Biological characteristics and spawning specifics of the northern pike

The eggs are semi-pelagic, with an oil droplet, non-adhesive, 0.75 – 0.92 mm in diameter. Spawning occurs on sandy or pebbly soil at depths of 0.5 – 3.0 m. Absolute fecundity ranges from 50,000 to 5 million eggs.

Widely distributed in fresh waters of the northern regions of Europe, throughout water bodies of the Arctic and temperate zones, in the basins of the Baltic, White, and Caspian Seas, and in the basins of all Siberian rivers from the Ob to the Anadyr along their entire length.

Pike (Esocidae family). The pike's body has an elongated, arrow-like shape. The head is strongly elongated, with the lower jaw protruding forward. The teeth on the lower jaw vary in size and serve to grasp the prey. It is characteristic of pikes to undergo tooth replacement on the lower jaw.

Length up to 1.5 m, mass up to 35 kg (usually up to 1 m and 8 kg). The body is torpedo-shaped, the head is large, and the mouth is wide. The coloration is variable, depending on the environment: it can be grayish-green, grayish-yellow, or grayish-brown depending on the nature and degree of vegetation development; the back is darker, and the sides have large brown or olive spots that form transverse stripes. The unpaired fins are yellowish-gray or brown with dark spots; the paired ones are orange. It feeds primarily on fish.

In natural water bodies, female pike reach sexual maturity in their fourth, more rarely third, year of life, and males in their fifth.

Pike spawning occurs at a temperature of 3–6 °C, immediately after the ice melts, near the shore at a depth of 0.5–1 m. During spawning, fish come to shallow waters and splash noisily. Typically, the smallest individuals spawn first, and the largest ones last. During this time, pikes stay in groups: 2–4 males per one female; up to 8 males near large females. The female swims in front, and the males follow her, lagging about half a body length behind. They either press against the sides of the female or try to stay directly above her back. During this time, the dorsal fins and the upper parts of the fishes' backs are constantly appearing above the water.

During spawning, pikes rub against bushes, roots, stems of reeds and cattails, and other objects. The fish do not linger in one place for long; they constantly move around the spawning ground and lay eggs. At the end of spawning, all individuals of the group that have spawned dart in different directions, causing a loud splash; at this moment, females often jump out of the water into the air.

Depending on its size, a single female pike can lay from 17,500 to 215,000 eggs. The eggs are large, about 3 mm in diameter, slightly adhesive, and may attach to vegetation but easily fall off when shaken. After 2–3 days, the stickiness disappears, most of the eggs roll off, and their further development occurs on the bottom.

The pike's diet consists mainly of various fish species, including roach, perch, ruffe, silver bream, rudd, gudgeon, loach, stone moroko, bullhead, and sometimes even members of their own species. There are known cases where pikes have grabbed and pulled under water ducklings, as well as mice, rats, sandpipers, and squirrels crossing rivers during their migrations. Large pikes can attack even an adult duck, especially during the molting period when these birds do not lift off the water into the air. It is worth noting that the pike's victims are often fish whose length and weight reach 50%, and sometimes even 65%, of the predator's length and weight.

In the diet of medium-sized pikes, about half a meter long, numerous and low-value fish predominate; therefore, the pike is a necessary component of rational fish farming in lakes; due to its absence in lakes, the population of small ruffe and perch increases sharply.

Eel (Anguillidae family) is a predatory fish with an elongated, snake-like body that is more or less rounded in the front part and compressed from the sides from the anal opening to the tail. It is covered with a layer of thick mucus, making it very slippery. The dorsal, tail, and anal fins form a ribbon-like fringe that covers more than half the length of the fish.

The rays of all fins are covered by skin. The pectoral fins are wide but short; the pelvic fins are absent. The scales are very small, almost hidden in the skin, and extend onto the head and fins. The head is small, conical in shape, and somewhat flattened.

In the CIS, the European eel is most often found in the water bodies of the Baltic Sea basin. Through canals, it also penetrates into other basins.

In the lakes of the Volyn and Rivne regions, eels reach a length of 80–100 cm, and their weight is often 2.5–3 kg. In the water bodies of Belarus, eels are found with a length of up to 115 cm and a weight of up to 3 kg. Males are smaller than females.

Eels feed only during the warm season, mainly at night; during the day, they bury themselves in the soil, leaving only their heads exposed. With the onset of frosts, they stop feeding until spring. In the intestines of eels, one can find worms, leeches, mollusks, larvae of mayflies, dragonflies, caddisflies, higher crustaceans, and fish (perch, ruffe, rudd, roach, bleak, etc.). The composition of the eel's diet and the prevalence of specific animals in it depend on the eel's age, the availability of these organisms, and the season of the year.

Eels move in a serpentine manner, relatively slowly. When in danger, they quickly bury themselves in silt or hide in all sorts of shelters. In damp places, eels can live for a long time without water. They are capable of moving through grass, especially in dew or after rain, and even on wet gravel or cobblestones, but they travel only short distances on land.

