Downstream migration (DSM) of young fish is an efficient dispersal method in rivers. Regulation of rivers transforms them into lotic–limnetic cascades fragmented by dams. Novel conditions of transformed riverine habitats influence the routes and behavior of migrants, creating hazardous ecological situations, which lead to enhanced mortality and reduced fitness of fish. To mitigate these effects and develop efficient management programs, the gaps in our knowledge on the ecology of fishes in transformed rivers should be filled. Most studies on the ecological effects of river regulation on DSM are focused on relatively small areas adjacent to dams. We believe that upstream sections of reservoirs also significantly impact the survival of juvenile fish and the intensity of their transportation to dams and water intakes; thus, these sections deserve special consideration. This review addresses ecological effects of river regulations, focusing on the heterogeneity of habitats as one of the key factors controlling DSM throughout the novel system of the human-made reservoir. Our analysis shows that habitat heterogeneity influences foraging, defense, and migratory behavior at different spatial scales. In reservoirs, the topography of the whole reservoir (macroscale), stretched hydraulic gradients between migratory and residence habitats (mesoscale), and local shelters and landmarks (microscale) may hamper DSM. Homogeneous open waters are less suitable for feeding, communication, defense, and other behaviors of migrants than heterogeneous inshore habitats. We distinguish three main ecologically distinct zones within reservoirs and describe their specific modifications of DSM. Fish from different taxonomical and ecological groups, e.g., Cyprinidae and Percidae, were found to respond differently to newly emerged structures and modified habitat heterogeneity. When creating fish-friendly migration routes and developing conservation measures for fish populations in regulated rivers, not only transformed habitats but also the characteristics of different ecological groups of fish should be taken into account.

Downstream migration (DSM) of young fish is an efficient way of their dispersal in rivers. Regulation of rivers transform them into lotic-limnetic cascades fragmented by dams. Novel conditions of transformed riverine habitats influence migration routes and behavior of migrants, creating hazardous ecological situations, which lead to enhanced mortality and reduced fitness of fish. The review is aimed at ecological effects of river regulations on DSM, taking into account not only entrainment, damage and mortality of migrants in the water abstraction systems at the dam, but also modifications of DSM in the whole reservoir and within its sections. Habitat heterogeneity/complexity is one of the key factors controlling DSM both in natural and regulated rivers. It influences foraging, defense, migratory behavior at different spatial scales. In reservoirs, topography of the whole reservoir, stretched hydraulic gradients between migratory and residence habitats, widen inshore areas rich of shelters and landmarks provide higher retaining capacities for migrating fish. Homogeneous open waters are less suitable for feeding, communication, defense, and other behaviors of migrants than heterogeneous inshore habitats. Three ecologically distinct zones were recognized in the reservoir, where specific modifications of DSM were described. Fish from different taxonomical and ecological groups, e.g. Cyprinidae and Percidae, respond differently to the newly emerged structures and modified habitat heterogeneity. When creating fish-friendly migration routes and developing conservation measures, we have to take into account not only transformed habitats, but also characteristics of different ecological groups of fish.

The size, age, and sex structures of pikeperch Sander (L.) from Lake Necheritsa (Sebezhsky National Park) have been studied, as has the linear growth using back-calculation according to the Leo direct dependence function. The growth of pikeperch is described formally by linear equation L(t) = 5.4x + 0.2 (cm). The length frequencies of individual age groups are normally distributed, which corresponds to ideas about linear fish growth. The population is represented by 13 age classes; the maximum length of an individual in the sample is 872 mm. The analysis of the diet showed the presence of 6 mass fish species observed in the catch, including the young pikeperch; the ratios of individual species in the diet and the sample indicates an absence of electivity of nutrition. According to the estimates, the proportion of pikeperch in the ichthyocenosis in the period 1989–2022 in Lake Necheritsa has increased ∼8 times. A comparative analysis of the linear growth of pikeperch from populations of different regions under environmental conditions, differing in a number of values of key parameters, is carried out. Growth rates of pikeperch from southern populations are generally higher and more consistent with asymptotic growth, while the size limits and life expectancy are generally higher in northern populations. A hypothesis has been tested about the causes of differences in the nature of linear growth.

