Biological characteristics and breeding of herbivorous commercial fish species

Grass carp is a herbivorous fish inhabiting the lower reaches of the Amur River and the rivers of China; it reaches a length of 1.2 m and a mass of more than 32 kg. It has sharp pharyngeal teeth adapted for grinding vegetation. It consumes not only soft underwater vegetation but also terrestrial plants, moving onto river floodplains and floodplain lakes.

Grass carp reaches sexual maturity at the age of 7–9 years; it spawns at a water temperature of 26–30 °С in river channels in areas with fast turbulent flow, releasing from 100,000 to 800,000 eggs. It is successfully farmed in ponds and reservoirs. It has been acclimatized in the European part of Russia. To gain 1 kg of weight, a grass carp needs to consume 30–70 kg of vegetation.

Black carp. It is distributed in the basins of the Amur, Ussuri, Lake Khanka, and the rivers of China. Its body color is dark, almost black; it reaches a length of more than 1.2 m and a mass of 36 kg. It spawns in river channels in June-July at the age of 7–10 years. Black carp feeds on mollusks, crushing their shells with powerful pharyngeal teeth, as well as on aquatic insects and shrimp. It has been acclimatized in the European part of Russia and Central Asia.

Silver carp is a herbivorous fish, a large schooling pelagic fish, reaching in some cases more than 1 m in length and more than 2.0 kg in body mass. It grazes in lakes and reservoirs, and for spawning, it enters inflowing large rivers and canals. It reaches sexual maturity at 3–4 years of age. Spawning occurs after the water temperature reaches 18–20 °C and coincides with a sharp rise in river water levels in May-June.

It releases its eggs in the current in areas with whirlpools. The eggs are pelagic; they swell in the water, increase in size, and develop while drifting downstream. If they enter stagnant water, the eggs sink and perish. For the eggs to develop, they require 100 km or more of river channel flow. Fecundity is high:

• in large broodstock weighing over 20 kg – up to 3 million eggs;
• in pond fish weighing up to 8 kg – up to 1 million eggs.

It feeds on phytoplankton.

Bighead carp is similar in body shape to the silver carp. The bighead carp has a larger head, eyes set wider apart, a less deep body, and longer pectoral, pelvic, and caudal peduncles. The color is significantly darker, and adult fish have dark spots on the sides of the body. Juveniles have golden sides. It is a large fish, up to 146 cm long and weighing up to 32 kg.

In terms of its lifestyle, it has much in common with the silver carp, but it is more thermophilic. Its main food source is zooplankton, but by autumn, the proportion of phytoplankton, including blue-green algae, increases in its intestines. Due to its diet (zooplankton), its intestine is shorter than that of the silver carp. It matures at different ages in different water bodies; on average, it becomes sexually mature at the age of 4 years with a length of 80–90 cm and a mass of 8.5–10.0 kg. It spawns later than the silver carp, at the end of May. Fecundity is 600,000–1,000,000 eggs. Optimal temperatures for egg development are 18–25 °С. The eggs are demersal-pelagic and are released in several portions.

Asp mainly inhabits rivers; it is a large fish, reaching a mass of 12 kg and 80 cm in length. The color is silvery, sometimes dark-silver; the dorsal and caudal fins are gray with a black border, while the lower and side fins are reddish. The iris of the eye is yellow with a green spot in the upper part. The body is elongated, slightly compressed from the sides, with an upward-facing mouth, a short anal fin starting behind the pelvic fin, small scales, and two rows of pharyngeal teeth arranged in three and five per row.

Being the only toothless predator in the entire cyprinid family, the asp feeds on small fish and fry. The asp's mouth has a protrusion on the lower jaw that fits into a notch on the upper jaw. This helps it hold the captured fish. There are no teeth on the jaws.

The asp is a thermophilic fish and leads an active lifestyle only during the warm season. The asp becomes sexually mature in the 4th–5th year of life; it spawns in late April or May in river channels on rocky shoals and sandbars. After spawning, it rests for a long time – 12–15 days.

