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Article

Effects of Flow Velocity on the Dynamics of Juvenile Fish Habitats in River Meanders of the Irtysh River

by
Andrey A. Chemagin
1,*,
Elena I. Popova
1 and
Martin Schletterer
2,*
1
Tobolsk Complex Scientific Station of Ural Branch of the Russian Academy of Sciences (UrB RAS), Akademika Yuriya Osipova Street 15, 626150 Tobolsk, Russia
2
Institute of Hydrobiology and Aquatic Ecosystem Management, University of Natural Resources and Life Sciences, Vienna (BOKU University), Gregor-Mendel-Straße 33, 1180 Vienna, Austria
*
Authors to whom correspondence should be addressed.
Diversity 2025, 17(1), 68; https://doi.org/10.3390/d17010068
Submission received: 6 December 2024 / Revised: 10 January 2025 / Accepted: 16 January 2025 / Published: 19 January 2025
(This article belongs to the Special Issue Socioecology and Biodiversity Conservation—2nd Edition)
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Abstract

:
Understanding the spatial distribution of freshwater fish in heterogeneous aquatic environments is crucial for understanding riverine ecosystems and the rational use of aquatic biological resources. This study investigates the distribution patterns of juvenile fish in the lower reaches of the Irtysh River, including hydrodynamic conditions during different water level regimes. With hydroacoustic surveys, we assessed fish density and distribution in two wintering riverbed depressions during the spring flood and summer low water period. The main fish aggregations consisted of Cyprinidae and Percidae, with juveniles predominantly occupying areas with reduced flow velocities (0.15–0.75 m s−1). Correlation analysis showed strong direct relationships between the area occupied by juvenile carp and perch and zones with specific flow velocities. The study highlights that hydrodynamic characteristics, particularly flow velocity, are key factors influencing the distribution and aggregation of juvenile fish. These findings underscore the importance of considering hydrodynamic factors and species-specific traits in understanding fish distribution patterns and in managing freshwater ecosystems effectively. This research contributes to the understanding of the multifunctional roles of riverbed depressions in supporting juvenile fish populations and emphasizes the importance of hydroacoustics to predict fish distributions in dynamic aquatic environments.

1. Introduction

Studying patterns and determining factors influencing the spatial distribution of aquatic organisms in heterogeneous aquatic environments [1], including large rivers, is one of the main problems in ecology. Currently, the spatial distribution of freshwater fish is studied primarily on a large scale, while studies of local ecological factors and their dynamics are rare [1,2]. The distribution of organisms in the heterogeneous environment of a river is uneven [3] due to the selection of optimal strategies for using the habitat to increase individual survival [4,5,6].
The spatial distribution of freshwater fish is determined by various factors: physicochemical parameters [7,8,9], availability of resources and habitats [10,11,12], predator–prey interactions [13,14,15], intraspecific relationships, and individual habitat preferences [16].
Knowledge about spatial distribution patterns of fish is necessary for a full understanding of ecosystem functioning [1] and rational use of aquatic biological resources [17]. One of the optimal methods for observing the spatial distribution of fish in aquatic habitats are hydroacoustic surveys [2,18]. This method has proven effective for quantitative real-time observation of fish populations with high spatiotemporal resolution [19,20,21]. When used in conjunction with geographic information systems (GIS), it creates an ideal opportunity for mapping and modeling the spatial distribution of fish [1], including small pelagic ones [2]. At the same time, the variability of the river channel and its sinuosity [22] provide a wide variety of microhabitats to which fish evolutionarily adapt [23]. The swimming ability of fish is a key factor determining their survival in the natural environment and ability to make various movements: short and long-distance, ensuring dispersal, habitat selection, interaction with food objects and predators, as well as spawning migrations [24,25]. When the hydrological phases change, for example in the case of increased flow velocity characteristics, fish individuals have to change their swimming regime, exerting greater physical effort and intensifying metabolism for additional energy expenditure [26,27].
The aim of this study was to determine the distribution patterns of juvenile fish, considering hydrodynamics in the riverbed depression areas during different water level regimes (during flood and low water), specifically comparing high-velocity zones and juvenile fish concentration areas in a large river system.