Biological features and reproduction of commercial fish species

Effective fish farming requires a precise understanding of the life cycle and habitat conditions of each species. Such commercial fish as the European eel and the so-iuy mullet possess unique biological characteristics that determine their migrations, water temperature requirements, and spawning times. Considering these features allows fish farmers to correctly plan harvesting and control the population status in water bodies.

The life cycle of the European eel is closely linked to long migrations from freshwater bodies to the marine environment. The development of this fish from larva to adult passes through several stages:

1. Reaching sexual maturity in fresh waters at the seventh to ninth year of life and subsequent departure to the sea.
2. Spawning at a depth of 400–500 m in the Sargasso Sea (southern part of the Atlantic Ocean) in April – May, after which the adult eels die.
3. Hatching of transparent leaf-shaped larvae in late winter – early spring and their drift with currents toward the coasts of America and Europe.
4. Transformation of larvae near European shores into glass eels by the autumn of the third year of drifting, upon reaching an average length of 7.5 cm.
5. Entry of the glass eel into fresh waters, where it lives and grows for 9–15 years.

The so-iuy mullet (Mugilidae family) is a schooling fish that enters rivers in the autumn for wintering and returns to the sea in early spring. This species was successfully acclimatized in the Sea of Azov in the second half of the 20th century, where its catch volumes now amount to tens of thousands of tons. In the Black Sea, the so-iuy mullet feeds primarily on Nereis worms, as well as other small benthic invertebrates.

• Maximum mass of so-iuy mullet — 12 kg
• Water temperature for spawning — 18–24 °С
• Age of maturity for males — 4 years
• Age of maturity for females — 5 years
• Average weight of an adult — 7 kg

So-iuy mullet spawning occurs near the coast over shallow depths. The breeding period is prolonged due to the different timing of the arrival of broodstock at the spawning grounds and potential two-stage egg laying. During the reproduction process, one large female is accompanied by several males, fertilizing the pelagic eggs.

Influence of fish body shape on hydrodynamics and housing conditions

The body shape of a fish directly determines its motor activity, swimming speed, and preferred habitats. The interaction of streamlined contours, fins, and musculature allows fish to maximize the use of hydrodynamic forces. For a fish farmer, these parameters are important for assessing the adaptability of species to currents, stocking density, and the design of flow-through structures.

Differences in lifestyle have led to the formation of two main modes of fish movement. Benthic species with elongated bodies move by means of lateral undulating bends of the entire trunk, with the speed of their movement being relatively low. Most other fish move with the help of frequent oscillations of the rear part of the body and a powerful caudal peduncle, while the front part acts as a water splitter.

In the process of evolution, fish have developed different types of body structures adapted to specific environmental conditions:

Fish growth control and exterior evaluation

Regular monitoring of fish weight and linear growth allows for timely adjustment of keeping conditions, assessment of feeding efficiency, and productivity forecasting. To this end, systematic test fishing is carried out at the farm. The frequency of sampling depends on the age of the fish — from frequent measurements at early stages to seasonal monitoring in adult individuals.

• Larvae and fry (first 15 days) — every 2–3 days
• Fry older than 15 days — every 10 days
• Two-year-olds (test fishing) — every 10–15 days
• Adult fish — in spring and autumn

Different tools are used for measurements depending on the size of the objects. Larvae and small fry are examined under a microscope using an ocular micrometer. Larger juveniles are measured with calipers, a standard compass, or a ruler, while adult fish are measured with a ruler or a special measuring board.

To assess body constitution and productivity direction, five main parameters are determined: total length (L — from the tip of the snout to the vertical of the end of the longer lobe of the caudal fin), body length excluding the caudal fin (l — from the tip of the snout to the end of the scaly cover), head length (C — to the posterior edge of the gill cover), maximum body height (H — in front of the dorsal fin), and body girth (O — at the first ray of the dorsal fin).

Based on these measurements, exterior indices are calculated:

• Body height index: ratio of body length to height (l / H).
• Relative body thickness index: ratio of maximum thickness to length (m / l × 100 %).
• Big-headedness index: ratio of head length to fish length (C / l × 100 %).
• Compactness index: ratio of body girth to fish length (O / l × 100 %).

Growth rate is determined in absolute values (weight gain in grams per day) or relative percentages, which show the intensity of the process. For example, in carp, the relative growth rate is maximal at the larval stage, while the highest average daily absolute weight gain is observed at the age of 3–5 years.

Anatomical markers: mouth type, lateral line, and fins

The structure of the oral apparatus and sensory organs determines the feeding behavior of the fish and its ability to navigate in the water body. The position of the mouth is an important systematic trait that indicates the water layers in which a given species feeds.

