Biological characteristics and classification of fish and cyclostomes

For efficient fish farming, it is essential to accurately understand the biological characteristics and ecological requirements of the species being raised. Fish are a vast group of gnathostome vertebrates characterized by gill breathing at all stages of development. The cultivation technology, pond design, and selection of feed base depend on the anatomical structure, type of skeleton, and the species' tolerance to water salinity.

• Global fish fauna — more than 20,000 species
• Ichthyofauna of Russia — about 1,500 species
• Freshwater species in the Russian Federation — 300 species
• Global diversity of lampreys — about 40 species
• Body length of lampreys — from 10 to 100 cm

Typical fish are cold-blooded animals with a streamlined body. They obtain oxygen from water through gills (or auxiliary respiratory organs), possess jaws, have skin covered with scales, and reproduce by spawning. Movement is facilitated by fins: four paired ones (pectoral and pelvic), as well as unpaired ones — anal, caudal, and one to two (rarely three) dorsal fins.

In zoological taxonomy, the highest group is the class, and the lowest is the species (individuals with common hereditary traits from the same habitat). The complete classification hierarchy is structured as follows:

• classes;
• superclasses;
• orders;
• suborders;
• families;
• genus;
• species.

All commercial aquatic organisms are divided into two unequal classes: cyclostomes (which include lampreys) and true fish. Based on the degree of skeleton ossification, fish are divided into cartilaginous (sharks, rays), chondrostean (sturgeons), and bony fish (the majority of farmed species).

Ecological groups of fish by lifestyle and habitat

In aquaculture, the choice of rearing technology directly depends on the ecological group to which the fish belongs. Based on their natural habitat and migration routes, commercial species are divided into four main categories. Each is adapted to a specific level of salinity, water flow, and water density.

Marine fish (about 11.6 thousand species) live in salt water. They are classified by habitat zones:

• pelagic — live in the water column (anchovies, mackerel, tuna, ocean sunfish);
• benthic — inhabit the bottom substrate (flounder, ray, sculpin);
• by distance from the shore — oceanic (open ocean: tuna, flying fish), neritic (coastal zones: gobies, flounder, most sparids), and deep-sea (bathypelagic: lanternfish, anglerfish; and benthic).

Freshwater fish (about 8 thousand species) permanently reside in fresh water. Like marine fish, they are divided into pelagic, bottom-dwelling, and benthic inhabitants. Based on requirements for water flow, they are divided into two groups:

• rheophilic — adapted to living in currents (trout, marinka, char);
• limnophilic — live in standing water (carp, crucian carp, bream, common carp, weatherfish).

Anadromous fish perform regular migrations from one environment to another for spawning. Some species move from seas into fresh rivers to reproduce (salmon, herring, sturgeon). Others, such as eels, make the return journey — leaving freshwater bodies to spawn in the sea.

Brackish-water fish inhabit river estuaries, desalinated lagoons, and inland seas with reduced salinity. The specifics of their life cycle determine the regime of feeding and reproduction. Fish farmers divide this group into two categories:

• semi-anadromous — feed in brackish water, but enter lower reaches of rivers for spawning (whitefish, roach);
• true brackish-water — permanently live in desalinated marine zones (gobies, river flounder, pipefish).

Classification of commercial fish families

For successful planning of fish farm operations, it is necessary to accurately know the systematics of the objects being raised. Families and genera of fish are grouped by their biological similarity, which directly determines the requirements for water quality, types of feed, and housing conditions. A detailed classification of the main commercial groups used in fish farming practice is provided below.