2. Materials and Methods

2.1. Research Area

The Irtysh River is a left tributary of the Ob, originating on the southwestern slopes of the Mongolian Altai. Known as the Black Irtysh, it flows into Lake Zaysan, traversing China (525 km), Kazakhstan (1835 km), and Russia (2010 km), including Omsk Oblast (1132 km), Tyumen Oblast (639 km), and Khanty-Mansi Autonomous Okrug (239 km). The total length of the Irtysh from its source is 4370 km, making it the world’s longest tributary [28]. The mean discharge of Irtysh River at Tobolsk is 2140 m³ s−1 and the mean annual flow amounts 39.1 km³ a−1; at Khanty-Mansiysk (about 20 km upstream the mouth into Ob river) about 2800 m³ s−1 and 88.3 km³ a−1, respectively.
The Irtysh River channel in its lower reaches has a highly meandering character, with multidirectional currents and circulations [29]. The sinuosity of the channel and features of the riverbed relief create heterogeneity in the environment’s hydrodynamic characteristics [29]. At sharp bends (meanders), intense soil erosion processes can be observed, leading to the formation of riverbed depressions (“wintering riverbed depressions”). In these wintering holes, which serve as “temporarily limited biotopes”, valuable fish species concentrate during the winter period, including sterlet (Acipenser ruthenus Linnaeus), Siberian sturgeon (Acipenser baerii Brandt) and nelma (Stenodus leucichthys nelma Pallas). However, it has been shown that fish aggregations in such areas are recorded year-round, allowing the biological role of riverbed depressions to be defined as multifunctional—serving as rearing areas for juveniles, feeding grounds, and wintering sites for fish [30].
The survey was conducted in two wintering riverbed depressions, located in the lower reaches of the Irtysh River (Tyumen Oblast, Western Siberia). Observations were made during the active phase of the spring–summer flood on 30 May 2022, and the summer–autumn low water period on 31 July 2022.
The Gornolsinkinskaya wintering riverbed depression, formed by circulatory currents on a sharply curved meander, is located at 58.731950° N, 68.698582° E (Figure 1). The study area covered 41 hectares, with depths reaching over 40 m (Figure 1c).
The Ukinskaya wintering riverbed depression was formed as a result of erosion and the channel processes of an omega-shaped meander of the Irtysh River, located at 58.857111° N, 68.742535° E (Figure 1). The area of the studied site was 75.9 hectares, with depths reaching more than 20 m (Figure 1c). This river section with fish aggregations was recently identified [31] and a justification for its inclusion in the official list of wintering riverbed depressions of the Irtysh River was made, with the aim of preserving aquatic biological resources by fish protection authorities and preventing illegal fishing, primarily of sturgeon and whitefish.

