The use of mazes has become a common method in fish behavior research, serving as an objective approach to assess mechanisms of spatial learning and memory. To study the possibilities of objective control over the learning process of fish <i>Danio rerio</i>, a small aquarium maze (35×25 cm with a 10 cm wall height) was designed. The specimens were fed for 10-12 days, measuring the time taken to reach the feeding location. It was found that the reduction in time from initial to final values for the fishes' passage through routes of different complexity followed an exponential function <i>y=ae<sup>bx</sup></i> with corresponding coefficients. A mathematical index of route complexity (IRC) is proposed, reflecting the product of the probability coefficients of the fish passing bifurcation points along the route, multiplied by a species coefficient. The latter is determined as the ratio of the length of the true path between the starting and finishing points and average speed of the biological subject. IRC indicates the minimal possible time to reach the goal, representing the initial time spent on the task before the learning process starts. The index of maze complexity (IMC) for fishes can be calculated as the sum of route index values under the non-intersecting and non-repeating corridors or by summarizing the probabilities of bifurcation points and the total corridor length. The data on feeding behavior and spatial orientation of the studied fish confirmed the positive correlation between the calculated IRC values for both simple and complex maze routes. Various aspects of fish intelligence can be studied by comparing mathematical parameters of maze complexity with task completion times.

The taxonomic structure of phytoplankton and regularities of inter-annual and seasonal dynamics of its cell numbers and biomass were studied in different sections of the Zaporizhzhya Reservoir during vegetation seasons in 20192021. The highest phytoplankton numbers at all stations were registered during vegetation season in 2020, whereas the highest biomass - in 2019. During almost the whole period of investigations, Cyanobacteria dominated forming water bloom. It has been found that the content of heavy metals (lead, zinc, copper, iron, manganese, and nickel) significantly influences the hydroecosystem of the Zaporizhzhya Reservoir. It has been shown that the content of artificial radionuclides <sup>137</sup>Cs and <sup>90</sup>Sr in phytoplankton of the Zaporizhzhya Reservoir was rather low. The specific activity of natural radionuclides (<sup>226</sup>Ra, <sup>232</sup>Th and <sup>40</sup>K) was 80-320 times higher than that of artificial ones, which is accounted for by their higher concentration in the ecosystem of the Zaporizhzhya Reservoir. The correlation analysis of the relations between hydrochemical, toxicological, radioecological parameters and species richness and quantitative indices of phytoplankton development was performed. The main factors influencing phytoplankton development in the Zaporizhzhya Reservoir at the present time were revealed.

The fractional composition of blood serum of crucian carp (<i>Carassius gibelio</i> Bloch.) and pike (<i>Esox lucius</i> L.) under the impact of 2 and 5 maximum permissible concentrations (MPC) of cobalt was studied. Increased concentrations of cobalt ions caused changes in both total content of serum proteins and their fractional composition. Under the impact of 2 and 5 MPC of cobalt ions total protein content in blood serum of crucian carp increased by 18.9 and 24.8%, respectively, whereas in pike it confidently decreased by 15.8% under the influence of 5 MPC. In crucian carp, 5 MPC of cobalt did not cause significant changes of albumin and γ-globulins content, whereas 2 MPC caused decrease of α<sub>1</sub> and α<sub>2</sub> -globulins and increase of β-globulins content. In pike, 2 MPC caused confident increase of albumin and α<sub>2</sub> -globulins content and decrease of the β- and γ-globulins, and5 MPC caused increase of β-globulins and decrease γ-globulins content. Significant increase of the albumin/globulin ratio was noted only in pike blood serum under the impact of 2 MPC of Co<sup>2+</sup> ions. Further studies of the adaptive responses of the blood system of the freshwater fishes are promising for both assessment of their organism health and biomonitoring of hydroecosystems in terms of cobalt intoxication.

The article deals with assessment of the ecological state of the Vorskla River within the Sumy region limits using the visual test methodology. The purpose was to adapt the test methodology for assessment of the ecological state of a river at the key sites and the certain area, and development of the water-protection measures. According to the test methodology, the ecological state of the Vorskla River at key points and within the studied section "Klimentovo - Buimerivka" was determined mainly as "good" and "still good", at three points - as "moderate", and ecological state of the mouth areas of the tributaries was assessed as "moderate" (Sosonka, Gusynka, Oleshnia rivers) and "poor" (Okhtyrka River). It was proposed to delineate the water protection zones and coastal protection zones as natural biofilters, introduction of strict penalties for violation of their integrity and pollution, prevention of construction in the coastal protection zones, and facilitation of their rehabilitation.