• Family Acipenseridae (sturgeons):
  • Genus Huso — Kaluga, Beluga
  • Genus Acipenser — ship sturgeon, sterlet, stellate sturgeon, Amur sturgeon, Russian sturgeon
  • Genus Scaphirhynchus — Amu Darya shovelnose sturgeon, Syr Darya shovelnose sturgeon
• Family Salmonidae (salmonids):
  • Genus Oncorhynchus — pink salmon, masu salmon, coho salmon, chum salmon, chinook salmon, sockeye salmon
  • Genus Salmo — Atlantic salmon, White Sea salmon, Black Sea salmon, Caspian salmon, Aral salmon, lake salmon, brown trout, lake trout
  • Genus Salvelinus — char, Arctic char
  • Genus Hucho — taimen, Sakhalin taimen, Danube salmon
  • Genus Stenodus — inconnu, sheefish
  • Genus Coregonus — whitefish, broad whitefish, Arctic cisco, peled, vendace
• Family Thymallidae (graylings):
  • Genus Thymallus — graylings
• Family Esocidae (pikes):
  • Genus Esox — northern pike, Amur pike
• Family Percidae (perches):
  • Genus Sander — zander, Volga zander
  • Genus Perca — perch, zingel
  • Genus Gymnocephalus — ruffe
• Family Siluridae (Sheatfish):
  • Genus *Silurus* — Wels catfish
  • Genus *Silurus* (Amur group) — Amur catfish
• Family Anguillidae (Freshwater eels):
  • Genus *Anguilla* — European eel
• Family Gadidae (Cod):
  • Genus *Lota* — Burbot
• Family Gobiidae (Gobies):
  • Genus *Neogobius* — Round goby, monkey goby, syrman goby, sand goby
  • Genus *Benthophilus* — Hell's angel (tadpole goby)
  • Genus *Benthophiloides* — Tubenose goby
  • Genus *Benthophilus* (three-toothed) — Three-toothed goby
• Family Clupeidae (Herring):
  • Genus *Alosa* (Caspian-Black Sea group) — Black-backed shad, Volga shad, Caspian shad, Black Sea shad, Danube shad, Azov shad
  • Genus *Clupeonella* — Kilka, Abrau sprat
  • Genus *Alosa* — Twaite shad
• Family Pleuronectidae (Righteye flounders):
  • Genus *Platichthys* — European flounder, Baltic flounder, White Sea flounder, Arctic flounder, Pacific flounder
• Family Mugilidae (Mullet):
  • Genus *Mugil* — Flathead grey mullet, So-iny mullet
• Family Cyprinidae (Carp):
  • Genus *Cyprinus* — Common carp
  • Genus *Rutilus* — Common roach, Astrakhan roach, vobla, kutum
  • Genus *Diptychus* — Naked carp
  • Genus *Leuciscus* — Common dace, Siberian dace, common chub, ide
  • Genus *Phoxinus* — Minnow
  • Genus *Scardinius* — Common rudd
  • Genus *Mylopharyngodon* — Black carp
  • Genus *Ctenopharyngodon* — Grass carp
  • Genus *Aspius* — Asp
  • Genus *Tinca* — Tench
  • Genus *Luciobarbus* — Barbel
  • Genus *Chondrostoma* — Common nase, Kuban nase, Volga nase
  • Genus *Hemibarbus* — Yellowfin barbel
  • Genus *Hemibarbus* — Fine-scaled barbel
  • Genus *Pseudorasbora* — Stone moroko
  • Genus *Gobio* — Gudgeon
  • Genus *Capoeta* — Capoeta
  • Genus *Barbus* — Barbel
  • Genus *Schizothorax* — Marinka
  • Genus *Alburnus* — Shemaya, Azov shemaya, Black Sea shemaya
  • Genus *Alburnus* — Common bleak
  • Genus *Blicca* — Silver bream, Transcaucasian silver bream
  • Genus *Abramis* — Common bream, Eastern bream, white-eye bream
  • Genus *Vimba* — Vimba bream
  • Genus *Erythroculter* — Topmouth culter, redfin culter
  • Genus *Pelecus* — Sabre carp
  • Genus *Rhodeus* — Bitterling, Amur bitterling
  • Genus *Carassius* — Crucian carp, Prussian carp
  • Genus *Hypophthalmichthys* — Silver carp, bighead carp

Biological characteristics and reproduction of sturgeon

The anatomical structure of sturgeon has a number of distinct features that are important for practical fish farming. Their body is spindle-shaped and covered with five rows of large bony scutes: one dorsal, two lateral, and two ventral. Small bony plates and granules are scattered between them. The sturgeon skeleton remains cartilaginous throughout its entire life; they have no spine, and a flexible notochord performs its functions.