2.2. Field Work and Analyses

The hydroacoustic survey for the assessment of fish density and distribution in the wintering riverbed depressions was carried out with the computerized hydroacoustic system “AsCor” with a vertical view (LLC «Promgidroakustika», Petrozavodsk, Russia) (Figure 2). The system’s operation is based on the use of a serial Furuno LS 4100 echosounder (Furuno Inc., Ashihara-cho, Nishinomiya City, Hyogo, Japan), with operating frequencies of 50 and 200 kHz, with viewing angles of 46° and 14°, respectively, [32]. During the survey, recordings of fish distribution and tracks, along with navigation tracking data from an external satellite navigation receiver, were stored on the field tablet iX103 (Xplore Technologies, Austin, TX, USA). This system is used frequently in inland freshwater bodies [33].
The hydroacoustic survey recording is processed frame by frame using a special software application «AsCor Expedition, 2014», with 1 frame consisting of 100 signal transmission-receptions. The processing program algorithm assists in determining fish body length using an equation used for mass fish species of the Lower Irtysh [19], as well as classifying registered fish into 4 groups of mass fish species of the Lower Irtysh based on the shape of the reflected echo signal: cyprinids (two-chambered swim bladder), percids (asymmetric swim bladder), pike and whitefish (symmetric swim bladder), unidentifiable (sturgeons and burbot, lacking a distinct swim bladder shape).
During the recording of the hydroacoustic survey file, the envelope curve of the echo signal with filtered noise is continuously analyzed. Based on the measured pulses, single and multiple targets are identified. This allows for the simultaneous use of echo counting, i.e., counting single targets, as well as echo integration, i.e., measuring the strength of the target for the entire aggregation and determining its density. This process accounts for the number, density (individuals/ha−1), and size of fish (target strength) in situ.
To conduct the hydroacoustic survey in the studied river sections, movement was carried out according to established methods [34] along a grid of zigzag tacks (Figure 3). Before conducting the echo sounding survey, the antenna is calibrated using a standard copper sphere with D = 45 mm and TS = −39 dB [21].
To conduct the hydrometric survey in the studied areas of riverbed depressions, flow velocity measurements were taken using an ISP-M flow velocity meter (LLC «Gidrometeopribor» St. Petersburg, Russia) at points of predetermined cross-sections using the Google Earth Pro 6.3 application, with distances between points of 80–100 m. The flow velocity meter consists of a measuring part, a micro-propeller (120 mm diameter) with a horizontal axis of rotation, and a signal converter of the propeller, which translates the rotation frequency of the propeller into flow speed.
The marked points were transferred to a Garmin 62Stc navigator (Garmin Ltd., Schaffhausen, Switzerland), which was used to navigate to given points and control the location. To move from point to point, the ISP-M was put in transport position, and when the small craft was positioned at the specified points on course, flow velocity measurements were taken at a depth of 2 m. This depth corresponds to the zone where beams from the hydroacoustic complex with vertical echo sounding are deployed. The hydrometric propeller (ISP-M) on a metal rod was preliminarily installed in a rotary-extendable device located in the bow of the small craft before measurements (Figure 4). To avoid errors in flow velocity measurement, the absence of small craft drift was controlled using a GPS navigator and visually, relative to the shores. Flow velocity measurement at one point was performed for at least 3 min. The measured flow velocities stored in the device’s memory were then exported as a TXT file to a computer via USB cable.
Horizontal projections of the distribution of velocity zones and juvenile fish in the studied water areas were performed using Surfer 9.0 software (Golden Software, Golden, CO, USA), with the geostatistical interpolation Kriging method [33,35], widely used in fishery and ecological research [36,37,38]. Preliminary calibration of the maps of the studied water areas was performed in the MapViewer 10.0 application, using imported satellite images of the area from Google Earth Pro 6.3. Analysis of the correlation (Pearson correlation coefficient) between fish distribution and zones of different velocity areas was performed using Statistica 10.0 software (StatSoft Inc., Tulsa, OK, USA). The normality of the data distribution was preliminarily determined (Kolmogorov–Smirnov and Shapiro–Wilk tests).
Data on flow speed zones and fish habitat areas were collected simultaneously for two riverbed depressions. For example, the values of habitat area use by Cyprinidae were compared (VS) with the values of zones for each class of flow speed.
We consider the distribution of fish over the entire water area of the riverbed—coastal zones and midstream. The midstream or thalweg are the fastest and deepest parts of the riverbed. The coastal zone of a river is the section along the coastline (close to the littoral), with flow velocities close to “0”.
To control the species composition of the fish population, control fishing was conducted using a small-mesh (3–5 mm) net (“spider net”, Supplementary S3).
Fishing was carried out to control the species composition of the fish population. Control fishing was conducted on 28 May 2022, 29 May 2022 (flood), 29 July 2022, and 30 July 2022 (low water). Fishing was carried out near the shores at a distance of up to 50 m, taking into account the possibility of anchoring a small craft. For one control fishing effort, the fishing gear was lowered into the water to a depth of up to 3 m, and the fish were allowed to settle for 30 min before the “spider-net” (dip net) was quickly lifted. A total of six catches were made on each side (left bank, right bank). During the flood as well as during low water periods, 12 catches were made in each riverbed depression. It is necessary to note that fishing was conducted in coastal zones with slowed current. Due to the impossibility of anchoring the small craft at greater depths in the main channel, control fishing of young fish was not carried out there. Apparently, the number of caught fish in the main channel would be significantly lower, and the overall fish density comes from hydroacoustic surveys.

