Meander reshaping – the formation process of a wintering riverbed depression of fish

The study of the spatiotemporal distribution of fish is an important and poorly studied aspect of the ecology of aquatic organisms. The research work was performed using the modern hydroacoustic method and geographic information systems. A section in the lower reaches of the Irtysh, a large transboundary Siberian river (in Western Siberia, Russian Federation), was studied. It has a strong development of meandering. The merging (i.e., reshaping of closely spaced meanders and erosion-accumulating channel processes) results in development of wintering riverbed depression, which is a critical “temporal bottleneck” during the winter period of the fish life cycle. The average density of fish in the study area in summer and autumn was 8,031 and 9,194 individuals per ha, respectively. Analysis of the distribution of fish showed that the distribution in the horizontal aspect had a more aggregated character in the autumn. In the vertical aspect, it had a more surface (pelagic) character. The ichthyofauna in the water area of the riverbed depression is mainly represented by cyprinids. The loop-shaped evolution of the channel formed a section of the river with multidirectional, circulating, and counter-current flows. It also created zones with depths exceeding 20 m and aggregations of fish. These features characterize the studied water area as a wintering riverbed depression of the fish of the Lower Irtysh. This section of the river should be included in the list of protected wintering biotopes of fish in the West Siberian fishery basin, which will ensure the conservation of fish at the critical stage of the life cycle.

DIEL AND INTER-SEASONAL DYNAMICS OF SPATIAL DISTRIBUTION OF FISH IN WATER AREA OF WINTERING RIVERBED DEPRESSION

To study the dynamics of the spatial distribution of fish under ice cover the research was carried out in the water area of the riverbed depression located in the lower reaches of the Irtysh River (Western Siberia, Tyumen region). The research was conducted by using the hydroacoustic method based on a computerized hydroacoustic software-hardware complex. It was found that, in the diurnal aspect, throughout the entire observation period (December - April), the number of fish in the riverbed increases during the dark period of the day, and during the light period it decreases. The dominant groups of fish change with the change of the period of the day: December - cyprinids prevail in the dark period of the day, in March and April - in the light period of the day, the rest of the time the group of percids predominated. With an increase in water temperature from December to April from 3.7 to 6.5 °C, it was noted that the density of fish decreases from 184 to 8 sp/hect. in the daytime and from 2157 to 91 sp/hect. at night (dark period), respectively. Fish explore the horizons of the water column of the riverbed depression during the day and night from the surface to the bottom with the highest density indicators near the surface, except - the daytime in December. With increasing density of fish at nightfall at the same time, there was an increase used water area of riverbed depression. In the vertical aspect, cyprinids avoid predators (pike, in-connu, burbot) in the diurnal dynamics; the distribution of cyprinids has a significant negative cor-relation with the distribution of the second group of fish. Thus, the features of the vertical and hori-zontal distribution of fish in the diurnal dynamics throughout the entire observation period are the strategy of their survival: for prey - escape from predators, for predators - search for food objects

LOCALITY OF FORMING WINTERING RIVERBED DEPRESSION IN IRTYSH RIVER

The article describes a new locality of the formed wintering riverbed depression discovered in the erosion zone of the isthmus of conjugate meanders of the Irtysh river for the rational use and effective protection of aquatic biological resources in places of their concentration in the Irtysh river basin (Western Siberia, the Russian Federation. The morphology of the riverbed depression was mapped (relief and bathymetric maps). The maximum depths in the channel depression exceeded 27 m. The channel depression and 4 control river sections were studied. The analysis of hydroacoustic surveys showed that in the wintering riverbed depression there is, according to the statistical data, higher fish density compared to the control areas: in the water area of the riverbed depression the average fish density made 4 524 species/ha, in the control sections 245-2091 sp./ha. The cluster analysis based on the fish density and the size-taxonomic composition of the ichthyofauna has revealed that the riverbed depression is being separated into a cluster, but at the same time goes to the control section No. 4, which is located downstream, where further erosion of the river channel is registered. In this regard, the minimum protection zone of the wintering channel depression should include the water area of the channel depression and the water area of control section No. 4. In the water areas of the riverbed depression and control sections in the spring period, cyprinids dominate - up to 65% of the total number of fish, percids are represented to a lesser extent - 36.76%, coregonids and pikes - up to 18.18, sturgeons - up to 9.31%. The given locality of the wintering riverbed depression must be included in the list of protected river sections of the Fishing Rules for the West Siberian fishery basin for developing the effective strategy for the protection, restoration and rational use of aquatic biological resources, including rare and endangered fish species of the Irtysh River and the whole Ob-Irtysh basin

