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The clonal progeny of a female hybrid sterlet Acipenser ruthenus × kaluga A. dauricus and the same-age backcross hybrids (sterlet × kaluga) × kaluga have been analyzed for variability in morphometric traits. The heritability indices have been inferred from the dispersion values obtained. It has been shown that the clonal progeny does not differ from the backcrosses by lower variability in morphological traits, which is explained by the manifestation of the phenomenon of the relationship between heterozygosity and stability of development. The low values of the heritability index estimated for most traits agree with the general idea of the high modification variability in measurable traits. The tendency of the traits having a diagnostic value to a higher heritability is hardly expressed in the material studied, which is possibly due to the small number of the analyzed characteristics.

We studied the morphological structure of dorsal scutes in adult individuals of Acipenser ruthenus. We determined the morphometric indices of scutes, the values of which are different in males and females. We created a system for sex identification in sterlets based on indices and criteria indicative of the shape of the first five scutes. The results can be used for developing a method for intravital sex identification in sturgeons at earlier stages of development.

We studied the activity of the digestive enzymes, qualitative composition, and bacteria genera ratio in the intestinal chyme of carp Cyprinus carpio and sterlet Acipenser ruthenus grown on an artificial diet in cage culture. It was shown that the inclusion of live bacteria Bacillus subtilis in the probiotic ProStor had a positive impact on the size and the structure of the microbial community in intestinal chyme, on the activity of some digestive enzymes, and on the digestibility and the weight gain of carp and sterlet. The total amount of micro-organisms in the carp chyme was by 13% higher and in sterlet it was 3.5 times higher in the experimental groups compared to the control. In the studied species of fish, the increase in the activity of digestive enzymes was accompanied by the decrease in nitrogen, ammonia, and urea. Presumably, it happened because of better absorption of protein and more intensive use of nitrogen metabolites by intestinal microflora for the synthesis of their own biomass. The average daily weight gain of carp and sterlet was 25 and 35% higher, respectively, in the experimental groups than in the control.

The effect of testosterone on composition of leucocytes in the peripheral blood and immunocompetent organs of sterlet, Acipenser ruthenus, have been studied. It has been revealed that injection of physiological solution into the fish of control group and of testosterone into the experimental fish result in the changes in the proportions of certain types of white blood cells in the leucograms. The fish injected with hormone responded by decrease in the content of lymphocytes and increase in neutrophils and monocytes. The changes in the leucograms were most pronounced in peripheral blood and least pronounced in spleen.

The present paper provides the results of the study on feeding of juvenile (underyearlings and age 2+) sterlet (Acipenser ruthenus) caught in the Danube River in 2002–2003. The full spectrum of food items found in the fish stomachs was identified. The seasonal patterns of food composition are revealed. It is shown that juvenile sterlet in the Danube feeds mainly on larvae of chironomids and trichopterans, amphipods, and leeches. The organisms typical for lithophilic and lithorheophilic biocenoses play considerable role in the feeding of sterlet. Mean daily gains of body length and weight in the sterlet underyearlings are calculated.

Effect of dexamethasone (2.0 mg/kg) on activity of enzymes hydrolyzing proteinaceous and carbohydrate food components in the intestine of the starlet Acipenser ruthenus is studied. Dexamethasone modifies the activity of proteinases more than of glycosidases. As a rule, the hormone significantly decreases the level of proteolytic and general amylolytic activity of the intestinal mucosa and chyme in comparison with intact fish on the first day into the experiment and increases it on the 7th or 14th day. The dynamics of activity was different in enzymes of different chains (glycosidase and proteinase) and preparations (the mucosa and chyme).

Based on long-term materials, spawning, postspawning, and downstream migrations of the Lower Volga sterlet Acipenser ruthenus were studied. The dates, extent, pattern (two-step and one-step) and main parameters of the spawning and postspawning migrations of the previously separated three populations of the Lower Volga starlet—Volgograd and eastern and western delta ones were established. The main differences of the downstream migration of this potanadromous species from that of anadromous species of acipenserids were revealed. It was found that, after downstream migration, juveniles of sterlet form a continuous chain of aggregations from the dam of the Volgograd Hydraulic Power System to the Caspian Sea. Each link of this chain acts as a center of formation of local groups of sterlet that are components of the distinguished populations. It is shown that the observed distribution of sterlet in the lower reaches of the Volga is determined by the location of the spawning grounds of individual populations and the extent of downstream migrations of prolarvae and larvae. The total extent of migrations of sterlet is small (it does not exceed 250 km), which is a cause of the relative isolation of the previously distinguished populations. The stocks of Volgograd sterlet are exhausted at the present time; the recovery of its stock is possible, first of all, at the expense of a decrease in illegal fishing. The resources of the eastern and western delta populations of sterlet are in a satisfactory condition.

