The water area of our planet is inhabited by about 20,000 different species of fish. About a tenth of them are fishing. In addition to food, commercial fish provide us with medicines, industrial raw materials, industrial fats, fertilizers, pet food. Commercial fish are divided into freshwater, migratory and marine.
Freshwater fish, whose share in the global catch is about 11%, lives in rivers, lakes and ponds. Catfish, cyprinids and perch fish have the highest commercial value.
The largest fish that live in rivers are the Amur Kaluga (a representative of the sturgeon family), whose weight can reach 1000 kg and a length of 5 m, Chinese psephurus (length up to 7 m), European catfish, growing up to 5 m in length and over 300 kg weight, South American arapaim (200 kg weight, more than 4.5 m in length). But such giants are not common. Most freshwater fish that live in domestic reservoirs are of medium size.
Freshwater fish for commercial purposes are partially bred in special artificial ponds - for example, trout, carp, tilapia, grass carp, silver carp. Of those living in rivers and lakes, pike, catfish, common carp, crucian carp, bream, perch, and roach are most important for catching.
In natural reservoirs, freshwater fish lives mainly near the coast, where there is a lot of food and convenient places for spawning. In the summer in shallow water you can always see flocks of fry. In the immediate area there live fish that are not too demanding on the quality of water, i.e., its purity and oxygen content. This is pike, crucian carp, tench, roach. A little further live perch, bream, ide. Catfish, zander and salmon choose the most remote and cleanest places.
Such species of river fish as trout, char, pinching are found in streams. In small lakes, mainly pike, roach, ruff and loach are found. Large lakes in the number of species are not inferior to sea water bodies. The number of fish species there exceeds several tens.
The oxygen content in water is the most important factor affecting the diversity of the composition of its inhabitants. Only the most unpretentious fish are found in water bodies poorly saturated with oxygen, since in the summer heat and in severe cold many of their species die out of asphyxiation. Lines and crucian carp can exist in oxygen-poor small ponds that freeze through in winter. Ruffs tolerate oxygen hunger well, but require clean running water.
The most demanding for high oxygen content are salmon, zander, sturgeon.
Freshwater fish also react to temperature fluctuations. Some of its species are unable to exist at temperatures below a certain. So, for fish of the carp family, this figure is + 10 ° C. There are fish that do not tolerate high temperatures (whitefish, burbot, and pale). In the heat, they hide under the stones and crawl out only with the onset of cold weather. Other fish can well exist in cold water and survive even in freezing water bodies. In frosty winters, individual fish species may fall into an anabiotic state.
Freshwater fish is also sensitive to sunlight. Here, too, there are lovers of lighted and dark ponds. Lamprey prefers darkness, and roach, for example, gathers in sunny, well-lit places. Salmon fry climb into stones from bright light.
Experienced fishermen can determine what kind of fish flashed in the water depth not only in appearance, but even in a characteristic splash, because each of them moves in its own way.
Fishing in our country is a favorite national type of recreation, which includes elements of competition, sports excitement, communication, a surge of adrenaline, fresh air and the practical side of the matter in the form of a legal catch. Nowadays, there is a practice of fish farming in special ponds with the subsequent sale of licenses for visiting and catching fish in them.