В экспериментальной кольцевой гидродинамической установке определены индивидуальные параметры перемещения у данио Danio rerio и серебряного карася Carassius gibelio при их голодании в течение 12 сут. Выявлены две тактики поведенческих ответов рыб на голодание. Тактика однонаправленных ответов проявлялась в движении особей в одном направлении относительно течения на 2–5-е сут голодания. Тактика разнонаправленных ответов проявлялась в образовании на 10–12-е сут голодания групп рыб, перемещающихся в разных направлениях относительно течения. Рассмотрены популяционные преимущества таких поведенческих тактик ответа рыб на неблагоприятный фактор.

The results of the study of local populations of the resident form of the European river lamprey Lampetra fluviatilis in medium and small watercourses (tributaries of the Pola and Msta rivers), located at a distance of 700 km from the sea, in the native part of the species range, are presented. Such populations are still poorly studied, despite the fact that they ensure the preservation of the range under conditions of fragmentation of river system. The captured adults are typical representatives of the resident form, with a slight variation in the dentition. The population density of lamprey larvae in most of the studied habitats is low and average in a small part of them. The habitat conditions of ammocoetes are determined to be optimal, however, due to the peculiarities of small watercourses, their habitats are vulnerable to the negative effects of environmental factors.

This paper describes typical habitats of European river lamprey (Lampetra fluviatilis (L., 1758)) larvae in the rivers of Leningrad Oblast. The density of ammocoetes and abiotic components of biotopes, as well as the structure of benthic and planktonic algae communities, zooplankton, and macrozoobenthos, are estimated. It is shown that the preferred grounds for larvae are sands <0.25 mm. Oligochaeta and Chironomidae larvae reach high levels of quantitative development together with ammocoetes. Algae and zooplankton communities are not decisive for lamprey larvae and have expressed seasonal patterns with peaks of abundance in the spring. The part of lamprey larvae in the abundance of benthic coenoses can reach significant values—the part in the total biomass of macrozoobenthos is 30–80

Individual movement parameters for the zebrafish Danio rerio and the Prussian carp Carassius gibelio during starvation for 12 days have been determined in an experimental hydrodynamic installation circular tank. The fishes represent two tactics of behavioral responses on starvation. The tactics of unidirectional responses have been manifested in the movement of individuals in one direction relative to the water flow on days 2–5 of starvation. The tactics of multidirectional responses have been manifested on days 10–12 of starvation as movements in different directions relative to the water flow. The population advantages of such behavioral tactics of fish response to an unfavorable factor are considered.

Signs of early maturation have been revealed in 50

Fish distribution in different areas of Bratsk Reservoir has been studied using hydroacoustic equipment. The average abundance of fish decreases (r = –0.70; p < 0.05; R2 = 0.49) from the near-dam reach (214 ± 34 ind./ha) to the water area in the upper reaches of the reservoir (113 ± 43 ind./ha). A similar relationship is observed in the dynamics of the abundance of early juveniles and the immature fish population. However, such relationship has not been found in fish with a size of more than >100 mm, which with a low abundance are relatively evenly distributed along the longitudinal axis of the reservoir (10.5 ± 6.6 ind./ha). The common perch (Perca fluviatilis Linnaeus, 1758) is the most abundant species in the reservoir, its highest concentrations are recorded in the lower lacustrine part of the reservoir (139 ± 22 ind./ha). Less abundant cyprinids (Cyprinidae) (41 ± 15 ind./ha) and whitefishes (Salmonidae, Coregoninae) (12 ± 4 ind./ha) are more evenly distributed along the longitudinal axis of the reservoir. Temperature stratification of the water column is an important factor in the ecological differentiation of the fish population in the reservoir. The major part of the ichthyomass consisting of relatively warm-water fish species from the families Percidae and Cyprinidae (common roach (Rutilus rutilus (Linnaeus, 1758) and common bream (Abramis brama (Linnaeus, 1758)) is concentrated in the warming up epilimnion. The fish population in the cold hypolimnion consists of single individuals of the Baikal omul (Coregonus autumnalis (Georgi, 1775)) and peled (C. peled (Gmelin, 1789)). Transverse distribution of fish depends on the presence of a runoff flow. At relatively high flow rates, fish begin to avoid the midstream part in the studied sites of the reservoir and concentrate in the floodplain areas (R2 = 0.54; p < 0.0001).