3. Results

It was established that the area of the studied river sections was 75.9 and 41 hectares for the Ukinskaya and Gornoslinkinskaya riverbed depressions, respectively. During the spring–summer flood period, the water level in the Irtysh River, according to data from the nearest hydrological station “Uvat” (40 km downstream), was 437 cm (30 May 2022). When conducting research during the summer–autumn low water period, the water level was 203 cm (31 July 2022), i.e., the water level drop in the Irtysh River channel was 234 cm. During the hydroacoustic survey, it was found that during the flood period, the density of fish concentrations in the riverbed depressions was slightly lower compared to the period of low water in summer. During these periods, the fish density values in the Ukinskaya riverbed depression area were 4.83 and 5.61 thousand specimens/ha, and in the Gornoslinkinskaya riverbed depression area they were 3.98 and 4.37 thousand specimens/ha, respectively. Thus, there was an increase in density by 16% and 10%, respectively, (Table 1).
As a result of the taxonomic identification, it was established that the major part of the fish population in aggregations in riverbed depressions consists of representatives of the Cyprinidae and Percidae families, while fish from the Coregonidae, Esociidae, Acipenseridae, and Lotidae families were less represented. The proportions of cyprinids and perches in the Ukinskaya riverbed depression area during flood amounted to 51% and 33% of the total number of registered fish, while the proportions of whitefish and pike were 9%, and unrecognized fish (including sturgeon and burbot) were 7%. As the water level decreased, the proportions of registered taxonomic groups in the considered river section showed similar but slightly changed values compared to the previous period: 48%, 31%, 12%, and 9%, respectively, (Table 1). In the Gornoslinkinskaya riverbed depression area during the flood, the proportions of registered taxonomic groups were quite close to the values in the second studied river section, amounting to 51%, 34%, 8%, and 6%, respectively. During the low water period, the proportion of cyprinids and perches was 57% and 31%, while the proportions of whitefish and pike was 7.14%, and unrecognized groups were 4.59%. Thus, the main fish aggregations consisted of cyprinids and perches, with their total share in the taxonomic structure in the Ukinskaya channel pit area during flood and low water periods being 84% and 79%, respectively. In the Gornoslinkinskaya riverbed depression area, these values were 85% and 88%, respectively.
According to the data from the control fishing in the Ukinskaya and Gornoslinkinskaya riverbed depressions, the catch per unit effort (CPUE) during the flood period was 1–3 individuals, with a total of 12 catches in each riverbed depression. During the summer–autumn low water period, the CPUE was 2–5 individuals in the Ukinskaya riverbed depression and 2–6 individuals in the Gornoslinkinskaya riverbed depression. The average fish density in the specified riverbed depression areas during the flood period was 13.9 thousand individuals/ha, and during the low water period, it was 28.19 thousand individuals/ha in the Gornoslinkinskaya riverbed depression and 31.25 thousand individuals/ha in the Ukinskaya pit.
According to control fishing data, juvenile cyprinids were represented mainly by roach (Rutilus rutilus Linnaeus) and to a lesser extent by dace (Leuciscus leuciscus Linnaeus) and ide (Leuciscus idus Linnaeus), while juvenile perches were predominantly represented by perch (Perca fluviatilis Linnaeus) and slightly by pike-perch (Stizostedion lucioperca Linnaeus). Analysis of the size structure of the fish population in the studied areas revealed that the size group ≤10 cm was most abundant, with a share of 66% during flood and 74% during summer in the Ukinskaya riverbed depression. In the Gornoslinkinskaya riverbed depression, these values were 62% and 72% (Figure 5). The remaining portions of the fish population in the studied river sections during these periods are represented by groups of fish with body sizes >10 cm. Thus, the majority of fish in the area of riverbed depressions, both during the flood and during the low water period, are represented by juveniles of cyprinids and percids.
As a result of the features of the channel relief on the river meander at the Gornoslinkinskaya riverbed depression, as well as due to the hydrophysical interaction of river flows during the ω-shaped evolution of the channel at the Ukinskaya riverbed depression, zones of hydrodynamic shadow areas are observed in the watercourse, with flow velocities close to “0”. As a result of constructing horizontal projections of flow velocity distribution, it was established that a gradient of flow velocity is observed in the studied river sections, having peculiarities in the distribution of zones with different velocity areas. With a decrease in the water level in the river, a drop in flow velocities is also noted, resulting in an increase in the area of hydrodynamic shadow necessary for juvenile fish living in the river channel. In addition, there is a dynamic spatial distribution of points of high and low flow velocities in its horizontal projection in the studied river sections. Thus, in the waters of the Ukinskaya and Gornoslinkinskaya riverbed depressions during the flood, zones of velocities with characteristics reaching values of 1.2 and 1.3 m s−1, respectively, were recorded (Table 2).
