offspring. There are frequent cases where parents even eat their own eggs and especially juveniles. Cannibalism occurs in mosquitofish, saffron cod, and even carp. Therefore, it is advisable to remove broodstock from spawning ponds to preserve the juveniles. However, many fish species do care for their offspring. In most cases, the protection of offspring falls to the males.

Examples of parental care are interesting and diverse: the male stickleback builds a nest from pieces of grass blades glued together with kidney secretions. The nest initially has two openings, and after it is filled with eggs by several females, the male closes one opening and remains to guard it, aerating the water with fin movements. After the juveniles hatch, the male monitors them for several days to ensure they stay in the nest and returns those that swim out by catching them with his mouth.

Female tilapia carry eggs in their mouths and, for some time after hatching, take the juveniles into their mouths in case of danger. In pipefish and seahorses, eggs are incubated in a fold or pouch on the males' abdomen. Labyrinth fish build a nest from air bubbles and a saliva-like secretion. Although the juveniles appear in the nest after a day, the male guards it until the fish are fully developed. Building nests of varying complexity is not uncommon among fish.

Trout and salmon dig several pits in the soil and cover the laid eggs with sand and gravel using tail movements, creating so-called spawning redds. Some gobies and catfish build nests from pebbles and pieces of plants. The lumpsucker guards a clump of eggs laid near the surf zone, watering them with water from its mouth during low tide. The zander builds a nest from root pieces or by clearing a rocky area: it bites a hand extended toward the nest, and it cannot be driven away. By moving its pectoral fins, it creates a water current that washes silt off the eggs.

Some fish lay eggs; for example, the thornback ray (thornback skate) lays an egg encased in a dense horny capsule. It is a flat capsule with convex sides and keels. It is equipped with four horn-like processes at the corners and covered with bundles of threads. With the help of these threads, the egg becomes entangled in algae.

Under the tough "shell" of the egg, the embryo develops over 4.4–5.5 months. The "shell" protects it from predators and provides oxygen thanks to seawater penetrating the egg. After the end of the incubation period, the fry exits through a narrow slit in the egg and begins an independent life. At this time, it is usually already devoid of the yolk sac and, with a body length of 12–13 cm, is forced to seek food on its own.

A female thornback ray lays a total of several dozen to several hundred eggs over the summer. By winter, spiny rays migrate to deeper waters.

The most advanced form of parental care is viviparity. In this case, fecundity is usually from one to several dozen individuals. In essence, this is ovoviviparity with the retention of offspring in the female's genital tract until the yolk sac is absorbed. It is inherent to many shark species, and among bony fish—to the eelpout, rockfish, mosquitofish, guppy, and swordtail.

In the individual development of fish, a number of major periods can be distinguished, each characterized by properties common to different species:

In aquaculture, the accurate calculation of feeding rates and stocking density depends directly on the current developmental phase of the fish. At each stage of ontogeny, requirements for feed quality change drastically, as does the organism's susceptibility to environmental factors. Taking development periods into account helps the fish farmer minimize juvenile mortality in the early stages and accelerate stock turnover.

Stages of ontogeny, growth parameters, and maximum age of fish

1. Embryonic period. Lasts from fertilization until the transition to external feeding at the expense of the yolk sac. It is divided into the egg sub-period (development inside the membrane) and the free embryo or pre-larva stage (outside the membrane).
2. Larval period. Begins from the moment of transition to external feeding. The fish's appearance is not yet formed; specific temporary organs function, which will later disappear.
3. Fry period. The fish acquires its adult appearance, larval organs disappear, but the reproductive system is not yet developed. The main part of energy is spent on intensive growth.
4. Sub-adult period. Rapid development of gonads and secondary sexual characteristics begins, but the individual is not yet capable of reproduction.
5. Adult period. The fish is capable of reproducing offspring. Energy is spent mainly on the maturation of eggs and milt, as well as on the accumulation of reserves for wintering and migration.
6. Senescence period. Reproductive function fades, and the fish's growth in length practically stops or slows down sharply.

Growth potential varies enormously among different fish species, which is important to consider when selecting objects for stocking. Small species include the parasitic candiru catfish from South America (up to 5 mm), the goby of the Philippine Islands (7.5–14 mm), the Berg's goby from the southern seas basin (20 mm), as well as bleak, kilka, anchovy, and stickleback. Large species represent the greatest commercial interest for commercial farming. Among cyprinids, the grass carp, silver carp, and common carp reach impressive sizes — a historical fact of a carp weighing 45 kg being caught in the early 20th century has been recorded.

The lifespan of fish is closely linked to their biological characteristics and habitat conditions. In the wild, due to the pressure of predators and diseases, fish rarely reach natural old age, and their average lifespan usually does not exceed 10 years. Small species like anchovy or kilka live significantly less. However, large fish such as pike, catfish, eel, and carp are considered long-livers. Under controlled conditions in artificial reservoirs, in the absence of natural threats, longevity indicators increase several times over.

• Maximum age of beluga in the wild — about 100 years
• Lifespan of kaluga — 55–60 years
• European eel record in artificial conditions — 88 years
• Maximum age of carp in an artificial environment — 50 years
• Goldfish record in an aquarium — 41 years