Long-term influence (53 days) of long-term constant illumination (100 and 1000 lux illuminance) on the behavior of rainbow trout (Oncorhynchus mykiss) juveniles and the level of thyroid hormones and cortisol in their blood has been experimentally recorded. At 100 lux illuminance, the fish moved equally upstream and downstream; i.e., their behavior was to stay in the current habitat. At 1000 lux illuminance, the fish more often moved upstream; i.e., their behavior was to leave the current habitat. The experimental illumination regimes did not affect the thyroid hormones and cortisol levels in the fish blood.

The genetic variability of microsatellite DNA has been studied for the first time for five loci in the common bream Abramis brama (L.) from lakes Sebezhskoye and Necheritsa of the Sebezhsky National Park lake system (Pskov oblast). The results demonstrate the uniformity of estimates of the genetic variability of the bream in the lake system. Estimates of the allelic diversity of microsatellite loci and the expected heterozygosity of local bream groups do not differ significantly. The total genetic differentiation of bream is θ = 0.004, 95

Objective: to characterize age-length composition of fish population in the lake Ozeryavki at National Park «Sebezhsky» and to estimate the density and biomass of main species using A method for estimating fish density through the catches of gillnets Methods: a method for estimating density by catches of gill nets applied to the catches of different mesh size. Methods: a method for estimating density by catches of gill nets applied to the current data. Novelty: estimates of mortality of 3 main species, roach, white bream and perch, in one of the lakes of the national park were obtained for the first time; the density and biomass of 3 species are calculated and the proportion of mortality from predation is estimated. Results : instantaneous natural mortality coefficients for roach, white bream and perch corresponds to Z = 0.59 year-1, Z = 0.51 year -1 and Z = 0.71 year -1 , respectively; the general density of all age groups is: for roach 2.7 ind./m 3 , for white bream 0.9 ind./m 3 and for perch 2.0 ind./m 3 ; average biomass: for roach 14.3 g/m 3 , for white bream 4.5 g/m 3 , for perch 3.0 g/m 3 ; mortality rate from pike predation accounts for 0.34 of biomass of three species. Conclusion. The study of fish population of lake Ozeryavki as a typical medium-sized reservoir in the Sebezhsky National Park gives an idea of local ichthyocenoses parameters and aquatic ecosystem condition as a whole.

Despite being innate, schooling behavior is subject to the influence of external factors that change the condition of fish or the conditions of their environment and reception. Of the abiotic factors, the strongest influence is exerted by the illumination and the water turbidity, the abundance of visual landmarks, and the flow velocity. The effect of temperature and the gas composition of water (oxygen content) is much less pronounced; only the first information is available about the effect of water salinity. Among biotic factors, schooling behavior is significantly influenced by various social conditions (fish stocking density, social deprivation), feeding and defensive motivations of fish (fish gut fullness, presence of predators), and parasite infestation. The ability of fish to exhibit schooling behavior is reduced in the presence of virtually all pollutants studied.

Individual concentrations of dopamine, norepinephrine, epinephrine, free triiodothyronine, total protein, albumin, as well as alanine aminotransferase activity were measured in embryos and larvae of the Atlantic salmon Salmo salar . Two groups of embryos and larvae differed by biochemical parameters were revealed. The relationship between biochemical differentiation and migratory polymorphism in the Atlantic salmon during the period of primary juvenile dispersal is discussed.

Data on the role of brain regions (forebrain, midbrain and other structures) in the control of schooling behavior of fish has been systematized. Data have been presented on the influence of the presence in the food of certain substances (docosahexaenoic acid) accumulating in the brain on the rate of formation of schooling behavior in fish ontogeny. The neurohormonal system may be involved in the regulation of schooling behavior. The individual behavior of fish in a school depends on the lateralization of brain functions. Attention has been drawn to the extremely poor knowledge of the processes of central and hormonal regulation of schooling behavior of fish. Vision is the leading, and most often the only sensory system that enables fish to demonstare schooling behavior. Monomodality distinguishes schooling behavior from other complex forms of fish behavior. Visual deprivation or deterioration of the conditions for visual reception makes schooling reactions of fish difficult or completely impossible. Existing assumptions about the possible participation in schooling behavior of other sensory systems of fish—lateral line, hearing, olfaction, electroreception—have been critically analyzed. Strict evidence of the real involvement of these sensory systems in the mediation of schooling contacts in fish is still lacking.