Distribution of sturgeon in the River Irtysh

Using the modern hydroacoustic methods and the computerized software and hardware complex “AsCor”, the features of the distribution of fish of the Acipenseridae family in the lower reaches of the large transboundary river the Irtysh (Western Siberia) have been investigated: in the water area of the riverbed depression (turbulent flow) and control sections (laminar flow). The identification of sturgeon was carried out by the echometric method according to the shape of the swim bladder. It is shown that in the water area of the riverbed depression, increased density of fish is formed, the share of sturgeons being 4.9–5.8%. Among other fish species, the group of cyprinids always dominates. The significant excess of the average fish density in the riverbed depression, in contrast to the control, was 6–30 times. In the water area of the riverbed depression, the average fish density was 4,524, in the control sections 245–2,091 individuals/ha. The size structure of the group of sturgeon in the control sections was represented by individuals with body sizes less than 25–30 cm, and in the riverbed depression – by individuals of different sizes and ages with a body length to 35 cm. The water area of the riverbed depression is located in the meandering section of the river and is characterized by intense vertical vortex structures (whirlpools) and the presence of malfunctioning currents. Features of the hydrological characteristics of the riverbed depression are factors in the formation of aggregations of fish, since fish can use the energy of discrete vortices, low-velocity areas, and as a result, show preference for certain turbulence regimes.

Fish distribution in riverbed depression and conjugated reservoirs (effects of lighting and turbulence)

The paper describes the studies carried out by the hydroacoustic method in the open water period (spring) in the water area of the riverbed depression of the Irtysh River and its conju-gated water bodies (Western Siberia, the Russian Federation) in 2017. Collectively investigated reservoirs form a laminar-turbulent system, as they vary in intensity and direction of currents which form a certain turbulence and turbidity. It has been stated that in the dark time the fish population density increases in the waters of the reservoir with the maximal turbulence and turbidity rates - 166 NTU (the Irtysh river), and decreases in other water bodies (in the Konda river - 20.75 NTU and in the lentic section of the riverbed depression - 25 NTU). In the waters of conjugate reservoirs an increasing share of cyprinids is accompanying by a decrease in the proportion of predatory fish, while with an increase in predatory fish, on the contrary, the proportion of cyprinids decreases. A significant difference (P < 0.001) in terms of fish density was noted in the dark time. It was found that of the two factors (time of the day and turbulence) the second factor has a significant reliable effect (F = 18.518; P < 0.001) on the variable of fish population average density, both factors reliably interacting (F = 3.982; P = 0.037). There has been shown a transition of non-predatory fish to the site that is visually and hydrodynamically the most difficult for finding prey, which seems to be an element of reducing the risk of predation. Thus, there was found a significant influence of an abiotic factor of turbulence and, consequently, arising turbidity when interacting with the illumination factor on forming the increased fish concentration in the waters of the riverbed depression during the open water period.

Daily dynamics of vertical distribution and amount of sturgeons in riverbed depression in summer-autumn period

The article presents the results of studying the fish distribution in the water column of the riverbed depression using remote echometric sounding with a software-hardware acoustic system with a vertical view. The study was conducted in the lower reaches of the Irtysh River. In the appointed period (July-October) under a decrease in the level regime and water temperature the increasing density of fish in the water area of the riverbed depression is explained by fish gathering in the period preceding wintering. The share of sturgeons varied from 5.46 to 10.28% of the total fish density, the indicators of which were in the range 1.70-5.05 thousand sp/ha in the daytime, 2.78-6.77 thousand sp/ha in the night. There has been stated the daily vertical migration of sturgeons, which is more uniform throughout the water column in the night, including the near-surface and near-bottom water horizons. In the dark near-surface water horizons are most intensively explored by large fish (25-30, 30-35, > 35 cm). The trigger of the daily vertical migration of sturgeons is supposed to be the light brightness, despite the poor eyesight of the fish under study, i.e. changing vertical position of sturgeons occurs due to endogenous circadian rhythms, as in many hydrobionts. The recorded features of sturgeon distribution because of preference of the direction and intensity of various factors can contribute to the optimization of bioenergy losses in hard turbulent conditions of riverbed depressions.

Effects of temperature and water levels on dynamics of density and structure of the fish population of the channel-floodplain complex of a large river in the period of spring floods