During the low water period, the maximum flow velocities recorded in the studied water bodies reached values of 0.8 and 0.75 m s−1. Consequently, at lower water levels, higher velocity sections (0.8–1.3 m s−1) were either not recorded at all or their area significantly decreased, while the area of medium-speed sections (0.15–0.7 m s−1) increased considerably. The area of zones with flow velocities in the range of 0–0.30 m s−1 in the Ukinskaya riverbed depression increased, whereas it decreased in the Gornoslinkinskaya riverbed depression (Table 2, Figure 6).
As a result of the recorded dynamics of decreasing flow velocities and increasing areas with reduced velocity values, there is an observed increase in the area utilized by perch fish in the studied riverbed depressions. In the Ukinskaya riverbed depression, the area occupied by juvenile perch fish amounted to 14% and 1% of the total area of the riverbed depression, or 11 and 311 hectares, respectively, (Table 1). Similarly, an increase in the occupied area is noted for juvenile carp fish in this section, with the occupied area growing by more than two times: 20% and 48%, or 15 and 37 hectares during the flood and low water periods, respectively, (Table 1). The location of juvenile cyrpinid and percid fish aggregations in the Ukinskaya riverbed depression during the flood period was identical and was noted in the left bank part of the river, with flow velocity zones of 0–0.75 m s−1 ; additionally, a small number of fish were registered in the channel part of the river, also with reduced flow velocities of 0.40–0.75 m s−1 (Figure 7).
During low water, the localization of aggregations changed; cyprinids were recorded in the channel and right bank parts of the watercourse at flow velocity ranges of 0.40–0.75 m s−1. The left bank section of the riverbed depression’s water area with zones of 0.30–0.55 m s−1 was additionally developed by juvenile perch fish. (Figure 7).
In the Gornoslinkinskaya riverbed depression, the effect of changes in habitat utilization is most vividly expressed not in the increase in usable area but in the shift in aggregation locations for fry. Thus, during the flood period, the highest concentrations of carp and perch fry were located near coastal areas of the left bank part of the riverbed depressions: carp in the zones of current velocities of 0.45–0.70 m s−1, perch in the zones of lower velocities of 0.45–0.65 m s−1.
During low water periods, aggregations of carp and perch were additionally found in the open channel part of the watercourse; both on the channel and on the right bank, individuals of both families were distributed mainly in areas with current velocities of 0.25–0.50 m s−1. (Figure 8). It is noted that for young carp, there was a decrease in the area occupied from flood to low water: from 60% to 43%, or from 25 to 18 hectares. Conversely, for young perch, an opposite trend was observed—an expansion of the occupied area: from 29% to 39%, or from 12 to 16 hectares, respectively, (Table 1).
For perch, a strong correlation was also found between the area occupied in the riverbed depression and the relative area of zones with flow velocities of 0.65 and 0.70 m s−1 (r = 0.994 and r = 0.975, p < 0.05) (Table 3). When analyzing the relationship between the relative area occupied and flow velocity areas, a strong inverse relationship was found only for perch with flow velocity areas of 0.80, 0.90, and 1.00 m s−1 (r = −0.982, r = −0.984, and r = −0.989, p < 0.05) (Table 3). The analyses of the relationship between the relative area occupied and relative areas of flow sections revealed a strong inverse relationship only for perch with a flow velocity area of 1.00 m s−1 (r = −0.989, p < 0.05) (Table 3). For indicators of occupied riverbed depression areas by carp and flow velocity areas at 0.10 m s−1, a strong negative correlation was found (r = −0.999, p < 0.05), while for perch, a strong positive correlation was found, with flow velocity sections at 0.65 and 0.70 m s−1 (r = 0.960 and r = 0.959, p < 0.05) (Table 3). In all other cases, no significant correlation was detected.
The analysis established a strong correlation between the area occupied by juvenile fish in riverbed depressions and the relative area of fast-flowing sections, with values of 0.70 m s−1 for carp species and 0.65 to 0.70 m s−1 for perch species. A negative correlation was observed only for perch in areas with flow speeds of 0.80, 0.90, and 1.00 m s−1. Thus, it has been reliably shown that the studied fish groups preferentially inhabit flow speeds of 0.65–0.70 m s−1 and tend to avoid faster sections with speeds of 0.80–1.00 m s−1.