The sturgeon's mouth is located at the bottom of the head, extends in the form of a tube, and has no teeth in adult fish, which determines their type of diet. Four sensitive barbels are arranged in a transverse row in front of the mouth. The pectoral fins are equipped with a powerful front ray-spine. This anatomical feature is used to control the age structure of the herd.

Sturgeons are characterized by a long life cycle. With the exception of sterlet and shovel-nosed sturgeon, they reach sexual maturity late. Males of anadromous species mature no earlier than 10–12 years, and females — no earlier than 12–15 years. The most precocious are considered to be the Azov populations, which reproduce in the Don and Kuban rivers. Broodstock spawn several times in their lives, but not every year.

Fish species	Maximum age (years)
Beluga	100 or more
Russian sturgeon	50
Stellate sturgeon	30
Sterlet	20–22

During the spawning migration, anadromous species almost completely stop feeding. Spawning occurs in spring and summer on pebble or pebble-sand soils with a fast current. Spawning grounds are usually located on floodplains flooded by high water or on deep-water channel ridges.

• Water temperature for spawning — not lower than 15–20 °С
• Maturation of males of anadromous species — no earlier than 10–12 years
• Maturation of females of anadromous species — no earlier than 12–15 years

Development specifics of juveniles and sturgeon diet

For the successful reproduction of sturgeon, it is critically important to prepare a suitable substrate for spawning. Their adhesive eggs must firmly attach to pebbles or stones after fertilization. The incubation period is fast and takes from 2 to 10 days. The hatched larvae have a large yolk sac and initially develop exclusively at the expense of its nutrients.

1. Period of rest and larval feeding at the expense of yolk sac reserves.
2. Transition to external feeding on planktonic crustaceans (daphnia and cyclops) after the resorption of the yolk sac.
3. Introduction of mysids, gammarids, oligochaetes, and chironomid larvae into the diet of growing fry.
4. Transition to feeding on large benthic organisms or active predation (for certain species).

The behavior of juveniles depends on the biological group of the fish. Fry of anadromous species (beluga, stellate sturgeon, ship sturgeon, Russian and Atlantic sturgeons) migrate from rivers to the pre-estuarine areas of the sea in their first summer. Some juveniles of the ship sturgeon and Russian sturgeon may stay in river water for more than a year. Adult anadromous fish return to the sea immediately after spawning is completed.

The diet of most sturgeon species is based on benthic invertebrates: worms, mollusks, crustaceans, and insect larvae. Semi-anadromous species (Siberian and Amur sturgeons, kaluga) gain mass in the delta zones of large rivers — the Ob, Yenisei, Lena, and Amur. In spring, the sexually mature stock migrates upstream along the channels of these rivers for spawning.

• Starry sturgeon roe incubation — 2–5 days
• Starry sturgeon spawning temperature — 13–30 °C
• Sterlet maturation period — 5–8 years
• Starry sturgeon egg diameter — up to 3 mm
• Beluga life span — over 100 years

Commercial characteristics of main species

When organizing a pond or basin farm, it is necessary to consider the individual biological parameters of each representative of the sturgeon family. Differences in growth rates, maturation periods, and food preferences determine economic efficiency of fish farming. Comparative data for four commercially valuable species are provided below.

Fish species	Maximum dimensions (length / mass)	Age of maturation (males / females)	Adult feeding habits
Beluga	over 5 m / over 1000 kg	12–16 years / 14–20 years	Predator (anchovy, herring, gobies, red mullet)
Kaluga	up to 5.6 m / up to 800–1000 kg (usually 150–400 kg)	18–22 years (at a mass of about 80 kg)	Predator (chum salmon, pink salmon, herring, smelt, its own juveniles)
Starry sturgeon	over 2 m / up to 80 kg	9–13 years / 11–17 years	Typical benthophage (benthic invertebrates)
Sterlet	up to 100–125 cm / up to 16 kg (usually 1.5–2 kg)	5–8 years (for both sexes)	Benthophage (benthic invertebrates)

The beluga is one of the largest fish in fresh waters and leads a predominantly solitary lifestyle. Its torpedo-shaped body is protected by five rows of durable bony plates, the back is colored in a dark gray or greenish tone, and the belly is white. It spawns spring after spring at intervals of 2–4 years in the channels of freshwater rivers.