Chub is a freshwater fish from the cyprinid family. It reaches 80 cm in length and a weight of up to 8 kg. The massive head is slightly flattened on top, the forehead is wide, and the scales are quite large. It feeds on aerial insects, juvenile crayfish, fish, and frogs. Chub spawning occurs at a water temperature of 12–17 °С, predominantly in the southern regions of Europe in April, and in the north in May-June, not far from their habitats. Chub fecundity is 10,000–200,000 eggs. They reach sexual maturity at the end of the 2nd–3rd year of life, weighing 100–200 g.

The chub feeds on a wide variety of food, and in this regard, it can be classified as an omnivorous fish. Its diet consists of:

• insects;
• worms;
• mollusks;
• crayfish;
• frogs;
• fish eggs and fry;
• everything that falls into the water.

Large chub are very voracious predators; they not only consume gudgeon and bleak in large quantities, but also snatch anything that falls into the water, even mice.

Common bream is widespread in the water bodies of Europe and the basins of the North, Baltic, Barents (Pechora River), Black, Azov, Caspian, and Aral Seas. It inhabits standing and slow-flowing water bodies, but in the Caspian and Azov Seas, it forms a semi-anadromous form that feeds in the desalinated zones of the sea.

Sexual maturity in bream occurs at the age of 3–4 years; in northern regions, it spawns 100–340 thousand eggs during the spawning period; it feeds on amphipods, chironomid larvae, and mollusks.

Biological characteristics of fish in standing and slow-flowing water bodies

When designing polyculture in pond farms, bream is valued for its effective use of bottom food resources. The species has a high, strongly laterally compressed body and a long anal fin, which begins in front of the end of the short dorsal fin. On the belly behind the pelvic fins, there is a bare keel, and on the back behind the nape, there is a scaleless groove. Large dense scales and a blunt snout hanging over a small mouth are key features. In commercial practice, small individuals weighing less than 1 kg are called juvenile bream.

Tench is well-suited for stocking silted ponds and water bodies with periodic oxygen deficiency. It is a typical bottom dweller that buries itself in the silt for the winter and received its name for its ability to change color in the air. The fish's broad body is covered with small dense scales, and the small mouth is equipped with a pair of short barbels in the corners.

• Tench body length — up to 60 cm
• Tench body mass — up to 7.5 kg
• Spawning age — 3–4 years
• Working spawning temperature — from 18–20 °С and higher
• Female fecundity — 300–900 thousand eggs

The tench diet includes the following components:

• for juveniles — plankton, chironomid larvae, small crustaceans, and mollusks;
• for adults — benthic organisms extracted from the muddy soil.

Common roach is used for stocking slow-flowing and standing water bodies with abundant vegetation. In its natural environment, it is widespread in the Kuban, Siberia, and the Aral Sea basin, where it is easily recognized by the orange iris of its eyes. Spawning takes place in early spring at a water temperature of about 8 °С, when fish at the age of 3–4 years lay up to 140 thousand eggs on last year's vegetation. The average length of marketable roach is 25–30 cm with a mass of 150–200 g, and the maximum weight can reach 2 kg.

With age, the roach's feeding spectrum changes:

• juveniles consume zooplankton;
• adults switch to algae, insect larvae, and mollusks.

Common gudgeon inhabits clean freshwater bodies with a sandy bottom. Its color camouflaged to match the soil, its lower crescent-shaped mouth with barbels, and its schooling lifestyle allow the fish to effectively adapt to bottom conditions. Sexual maturity occurs in the 2nd–4th year of life, and intermittent spawning lasts from April to June. Sticky benthic eggs with a fecundity of 1–3 thousand settle on sand grains and become invisible.

The gudgeon's diet consists of benthic organisms:

• zooplankton and small benthos;
• various insect larvae and small mollusks;
• eggs of other fish at spawning grounds.

Conditions for reproduction and feeding of semi-anadromous fish species

When organizing fishing in the Caspian basin, the Caspian roach — a semi-anadromous form of roach — is of key importance. It grows up to 35 cm in length and gains a mass of up to 800 g. The herd feeds in the Northern Caspian in desalinated areas with a water salinity of 7–8%. For spawning, the fish enters the Volga, Ural, Kura, and other rivers at a water temperature of 10–20 °С, where it leaves 20–30 thousand sticky eggs on vegetation. The Caspian roach winters in holes in front of river mouths.