The studies carried out from 05.15.2023 to 12.31.2023 showed that some part of the fish population of 12 species moved from the lower section of the Dnieper River and probably from the Kakhovka Reservoir to the western part of the Dnieper-Bug estuary (liman). This was caused by destruction of the Kakhovka dam on 06.06.2023, and inflow of huge water masses from the Kakhovka Reservoir. After the disaster, the numbers and occurrence frequency of most fish species in the western part of the estuary significantly increased. In four species, the certain size groups were replaced by another. Two species have been detected in the estuary for the first time. Of them, one is a dangerous invasive species, and other was absent for 25 years. As a result of relocation further to the Black Sea, <i>Polyodon spathula</i> was found for the first time in the Dzharylgach Bay. <i>Serranus scriba</i> was registered in the Berezan estuary, and <i>Sander marinus </i>was found in the estuary after the 80 years of absence. Besides, part of <i>Sander marinus</i> population migrated from the Dnieper liman to the upper part of the Bug liman and upstream to the lower section of the South Bug River. <i>Tinca tinca</i> was also registered for the first time in the Berezan liman, probably owing to the expansion of its range to the northwestern part of the Black Sea. The destruction of fish breeding ponds in Crimea because of catastrophic storm on November 26-27, 2023 resulted in migration of many fishes to the Dzharylgach Bay.

The results of long-term studies of the role of humic substances in the binding of a number of metals (Al, Fe, Cu, Mn, Cr, Zn, Pb, Mo, Cd, V, and Ti) in anionic complexes in various types of water bodies of Ukraine are summarized and discussed in the paper. It has been shown that the share of complex compounds of these metals increases significantly with increasing the concentration of humic substances. The water bodies and watercourses with a rather high content of humic substances include primarily the rivers of the Prypyat River basin and the reservoirs of the Dnieper cascade. The relative content of each metal in these water bodies (in % of the total concentration of M<sub>dis</sub>) on the average was as follows: Al - 63.3 - 83.0, Fe - 58.6 - 76.4, Cu - 63.0 - 78.2, Mn - 52.3 - 60.0, Cr - 44.6 - 76.5, Zn - 39.8 - 58.5, Pb - 55.8 - 73.4, Mo - 63.4 - 80.3, Cd - 57.6 - 70.0, V - ≈ 60.0, and Ti - 33.2 - 62.5. The share of anionic complexes decreases with a decrease in the concentration of humic substances in the water. Its lowest values are characteristic of water bodies in the urbanized areas subjected to anthropogenic load and characterized by a relatively low content of humic substances. It is emphasized that for some metals, the results of modeling and field studies of coexisting forms differ significantly. This depends largely on the values of the conditional stability constants of complexes with humic substances used in the calculations. The peculiarities of complexation of oxoanions (Mo<sup>2-</sup><sub>4</sub> and HVO<sup>2-</sup><sub>4</sub>) with humic substances are considered separately.

Changes in the content of cortisol, triiodothyronine, thyroxine and glucose, the activity of AST and ALT and their ratio in the blood plasma of perch, roach, rudd and silver crucian carp from the Kiliya delta of the Danube River were studied in summertime. Hormone content was determined by enzyme immunoassay, and enzyme activity and glucose content - by standard methods. It has been shown that studies of the biochemical state of various ecological groups of fish make it possible to assess the quality of both surface and near-bottom water layers of the entire water body. The deteriorations of surface water quality and stressful circumstances result in an increase in the content of cortisol and triiodothyronine in the blood plasma of rudd. Under deteriorated environmental conditions, in fish from the near-bottom layers of water (roach and perch), partial liver damage is observed according to the activity of AST and ALT, and the De Ritis coefficient. Basically, all studied species actively use energy resources (glucose) to adapt to existing environmental conditions. For the vast majority of the studied indices, the highest coefficient of variation is observed in silver crucian carp, then rudd, and significantly less in roach and perch.

The present study assesses the water quality of the Man stream, a significant tributary of the Beas River in Shivalik Himalayas, Himachal Pradesh, by analyzing environmental variables, Water Quality Index (WQI) and fish species distribution using the Geographical Information System (GIS). Water samples were collected from two sites (upstream and downstream), over a year and various physiochemical parameters were analyzed. The study utilized the Canadian Council of Ministers of Environment - Water Quality Index (CCME-WQI) to evaluate water quality, depicting "fair" to "marginal" water quality status. Pearson’s correlation coefficient was used to assess the correlation between physiochemical parameters. A total of 23 fish species belonging to 6 orders and 9 families was reported, with Cypriniformes being the dominant order. Diversity indices revealed seasonal variations in fish communities, with the highest dominance index during monsoon and the Simpson and Shannon indices lowest during monsoon and nearly the same during pre-monsoon and post-monsoon with insignificant fluctuations. This comprehensive assessment provides valuable insights for fishery management and sustainable development in the region.

Monitoring of the genetic structure of the crucian carp <i>Carassius</i> settlements in the control water bodies over time, as well as experimental crossbreeding of species and biotypes of crucian carps made it possible to determine the main reasons for extinction of the unisexual populations of <i>Carassius gibelio </i>in the water bodies of Ukraine. The mechanisms of gynogenesis were thoroughly investigated, and experiments on the genome incorporation over gynogenesis were carried out. Special attention was paid to the study of deviations from the classic clonal reproduction and the hybrid posterity forming. The histological studies were carried out of the breeders' gonads in view of hermaphroditism and a series of experiments to confirm or refute the presence of the hermaphroditic individuals in populations. Conclusions were drawn regarding the probable autonomous existence of unisexual polyploid populations of crucian carp.