The kaluga is a representative of the beluga genus, distinguished by an enlarged first dorsal scute. The species exists in two ecological forms: semi-anadromous (enters the Amur from the estuary to spawn) and resident river form. The adult fish hunts spawning salmon and herring, easily swallowing prey weighing up to 5 kg. Due to the decline in the natural population, the reproduction of the kaluga is maintained artificially.

The starry sturgeon is easily recognizable by its very long, upward-curved snout, which occupies more than half of the head length. The dorsal scutes number from 11 to 14, and the lateral ones from 30 to 36. The fish prefers to spawn on pebbly sections of rivers in the period from April to August, laying from 35 to 630 thousand eggs.

The sterlet stands out among other sturgeons due to its short life cycle, which is about 30 years, and its quick achievement of sexual maturity. It lives exclusively in rivers and lakes of the European part and Siberia, including Lake Ladoga and Lake Onega. Distinctive external features of the species are a narrow elongated nose and long barbels reaching the mouth.

Biological characteristics of commercial sturgeon fish

For successful breeding of sturgeon in artificial conditions, it is important to consider their natural reproduction cycles and diet. The sterlet, Russian sturgeon, and shovelnose sturgeon have different habitat requirements, which directly affects the technology of their keeping. Knowledge of these parameters allows minimizing fish mortality at key stages of farming.

Sterlet spawning takes place in the spring period — from mid-April to early June. To deposit sticky eggs, females choose deep river channel sections with a fast current and a stony or pebbly bottom. Thanks to high adhesiveness, the eggs are firmly held on the substrate and are not washed away by the water flow.

The Russian sturgeon is represented by both anadromous and resident forms, living in the basins of the Caspian, Black, and Azov Seas. For reproduction, the fish enters the Volga, Ural, Terek, Sulak, Samur, Danube, Dnieper, Mzymta, Psou, and Don rivers. The shovelnose sturgeon, on the other hand, is a purely river inhabitant, adapted to the conditions of the Amu Darya and Syr Darya.

Fish species	Typical dimensions and weight	Age of sexual maturity	Fecundity, thousand eggs	Main diet
Russian sturgeon	Common weight — 15–25 kg (max. up to 80 kg), length — up to 2.3 m	From 6 to 19 years (on average — 10–12 years)	720–950	Invertebrates, mollusks, small fish
Sterlet	Stays at the bottom in deep areas	Not specified	15–50	Crustaceans, worms, insects and their larvae, eggs of other fish
Shovelnose sturgeon	Common weight — 2–3 kg (max. up to 4.5 kg), length — 60–90 cm (max. up to 100 cm)	Not specified	Not specified	Benthic organisms (benthophage)

An anatomical feature of the shovelnose sturgeon is its very long, flattened caudal peduncle, covered with strong bony plates resembling a carapace. The caudal filament in this species is poorly developed or completely absent. The fish has a large swim bladder and very small eyes, which is associated with its bottom-dwelling lifestyle in turbid river currents.

Biology and specifics of salmonid spawning

The salmon family unites anadromous and freshwater fish of the Northern Hemisphere, found in Europe, Northern Asia, North America, and the mountain streams of North Africa. These fish are characterized by extremely high ecological plasticity. They are capable of rapidly changing their coloration, body shape, and lifestyle depending on external environmental factors.

• Water temperature for sterlet spawning — 12–17 °C
• Maximum age of the Russian sturgeon — up to 50 years
• Feeding depth of Pacific salmon — up to 10 m
• Number of rays in the dorsal fin of salmonids — from 10 to 16

All salmonids are divided into two subfamilies: Salmoninae and Coregoninae. Whitefish (Coregoninae) are distinguished by peculiarities of their skull structure, smaller mouths, and larger scales. All members of the family are characterized by specific anatomical markers:

• one true dorsal fin and one adipose fin without rays;
• a vestigial state or complete absence of oviducts in females, causing the ripening eggs to fall directly into the body cavity;
• the presence of numerous pyloric caeca in the intestine;
• the presence of transparent eyelids in most species.