In most fish species, the transition to a schooling lifestyle occurs soon after the start of exogenous feeding. The age at which juveniles begin to show schooling behavior differs in different species and is not related to the level of schooling in adults, i.e., their belonging to obligate or facultative schooling fish. In marine fish passing through the metamorphosis phase, schooling develops, as a rule, after the completion of this process. Among freshwater fish, juveniles living in rivers begin to show schooling behavior earlier than juveniles in stagnant waters. By the time a school is formed, the level of development of sensory systems and locomotion in juveniles is sufficient for intra-school contacts and schooling swimming. As juveniles grow, the coordination of schooling reactions and the ability to maintain the unity of a school during maneuvering increase. Compared to adult fish, schools of juveniles are less uniform in size and species composition.

The content of thyroid and sex steroid hormones in the blood of immature and precocious trout Salmo trutta at the final stage of the formation of early maturing fishes (age 1+, 2+) in the population was determined. The formation period of early maturing males varies in different years in Alatsoya River (Karelia). Fish can reach sexual maturity at the age of 1+ or 2+. Immature and precocious trout at the age of 1+ and 2+ do not differ in the free and total triiodothyronine and free thyroxine content. Immature females and males aged 1+ and 2+ also do not differ in the level of sex steroid hormones. In contrast to immature brown trout, precocious males aged 2+ are characterized by an increased content of testosterone and a reduced level of estradiol-17β in the blood. It has been established that the final stage of the formation of early maturing fishes in the population is characterized by a weak involvement of the thyroid gland in the maturation process of males. Also, it was shown that the rate of conversion of testosterone to estradiol-17β in their blood was a significant decrease (4 times). This transformation in all studied females and males of brown trout is associated with their body length. The rate of formation of estradiol-17β in fish was increased with increasing of body length.

Schooling as an ecological phenomenon first arose in the early Teleostei about 200–220 million years ago; there are currently no rigorous data on the presence of this form of social behavior in older fish, including Elasmobranchii. The evolutionary path of the emergence of schools could pass through several successive stages from asociality to aggregations and proto-schools and then to true equipotential schools. Schooling fish follow the same path in their early ontogeny. This article discusses the motives for the emergence of schooling behavior (ensuring the protection and survival of ancient fish during their development of the pelagic zone) and the prerequisites for the emergence and development of schooling (characteristics that fish transitioning to schooling should have). It is quite obvious that schooling is not related to evolutionary age or phylogenetic relationships of taxonomic groups or individual species of Teleostei. The transition to a schooling lifestyle is due to a change in the ecology of fish in connection with the development of new habitat conditions. Having arisen, schooling behavior disappears and again and repeatedly arises independently not only in different groups, but also among closely related Teleostei. Evolutionarily acquired schooling can be transformed into other forms of social behavior when personalization appears. Information about the genetic nature of schooling has been given.

The article opens a thematic issue of the Journal of Ichthyology , dedicated to Dmitry Viktorovich Radakov, a researcher who made an outstanding contribution to the knowledge of schooling behavior of fish. The article deals with many terms and concepts that are widely used in scientific publications on the schooling behavior of fish, but still do not have a generally accepted definition—crowd, aggregation, shoal, school, flock. Attention is drawn to the difficulties caused by the fact that the use of these terms is most often based not on objective criteria, but on the preferences, views or beliefs of individual researchers. General ideas about the prevalence of schooling behavior in fish of different taxonomy, lifestyle, condition and age; about the importance of the visual structuring of the environment for the manifestation of schooling, about the difficulty of dividing fish into facultative and obligate schooling, about the importance of knowledge about schooling behavior for solving applied problems have been considered.

Regularities of arrangement of fish in schools have been considered. In migrating isotropic schools, the internal structure is most ordered, the fish in them are oriented strictly parallel to each other, the partners are located at an equal distance and with a characteristic (rhomboid) displacement relative to each other in the horizontal plane. The location of fish in such schools is influenced by the physical forces of hydrodynamic vortices created by swimming fish (hydrodynamic wake). In isotropic schools, fish maintain a certain distance between themselves (linear distance) and a certain displacement (frontal, vertical and horizontal) relative to each other. The average density of schools varies greatly and depends on the size of fish, the swimming velocity or the flow of water to overcome, the level of illumination and other factors. In large schools, the average density of fish is higher in the center and decreases towards the periphery of a school. Fish in schools form intra-school subgroups of three to five individuals, within which fish are placed strictly in a horizontal plane, or with a slight vertical displacement. The mutual arrangement of fish in a subgroup is constantly changing, and the distance between partners is less than the distance to any individual of another subgroup. Existing data on individual spatial preferences of fish in schools has been analyzed.