The Southern Scientific Center of the Russian Academy of Sciences has a hospital in the village of Kagalnik in the Azov region, on the shore of one of the small branches of the southern part of the Don Delta, Svinoi Girla. Since 2003, scientists have been setting up nets for fishing almost every day, and everything that is caught is examined and counted for comparison with what was caught two, three, five, ten years ago. At the last count, among other things, 12 species of fish marked as "alien" got into the net: Amur chebachok, atherina, white amur, silver carp, carp, tench, sunny perch, pilengas, carp, silver carp, pike and ide.

Sergey Dudkin, Associate Professor of the Department of Zoology at the SFedU Academy of Biology and Medicine, noted that this is a good reason to discuss the composition of the Don's ichthyofauna.

"There's nothing to be surprised about. For many thermophilic Black Sea species, the Sea of Azov has traditionally been a seasonal place for summer feeding, and some of these fish have appeared at the Pork Girl, at the confluence of one channel of the Don into the Taganrog Bay, the salinity of which is growing," said Sergey Dudkin.

At the same time, the Don itself remains bland. Its mineralization is mainly due to hardness salts – sulfates and bicarbonates of calcium and magnesium, rather than chloride and sodium sulfate, as in the sea. And an increase in sodium chloride salinity is possible only in short periods of time when water is being pumped from the Taganrog Bay into the Don by a "low-level" westerly wind. During the run-up, the salt water goes back into the bay.

According to the scientist, the pike has always lived in the lower reaches of the Don, in its channels, backwaters, thickets of underwater vegetation. They are not "alien" to Don yaz, lin. And carp, also known as carp, is a typical Don fish.

"Silver carp, white and mottled, as well as white amur, have long been massively introduced into commercial aquaculture in Southern Russia, including the Rostov region, since the second half of the 1960s, and since then they have always been celebrated in the Don. These herbivorous fish of the Far Eastern complex were specially imported from the Amur River basin to fish hatcheries created specifically for fish farming, for example, to the Donskoy zonal fish nursery in the village of Kuleshovka near Azov, which has now been liquidated," shared Sergey Dudkin.

The most massive importation of juvenile white Amur and silver carp was carried out in the 1970s. At the same time, the "Chinese form" of the silver carp was introduced with the young of these fish, which is better known to Don fishermen as a "hybrid". At the same time, by chance, an Amur chebachka, a pseudo―collector, was also brought into the Nizhny Don basin.

So these species have been living in the Don for over 50 years. They are allochthonous, but no fisherman would dare to call crucian carp, carp carp or white amur aliens.

Pilengas was specially imported from the Far East and successfully acclimatized in the Sea of Azov. It is a fast-growing marine fish, characterized by tender juicy meat without small bones. In recent years, pilengas has been the basis for the coastal fishing of all Russian fishermen in the Sea of Azov.

"Its local fishing history is also quite long ― pilengas was included in the list of commercial species in 1993, more than 30 years ago. And in the Don, in its lower reaches from the delta to the Kochetovsky hydroelectric complex, pilengas is quite common, even widespread. His acclimatization as a result of the efforts of AzNIIRH scientists and specialists from the Rostov Acclimatization Station, which has also already been eliminated, was very successful. Now it is the number one fish in the catches of coastal fishermen of the entire Azov region," explained Sergey Dudkin.

Atherina is a permanent inhabitant of the Black Sea, which makes feeding and spawning migrations to the Sea of Azov in summer. The usual mass view in it. The plucker is also a common species found in the tributaries of the Kuban. It could get to the Don through the Middle Egorlyk and the Veselovskoye reservoir. A close relative of the loach, which has always lived in the tributaries of the Don.

What is really alien, invasive, and harmful is the sun perch, a small, very beautiful fish from North America. In the Seversky Donets basin, this fish, apparently released by aquarists, has spread to the Don, and now it is increasingly appearing in catches. Sun perch reaches puberty early, is fertile, and is capable of causing significant harm to the local ichthyofauna by devouring eggs and eating fish fry. An active voracious predator.

"If we continue the topic of "alienness", then the list presented is very far from exhaustive. There are many Black Sea fish in the Sea of Azov. These are sea dogs, sea ruff, smurf, greenfish, seahorse, Black Sea gobies, sea cockerel, dark humpback ― dozens of "new" species. Some of them may appear briefly in the Don Delta. Not only fish communities are changing, but also other aquatic organisms such as mollusks, crustaceans, and microalgae. With increasing salinity in the Sea of Azov, a "new" ecosystem is forming right before our eyes," added Sergey Dudkin.

Moreover, the biomass of fish species populations and the number of specimens of native and alien fish are very different. In terms of biomass, intruders predominate in catches, and in quantitative terms, native fish species. This means that the native species are smaller, while the aliens are large.

The large autochthons of the Don: pike perch, bream, ram, and fisher — have been eliminated by poaching. And the remaining conditional Don herring is obviously smaller than the "new" ones ― pilengas, silver carp or white Amur. And there's plenty of aboriginal stuff in the Don, too: these are bleak, and verkhovka, and needle fish, and small sticklebacks, and a button trap, and other small things. There are a lot of individuals in terms of number, but only tears in terms of biomass.

"In the new realities, our rivers, lakes, and reservoirs have reached such a stage of ecological transformation and degradation that it is no longer possible to obtain high fish productivity in them without special fish population formation and intensive stocking. And it is possible to form a highly productive Ichthyocene by "aliens" ― this experience proved to be very successful," summed up Sergey Dudkin.

Many allochthonous fish species were imported specifically in order to improve the composition of the ichthyofauna or to make more complete and productive use of the food resources of reservoirs unused by local fish. Thus, the white carp filters microalgae from the water during feeding, purifying the reservoir from "blooming". The white amur eats vegetation in the reservoir, fighting overgrowth and waterlogging. Pilengas cleans up dead silt from the bottom. It is not for nothing that these species are called biological reclamation plants. They revitalize reservoirs, make the water clearer and cleaner.

And by eating "food" that none of the local fish needs, without artificial feeding, these species produce a highly valuable animal protein – fish meat. It is not for nothing that white amur, for example, occupies the first place in the world in terms of cultivation volume ― almost 6 million tons of live weight annually. And the white carp firmly holds the second place in the world in terms of cultivation in freshwater aquaculture.