4. Discussion

The increase in the area occupied by juvenile fish in riverbed depressions is primarily due to a decrease in flow velocity characteristics. As water levels drop, in our case study, the proportion of areas with average flow velocities less than 0.75 m s−1 significantly increases, while areas with flow velocities of 0.75–1.3 m s−1 almost completely disappear. The developed low-velocities zones in the riverbed depressions are similar in dynamic characteristics to resident (littoral) habitats, located in the riverbed at a distance from the coastline and at great depths; nevertheless, there is a concentration of juvenile fish here.
This feature is linked to juvenile fish preferring calm areas with low velocities [39]. For example, fish with small, elongated body shapes (representatives of the Cyprinidae family and Nemacheilidae), prefer zones of reduced speeds in the range of 0.15–0.51 m s−1 [40]. Additionally, it was established that the area occupied by fish decreases as flow speed increases in the central part of the riverbed (midstream), and fish tend to accumulate and migrate mainly near the riverbank [41]. This phenomenon is likely related to flow velocity characteristics exceeding critical velocities for fish during high discharge. Similarly, in our case, at higher water levels, flow velocities are higher, leading to a larger proportion of high-velocity areas; as flow discharge decreases, either these high-velocity areas diminish or are simply not recorded. Moreover, there is a shift in preferred velocity areas from the shoreline towards the river’s thalweg.
Considering the velocity characteristics and distribution of juvenile fish, it becomes clear that one of the key factors ensuring the formation of juvenile fish aggregations in riverbed depressions is the hydrodynamic characteristics of these sections. This is confirmed by a direct correlation between the area occupied by juvenile carp and perch and the sizes of zones with flow speeds of 0.50, 0.55, 0.65, 0.75, and 0.90 m s−1 (2 m below water surface) during flood vs. low water.
Simultaneously, the formation of fish aggregations in sections with lower velocitiesmay occur due to loss of orientation among juveniles at minimal velocities [42] and also due to the need for rest when moving into faster areas [41]. This bioenergetic strategy allows fish to avoid being swept away by currents due to muscle fatigue under heavy currents, e.g., with an increase of flow velocity, glycogen content in fish livers decreases while lactic acid levels in their blood and muscles significantly rise [27]. Ultimately, dynamics related to hydrodynamic factors cause significant metabolic changes in fish [27,43], leading to changes in behavior expressed through shifts in preferred flow zones and redistribution across riverbed depression areas. Spatial segregation among different fish species—specifically differences in selecting zones within riverbed depression areas—results from morphological characteristics inherent to these families since it has been shown [44,45] that even closely related species exhibit varying swimming capabilities. It has been established [46] that morphological differences affect their ability to perform precise maneuvers and powerful accelerations, while influencing energy expenditure during prolonged swimming. Lifestyle differences, metabolic rate variations, and morphological traits contribute to differing swimming capabilities and resting requirements among various fish species [46,47,48,49]. Furthermore, it has been noted [25] that critical speed metrics vary significantly depending on family and species classification.
The morphology of elongated-bodied fish like juvenile roach and bleak represents their adaptive response aimed at reducing hydrodynamic resistance and energy expenditure while navigating through currents [23]. Thus, a greater occupancy area among carp species can be explained by their superior ability to withstand water currents since this capability correlates with morphological traits associated with swimming activity [45]. Although a strong relationship between swimming ability and body size has been demonstrated [50,51], body length remains a significant variable explaining critical speed magnitude [25]. In our case, we compare occupancy within riverbed depression areas for juvenile perch and carp exhibiting similar size characteristics, hence we attribute distribution differences to pronounced morphological distinctions [25] which determine swimming ability and association with specific hydrodynamic environments [52,53] despite identical body length metrics among species [43]. It has been shown [25,49] that species with more elongated body shapes can minimize energy loss due to resistance at maximum swimming speeds.
The observed flow circulations within riverbed depression areas may disorient juvenile fish [53], potentially serving as an additional factor influencing aggregation formation. Moreover, close proximity between significantly differing speed zones typical for lentic and lotic aquatic systems within researched river sections promotes fish aggregations; many authors note [54,55] that higher species diversity and density are recorded during summer studies at inflow zones where streams enter lakes.