According to the shape of the opening, the mouth can also be transverse, oblique, or crescent-shaped. Next to it on the head and cheeks, genipores are often located — small sensitive pores or papillae in the skin. These are the simplest organs of the lateral line, most noticeable on scaleless areas.

The lateral line looks like a row of pores on the scales along the sides of the body. The number of scales in it is the most important systematic trait and is recorded by a formula, where the left number denotes the minimum and the right the maximum number of scales (e.g., 44–46). The fraction before this number indicates the number of scales above the lateral line (in the numerator, 4–8) and below it (in the denominator, 3–4). If the lateral line is incomplete or absent, the number of transverse rows of scales is counted during classification.

Fish fins consist of rigid indivisible spines or soft branched rays. Paired fins (pectoral and pelvic) act as rudders, brakes, and stability stabilizers, while unpaired ones (dorsal, caudal, and anal) provide propulsion. In salmonids, smelts, graylings, and catfish species, an adipose fin is located behind the dorsal fin — a skin outgrowth without bony rays, filled with fat and not involved in movement.

Fin formulas and species identification by exterior

Fish fins serve as crucial systematic traits used to determine species identity. Their rays are divided into unbranched (rigid spiny or flexible segmented) and branched, which split at the base or at the tip. For the convenience of description in ichthyology, Latin designations are used: dorsal fin — D, anal — A, pectoral — P, pelvic — V, caudal — C.

The fin structure formula is written without commas: the ray type is coded with Roman (unbranched) and Arabic (branched) numerals. If a fish has two separate dorsal fins, the values for each are separated by a comma. The specific structure of the fins is directly related to the lifestyle and swimming speed of the fish.

• Number of dorsal fins — from 1 to 3
• Formula DIII 8–10 — 3 unbranched and 8–9 branched rays
• Formula A 7–10 — 2–3 unbranched and 7–10 branched rays

Pelvic fins can be located on the belly far behind the pectoral fins, below them, or in front of them. Pectoral fins are usually located behind the gill covers, but in some species, they may shift higher or forward. In gobies, the pelvic fins or their muscular bases fuse to form a ventral sucker.

The shape of the caudal fin determines the hydrodynamics of the fish. A heterocercal tail with an enlarged upper lobe is usually combined with an inferior mouth, as seen in sturgeon and sharks. In flying fish, the lower lobe is more developed, which helps them push off from the water during a jump.

• Lunate tail — characteristic of tunas (the fastest swimmers);
• Forked tail — noted in herring;
• Emarginate tail — developed in carp, zander, and salmon;
• Rounded tail — typical for the sedentary burbot.

Protective skin barriers: scales, mucus, and signaling substances

The skin of a fish consists of an outer multilayered epidermis and an inner layer — the dermis proper with subcutaneous tissue where fat accumulates. Unlike terrestrial animals, the outer cells of the fish epidermis keratinize but remain viable. In the pre-spawning period, male cyprinids and whitefish develop "nuptial tubercles" on the head, gill covers, and sides — small white bumps that disappear after spawning.

Glandular cells of the epidermis continuously produce mucus — a crucial factor in fish protection. Fish with dense scales (salmonids, perches) secrete it in moderate amounts. Species with reduced scales or none at all (tench, weatherfish, catfishes) secrete mucus very abundantly. Tench mucus possesses potent bactericidal properties, making it exceptionally resistant to parasitic infestations.

Mucus performs a whole complex of vital functions in the fish's body. It reduces friction against the water, prevents the penetration of bacteria, and accelerates blood clotting in case of wounds. The mucous coating also participates in the excretion of substances from the body, water-salt metabolism, settles silt suspended in water, and carries a species-specific scent.

• Mechanical protection: reducing water resistance during movement;
• Bactericidal barrier: protection against parasites, fungi, and bacteria;
• Regeneration: accelerating blood clotting during skin injuries;
• Excretion of substances and osmotic regulation: controlling the penetration of salts and water;
• Environmental purification and communication: settling silt and releasing individual species scent.

How skin pigmentation reflects the condition of the fish and the environment in the water body

The coloration of a fish is a reliable indicator of its health and habitat conditions in a pond. Chromatophores are responsible for body color — star-shaped cells with many processes containing pigment granules. They are located in the lower layers of the epidermis and at the boundary with the corium. It is these cells that determine the entire diversity of fish coloration, which is most vivid in tropical species.

The variety of shades is achieved through a combination of different types of chromatophores:

• Melanophores — contain grains of black pigment;
• Xanthophores — contain grains of yellow pigment;
• Erythrophores — contain grains of red pigment;
• Guanophores (iridocytes) — do not have pigment grains but contain crystals of guanine, thanks to which the fish acquires a silvery coloration.

In addition to water parameters, internal factors of the organism influence the behavior of the pigment. Coloration changes with age and depends on the sex of the fish and its general condition, including hunger or the breeding period. Emotional state also has a strong influence: pigment cells react quickly to the fish's fear or excitement.