The diet of the Caspian roach includes:

• small mollusks;
• crustaceans and worms;
• chironomid larvae.

In the Azov-Black Sea basin, a similar niche is occupied by the Black Sea roach — a semi-anadromous subspecies of roach. The fish grows up to 50 cm in length with a mass of up to 1 kg. Spring spawning takes place in river deltas, freshwater lakes, floodplains, and wetlands, and fecundity ranges from 22 to 202 thousand eggs. After spawning, the population moves down to the sea and limans to feed, and concentrates in river mouths for the winter.

Reproduction of the Azov-Black Sea vimba bream requires monitoring of spawning ground parameters. Mature fish enter rivers from November to January with immature roe and spend the entire winter in freshwater. Spring spawning takes place in the Kuban riverbed at a water temperature of 18–25 °C. Vimba bream prefers pebble shoals at a depth of 25–50 cm with a current speed of 0.7–1.1 m/s. The female lays 40–300 thousand eggs, which, after adhering to stones, are washed by the current into the crevices between the bottom substrate. Adult fish reach 40 cm in length with a mass of up to 0.8 kg.

The diet of vimba bream consists mainly of benthic organisms:

• insect larvae;
• amphipods;
• mollusks.

Shemaya is among the valuable species of the Azov-Black Sea, Caspian, and Aral basins. It grows up to 40 cm with a mass of up to 800 g. Spring spawning occurs at night in the upper reaches of rivers on pebble shoals with a fast current at a depth of 20–40 cm. Fecundity ranges from 3 to 23.5 thousand eggs. During the feeding period, shemaya feeds on zooplankton and juvenile fish, but completely stops feeding during spawning.

For breeding in large flowing lakes, reservoirs, and fast rivers of the Baltic, Black, Azov, and Caspian basins, the sabrefish (chekhon) is used. This is a schooling semi-anadromous fish with a saber-shaped body, a straight superior mouth, and easily deciduous scales. The species avoids small rivers and often stays near locks. The most intensive growth is observed in the first years of life, and upon reaching sexual maturity, the rates of development drop sharply.

Percid fish are an important element of freshwater ecosystems. For a fish farmer, they are of interest both as valuable objects of cultivation (zander) and as biological ameliorators regulating the population of weed fish (perch, Volga zander). All members of this family have a similar structure: two dorsal fins (a spiny anterior and a soft posterior one), dense scales, and bristle-like teeth.

• Zander spawning temperature — from 7–8 °C
• Optimal spawning ground depth — 1.5–2 m
• Permissible current speed — up to 0.2 m/s
• Zander nest depth — 5–10 cm
• Perch male maturation — 1–2 years

Biological potential of perch, Volga zander, and ruffe

Perch is an active predator that quickly gains mass and, with a good food base, reaches a length of 40 cm and a weight of over 2 kg. The maximum recorded indicators for this species are 55 cm and 3 kg. With age, the perch's body grows more in height and thickness, which causes large individuals to acquire a pronounced "hump." Males reach sexual maturity at 1–2 years of age, females at 3 years and later. Spawning begins after the pike at a water temperature from 7–8 to 15 °C. Females lay eggs on last year's vegetation, snags, tree roots, or directly on the soil, and fecundity ranges from 200–300 to 600–900 thousand eggs. The peak of the perch's predatory activity occurs at the end of summer, when the juveniles of other fish grow up.

Volga zander is outwardly similar to the zander, but grows more slowly and has a number of differences: it lacks a hump, completely lacks lower canines, the snout is blunter, and the eyes are convex. The maximum length of the Volga zander is 45 cm, weight — 1.2–1.4 kg. In the Volga delta, spawning occurs in April — May. Larvae first feed on small zooplankton, and upon reaching a length of 40 mm, they switch to benthos. The predatory diet of the Volga zander is formed only in the second year of life, and its basis is the current year's brood of cyprinids and percids.

Intensive feeding of the Volga zander occurs in the spring (due to over-wintered yearlings) and autumn (due to grown-up fingerlings); in summer, the intensity of feeding drops. As for the common ruffe, it is a small fish with an average length of about 10 cm and a mass of 50–200 g, preferring a sandy or gravelly bottom. In commercial fish farming, the ruffe often acts as an undesirable food competitor for valuable fish species.