Pacific salmon are represented by six clearly distinguishable species:

1. chum salmon;
2. pink salmon;
3. chinook salmon;
4. sockeye salmon;
5. coho salmon;
6. masu salmon.

During the marine period of their life, these fish actively gain weight in the northern part of the Pacific Ocean up to the warm Kuroshio Current, entering the Sea of Japan, the Sea of Okhotsk, and the Bering Sea. They feed on small pelagic fish, crustaceans, pteropod mollusks, juvenile squid, and worms. During this time, their body is covered with silvery scales, and there are no teeth on the jaws.

Upon entering rivers, the fish develop a nuptial coloration: the body flattens, large hooked teeth grow on the jaws, palate, and tongue, and the jaws become distorted. The skin becomes thick and rough, the scales grow into it, and the silvery color is replaced by black, crimson, or purplish-red. In females, these changes are significantly less pronounced than in males.

1. The fish enters the rivers and selects non-silted areas with a pebble or gravel bottom, where underwater springs often flow.
2. The female faces against the current and, with sharp movements of the caudal peduncle, excavates a deep spawning pit in the soil.
3. Large red-orange eggs are deposited into the prepared nest, which the male fertilizes with milt.
4. The female covers the pit with pebbles, forming a spawning mound, under the protection of which the larvae develop until the yolk sac is absorbed.

After spawning is complete, the exhausted fish die en masse. Some of the spawners die right at the spawning grounds, while others are carried by the current toward the river mouths. Dead fish cover the banks and bottoms of water bodies, becoming a food base for birds and taiga animals.

As soon as the yolk sac is absorbed, the fry emerge from the mound and drift downstream, feeding on small aquatic invertebrates and insects that have fallen into the water. In some species, they do not linger long in the river, while in others, the river period extends up to one or two years.

Chum salmon. An anadromous fish of the Pacific salmon genus. Body length up to 1 m, weight up to 14 kg. Distributed in the northern part of the Pacific Ocean: off the Asian coasts and the American coasts — as far as Monterey Bay. From spring, while feeding, it migrates to the North. In small numbers, it reaches the rivers of the Arctic Ocean basin from the Mackenzie to the Lena. It feeds on crustaceans, pteropod mollusks, and juvenile fish (sand lance, smelt, herring, etc.). Matures at the 4th–5th year of life. For reproduction, it enters rivers. In the sea, the silver chum salmon has a silvery color, including the bases of the caudal fin rays, without stripes or spots. In the river, the coloration changes to brownish-yellow, with dark purple or dark crimson stripes.

Two forms of chum salmon are distinguished: summer (up to 80 cm in length), entering rivers from the first days of July until mid-to-late August; it predominates in the northern parts of the Pacific Ocean. Autumn chum salmon (up to 1 m in length, larger and more valuable) predominates in the southern parts of the range. Both forms enter the Amur River and the rivers of Sakhalin. The average length of a migrating chum salmon in Sakhalin is 61–65 cm, weight 2.7–3.3 kg; further north, the chum salmon is larger. Autumn chum salmon enters the Amur starting from the end of August and early September and travels much further up the rivers than the summer form. It often spawns under the ice. For spawning grounds, chum salmon selects quiet sections of small rivers, the bottom of which is covered with fine pebbles and gravel.

Eggs are deposited into pits dug in the soil, after which the female heaps a gravel mound up to 2–3 m long and 1.5–2 m wide over them. In the spring, the fry hatched from the eggs emerge from the spawning mounds and, without lingering in the river, migrate downstream to the sea.

Pink salmon. The smallest representative of the salmonids. The maximum length does not exceed 68 cm, and the weight is 3.0 kg, but the small size is compensated for by the large population numbers. It is widely distributed. Males are usually larger than females. Pink salmon mature in their second year of life. Spawning occurs along the main channel and in the lower reaches of large tributaries. Spawning grounds are located on riffles with clean water and on non-silted soil consisting of gravel and pebbles mixed with sand.