An analysis of swimming abilities—including determining critical speed as well as optimal and maximum swimming speeds—for different freshwater fishes, including roach and perch, revealed [56] that trout (Oncorhynchus mykiss Walbaum), roach, and carp (Cyprinus carpio Linnaeus) exhibited the highest swimming capabilities and endurance. The highest recorded critical speed values—1 m s−1 —were noted for large trout individuals capable of extensive movements. For roach under sizes <10 cm, critical speed values ranged from 0.5 to 0.6 m s−1, while for perch measuring around 10 cm, critical speed reached approximately 0.81 m s−1 [56]. In summarizing existing data on critical speeds, it was also established [26] that for roach, perch, and zander, these metrics hold high values compared to other freshwater species. Slightly lower critical speed values for roach and perch obtained under laboratory conditions [56] compared to preferred high-velocity zones recorded here cannot be entirely applied to individuals within natural habitats [57]; however, data obtained by [56] provide an overall understanding regarding swimming capabilities across various species.
It should also be noted that horizontal distribution among fishes within riverbed depression areas changed alongside water level dynamics, while juvenile preferences towards zones characterized by flow speeds around 0.75 m s−1 remained consistent. Notably, along with shifts in juvenile aggregation localization from flood towards low water, overall patterns regarding the positioning of more rapid sections [58] as well as dynamic axes within flows [59] along the outer banks (concave shore) of studied meanders remained unchanged.
Overall, distribution among fishes within lotic systems can be attributed to various abiotic factors, where we consider hydrodynamic factors as one key element influencing distribution patterns. This was previously demonstrated within a small tributary of the Irtysh River [60], indicating how hydrodynamic effects create environmental heterogeneity affecting both direct distribution patterns as well as indirectly through modifications on physical habitat characteristics. Nevertheless, distribution features among fishes within flows at family levels can primarily be explained through the pronounced morphological differences observed [61].
It should be noted that previous studies analyzing microhabitat selection among various fish species indicated reliable connections between variables, characterizing body morphology alongside habitat conditions including flow velocities for both cyprinids and percids, establishing significant relationships at p < 0.05. It becomes apparent that distribution patterns were aligned towards specific flow zones exhibiting average flows between ranges of approximately 0.45–0.70 m s−1 and lower flows around 0.25–0.50 m s−1 during floods and low flow, respectively. Since maintaining position within faster flowing zones requires greater energy expenditures, such areas are energetically unfavorable from a bioenergetics perspective.
In shallow water conditions, in particular in lakes with a thermocline, the presence of the “guiding effect” phenomenon of the redistribution of fish under the vessel’s hull is recognized [62]. This is caused by the simultaneous influence of vessel noise and low-frequency noise of parts of the trawl (fishing gear) [63] as well as the environment, primarily the influence of temperature, which can be enhanced by the halocline and bathymetric characteristics of the reservoir. We believe that in a lotic environment, as well as in the absence of trawl fishing during our acoustic surveys, the influence of these factors can be leveled out and reduced to minimum values due to the constant mixing of water masses by intense multidirectional currents. In addition, it is noted that the presence of predators can have a much greater effect on the distribution of schools of fish than the noise of the vessel and parts of fishing gear [63].
At the same time, it is known that hydroacoustic studies performed using high-frequency echosounders with an operating frequency of 455 kHz and a scanning beam provide the least error in the result [62]. However, in our study, with the goal to determine distribution patterns of fish aggregations of certain taxonomic groups, we used a dual-beam echosounder with operating frequencies of 50 and 200 kHz and beam opening angles of 46 and 140, respectively. The technique we use for the remote identification of fish by the shape of the swim bladder is based on the operation of an echosounder with the specified characteristics [19].
Our study revealed that the main abiotic factors are hydrodynamic ones—the intensity of currents and their direction. The effect of current velocities is confirmed by the distribution of fish in certain speed zones, and the number of juvenile fish in the riverbed is influenced by the dynamics of the area of suitable habitats, determined by the water level during flood and low flow. The Irtysh River Basin is inhabited by around 15 million people in China, Kazakhstan, and Russia, thus the river faces multiple anthropogenic stressors [64]. Among those stressors, pollution [65,66,67], hydrological changes due to hydropower use [68], river-borne cargo, and irrigation [69], as well as fisheries and poaching, [70,71] are relevant. Ecosystem services generated by fish populations are an important aspect with regard to socioecology [72]. In this context, our study contributes to the knowledge about fish habitats in the lower Irtysh River and recommends the protection of those important habitat sections to ensure biodiversity conservation.