Conditions for reproduction and organization of zander spawning

Zander is one of the most valuable objects of freshwater fish farming. In ponds and reservoirs, it usually reaches a length of 60–70 cm and a weight of 2–4 kg, although individual specimens can grow up to 1.3 m and weigh up to 20 kg. Zander matures in the 2nd–4th year of life with a body length of more than 30 cm. Spawning begins in the spring when the water warms up to 7–8 °C. For spawning, the fish choose quiet areas with a current of no more than 0.2 m/sec and a depth of 0.5 to 4 m (most often — 1.5–2 m).

For successful reproduction, the zander requires a suitable substrate: branched roots of sedges, reeds, rushes, water lilies, or willow or alder roots. The female prepares the spawning site, performing the work in a strict sequence:

1. Using water jets from its fin movements, it cleans the plant roots of silt and soil deposits.
2. It loosens hard patches of the bottom with its snout.
3. It forms a round or oval nest 5–10 cm deep with exposed plant roots.

After spawning, the female immediately leaves the nest. All care for the offspring falls to the male, who guards the clutch until the larvae hatch. It not only drives away predators within a radius of 50–100 cm but also constantly works its fins, creating a water current. This movement cleans the eggs of silt and ensures a constant supply of oxygen.

Ruffe and Catfish: Population Control and Keeping Conditions

The ruffe is an extremely voracious inhabitant of water bodies that can create stiff food competition for valuable commercial species. In the absence of its preferred food—bloodworms and amphipods—it easily switches to other organisms. Fish farmers should consider that the ruffe actively eats the eggs and fry of other species, and large individuals are often predatory. It feeds all year round, including the spawning period, and consumes a huge amount of resources.

• Feed consumption (per unit of body mass relative to bream) — 6 times higher
• Depth of egg-laying — 3 m and less
• Spawning temperature range — from 6 to 18 °C
• Number of individuals aged 1–3 years in the population — up to 93 %
• Duration of egg development (at 10–15 °C) — from 5 to 12 days

The ruffe reaches sexual maturity in its 2nd–3rd year of life at a body length of about 11–12 cm. It spawns from mid-April to June on a wide variety of substrates, making up to 2–3 clutches in one season, and does not guard its offspring. The fertility of a female ranges from 10 to 200 thousand eggs, depending on its size. In natural conditions, females live up to a maximum of 11 years, while males rarely exceed the seven-year mark.

To control the ruffe population in a water body, it is useful to introduce natural predators that actively feed on it. These include:

• zander;
• pike;
• large perch;
• burbot;
• eel;
• catfish;
• salmon.

The Wels catfish (European catfish) is the largest freshwater scaleless predator, which can reach a length of 5 m and a mass of 400 kg. In commercial pond farming, individuals weighing 10–20 kg are more common. The fish's body is covered with a thick layer of mucus, the skin is dark with a greenish tint, and the belly is white. The catfish has small eyes with black pupils and a flattened head with two long whitish barbels on the upper jaw and four yellowish ones on the lower. These barbels serve as tentacles for finding food in the dark and as bait for small fish.

The catfish inhabits rivers and lakes of the European part of Russia, except for the Arctic Ocean basin, and is also common in Europe, with the exception of southern countries, England, Scotland, Norway, and Ireland. Sexual maturity occurs in the fourth or fifth year of life. Spawning takes place in spring in the coastal zone among vegetation after the water warms up to 20 °C. The female lays eggs in a nest, and the male guards it until the fry appear.

After reproduction, catfish begin a post-spawning feeding frenzy that lasts until mid-summer, and with the first frosts, the fish may completely stop feeding. The predator's diet includes fish, crayfish, frogs, worms, waterfowl, and small animals. The color of the catfish varies from almost black to light yellow; albinos are occasionally found.

Burbot: Biological Requirements for Cold-Water Farming

The burbot is the only freshwater member of the cod family. The fish serves as a reliable indicator of water purity and prefers water bodies with a rocky-silty bottom and spring-fed water. Body color varies from dark brown to olive-black with light spots. The burbot has two dorsal fins (a short front one and a long rear one), a long anal fin, and small cycloid scales covering the body and head.