The article discusses the features of the dynamic process of migration of ichthyofauna from wintering sites to the wetlands of a large boreal river for feeding and spawning. The influence of factors of temperature and water level on various groups of fish is estimated, and the role of water bodies of various types during fish migration in a flooded floodplain is also established. The research was performed during the spring flood in the floodplain-channel complex of the Irtysh River (Western Siberia, Russian Federation) in the water area of the conjugate water bodies: the lotic and lentic parts of the riverbed depression and the mouth of the tributary – transit section of the migration pathway of fish to the flooded floodplain. The research work was performed by “AsCor” – computerized hydroacoustic complex, hydroacoustic survey data were processed in the laboratory. The “AsCor” complex remotely performs taxonomic identification of abundant species of fish at the family level by groups (Cyprinidae, Percidae, Coregonidae–Esocidae, Acipenseridae–Lotidae). It was found that after the breakup of ice cover, maximum densities of fish were observed in the wintering sections – the initial location of the spring migration. During the 30-days observation period in the wintering sections and conjugate water bodies, the density of fish decreased from 4 to 13 times due to their migration to the floodplain. The response to a decrease in the overall density of fish in the year-round functioning lotic part of the riverbed depression to the influence of factors of water level and temperature regimes was >0.300 in general for ichthyofauna and for all groups of fish registered by hydroacoustic method (Cyprinidae, Percidae, Coregonidae–Esocidae, Acipenseridae–Lotidae). We constructed regression models of changes in the density of fish in the wintering sections of the river depending on the considered factors. Use of ANOVA analysis of variance confirmed the adequacy of the choice and the acceptability of the constructed models. We established a high statistically significant inverse correlation between the density of the fish, the water level and temperature regime for the mouth of the Konda River and the lentic part of the riverbed depression, which are transit sections of the migration pathway of fish in the floodplain. The lentic part of the riverbed depression may perform the function of a temporary station of fish during migration to the floodplain, which is beneficial from the point of view of bioenergetics resources. During the observation period, in the river flows there was a decrease in the proportion of larger fish and an increase in smaller individuals; in the lentic part of the riverbed depression, on the contrary, there was a reduced proportion of small fish and increase in the proportion of larger individuals.This pattern is explained by the reduced risk of predation by small fish in more turbulent conditions, as well as by an initial entry into the flooded floodplain of larger individuals of fish for spawning, followed by the smaller ones for feeding.

COMPARATIVE ANALYSIS OF MORPHOMETRIC INDICATORS OF STERLET IN WINTERING HOLES OF THE LOWER IRTYSH

The article describes the results of morphometric studies of sterlet caught in water areas of two riverbed depressions of the Lower Irtysh (the Tyumen region) The studies were carried out in the period from 2015-2018, the riverbed depressions were remote from each other: Gornoslinkinskaya (the Uvat region), Kondinskaya (the Khanty-Mansiysk region). The geographic distance (the distance between the riverbed depressions is 445 km) and short migration length (about 200 km) prompted the assumption about the probability of significant discrepancies in the signs that suggest the allocation of any subgroups of sterlet. In the catches of sterlet in the Kondinskaya riverbed depression there were 93 specimens aged 1-4 years old. In the water area of the Gornoslinkinskaya riverbed depression 128 specimens were caught at the age of 1-7 years old. The comparison of sterlet groups from remote riverbed depressions was conducted using meristic signs: the number of bone plates (scutes) rays in the fins, the number of stamens on the first gill arch. According to the research results, it has been stated that there are no significant differences in the number of dorsal, lateral and ventral bone plates. Also, no differences were found in the number of stamens on the first gill arch. The greatest differences in the studied groups of sterlet were recorded in terms of the number of rays in the dorsal and anal fins. Additionally, the meristic features have been estimated by Mayr coefficient (1971), the value of which is equal to or greater than 1.28, which suggests the subspecific division. In our study, Mayr coefficient in all cases was less than 1.28, its maximum value (0.38) was established in studying the number of stamens on the 1-st gill arch. As for the remaining signs, this value is even smaller. Thus, geographical distance (over 400 km) of sterlet groups from the 2 riverbed depressions of the Lower Irtysh should not be taken as a reason to single out any subgroup.

VORTEX STRUCTURES (EDDIES) IN THE RIVERBED DEPRESSION AND FISH DISTRIBUTION

The article highlights the studies carried out in summer using the hydroacoustic method from the small boat with control fishing in the lower reaches of the Irtysh in the Gornoslinkinskaya riverbed depression. The method of research is based on using computerized hydroacoustic software and hardware complex “PanCor” (Promgidroakustika, LLC). Hydroacoustic records were processed in the laboratory. The features of fish distribution in a turbulent flow of the riverbed depression at the river bend have been recorded. Significant accumulations of fish in the areas of vertical vortex structures - eddies have been registered. Eddies are registered both visually and hydroacoustically. The recorded vortex structures are formed as a result of the hydrological features of the channel of the river studied section. There has been established a significant ( p < 0.05) high direct correlation (> 0.70) between the number of fish species from all taxonomic groups (Cyprinidae, Percidae, Coregonidae, Esocidae, Lotidae, Acipenseridae) and the height of the water column in vortex structures. The number of fish in the vortex zones reliably exceeds that number outside these zones; for Cyprinidae by 2.61times, Percidae - 1.47, Coregonidae-Esocidae - 2.80, Acipenseridae-Lotidae - 2.50. It has been stated that in the zone of eddies the percentage of small individuals and medium size fish (<20 cm) exceed the percentage outside the vortex structures. For larger fish groups (> 20 cm), on the contrary, the percentage is larger outside the vortex zones. The accumulation of fish in the whirlpool part of the riverbed depression occurs in the result of choosing the necessary habitat conditions that have a predictability component, as well as due to temporary drawing and holding the fish in the vortex zone.