The influence of cortisone on the part of leukocytes in the differential leukocyte count of peripheral blood of sterlet is described. The changes in particular groups of leukocytes are characterized quantitatively. To the hormone, the fish reacted by a decrease in leukocyte count, blastic (juvenile) cells, and eosinophils and an increase in neutrophils and monocytes. It is concluded that the stress hormone suppresses lymphopoiesis by the hormone-induced apoptosis of lymphocytes while, on the contrary, myelopoiesis activates it.

Trawl surveys made in the Lower Volga and the Caspian Sea revealed that the full life cycle of the Lower Volga sterlet is confined to fresh water and only single specimens living in the Volga-Caspian canal can live in the canal and swim to weakly saline and stagnant waters. Therefore, the sterlet living in the Volga-Caspian canal with its expressed current should be attributed to potamodromous forms. In consideration of the data on distribution, the variation of the complex of plastic characteristics, growth rate, and infestation with the parasite Amphilina foliacea it is concluded that within the Lower Volga population of sterlet three populations of the lower rank may be discerned: the Volgograd population living from the Volgograd Hydroelectric Complex dam to the Kamennyi Yar settlement; the eastern population living in the top of the Volga Delta and Buzan channel; and the western population in the Volga-Caspian canal. These populations differ in some plastic characteristics, linear growth rate, the part of fish infested with the parasite A. foliacea, and in the occurrence of anomalies in the structure of mature ovicells.

The dynamics of species composition of fish from the middle course of the Oka is considered. A total of 53 species of Cyclostomata and fish was previously recorded; however, only 43 species have been found at the present time. Due to different causes, ten species have disappeared, and ten new species have emerged. The species composition and structure of aboriginal fish population are characterized by relative stability; however, the total abundance of fish during the current decade, as compared to the 1970s, has considerably decreased. The sterlet Acipenser ruthenus, lake minnow Phoxinus precnurus, Volga zander Sander volgensis, and freshwater sculpin Cottus gobio belong to rare fish.

The species list of fish of the Upper Oka basin is verified. Data on biology of rare species are presented (western brook lamprey Lampetra planeri, sterlet Acipenser ruthenus, bystryanka Alburnoides bipunctatus rossicus, asp Aspius aspius, undermouth Chondrostoma variabile, bitterling Rhodeus sericeus, and fresh-water sculpin Cottus gobio koshewnikowi). Their morphological characters, distribution in the Upper Oka basin relative abundances of these species, and its changes during recent decades are also commented.

Thermoregulation behavior of satiated and hungry juvenile sterlet Acipenser ruthenus, the Siberian sturgeon Acipenser baeri, and the goldfish Carassius auratus was studied. Significant alterations of thermoregulation behavior appear in juvenile fish one day after food deprivation. Hungry fish preferred, on average, lower temperatures, their temperature range was broader, and the locomotor activity was higher than in satiated fish.

A list of ichthyofauna of the Lower Don basin based on modern material and comprising 71 species of fish and Pisciformes belonging to 16 families is provided. Considerable variation in the species composition and in the relative numbers of the aboriginal ichthyofauna that occurred within the last decades and the appearance of new invading species is noted. The assessment of the influence of damming and the seasonal redistribution of the Don River runoff on the conditions of reproduction of its ichthyofauna is given.

Species specificity of osmotic and ion regulation in five species of acipenserids connected in varying degrees with sea conditions is investigated. The objects of study were freshwater middle-Volga sterlet Acipenser ruthenus; Siberian sturgeon A. baerii, inhabiting the Lena River and migrating to the sea near the river mouth; and the following migratory acipenserids of the Volga-Caspian Basin (regular diadromous migrations): the Russian sturgeon A. gueldenstaedtii, the great sturgeon Huso huso, and the starred sturgeon A. stellatus. The study was carried out on immature specimens (of age 2+). The level of osmolarity and concentrations of cations (Na+, K+, Ca2+, and Mg2+) were determined in blood serum, urine, and liquid from the cavity of spiral valve in fish from the fresh water (control) and after their adaptation (period of seven to ten days) to sea water with salinity 12.5–12.7 and 18‰. The increase was found in the functional activity of the mechanism of osmotic and ion homeostasis in sturgeons according to the increase of the environmental salinity of a particular species. The freshwater sterlet adapted to the sea environment as osmoconformer. The other studied species were capable of maintaining a relative stability of osmolarity and ion concentration in blood serum at different levels while adapting to the sea environment; they changed from hyperosmotic (in fresh water) to hypoosmotic type of regulation. The kidney plays a more significant role in the removal of excessive Na+ in sturgeons than in bony fish. An increase in the sodium-reabsorption and magnesium-secretion functions of the kidney as well as the sodium-absorption function of the intestines was observed in sturgeons, relative to the increase of environmental salinity.