5. Conclusions

This study contributes to the understanding of the multifunctional role of riverbed depressions along river meanders. The local distribution of juvenile fish is related to their swimming abilities, the morphology of the channel and the use of low-velocity zones as recruitment areas.
The dynamics of the water level, accompanied by changes in areas and volumes of habitats in the floodplain–channel complex along the lower reaches of Irtysh River, include both an expansion and reduction of habitats, along with a change in flow velocities. The transition of juveniles from lentic to lotic water bodies is accompanied by a change in the characteristics of their habitats, first of all, regarding hydrodynamics—a change in the conditions from «still waters» to «flowing waters». The absence of low-velocity sections in the main channel would lead to a decrease in the survival rate of juvenile fish, which are evolutionarily adapted to such habitats in the floodplain–channel system. Thus, the importance of the presence of such sections for the functioning of ecosystems of floodplain–channel complexes is emphasized.
The studied river sections, which have a multifunctional biological role, should be provided with appropriate protection against changes in the morphology and relief of the riverbed. If anthropogenic transformation of the riverbed is necessary, water managers need to ensure the availability of the characteristics of velocity zones and their dynamics in the meandering sections of the river, in order to ensure fish recruitment in the floodplain–river complex.
The data obtained on the distribution of juvenile cyprinids and percids in the river meanders demonstrate the need for low-velocity sections and zones of hydrodynamic “shade”, providing the juveniles with fundamental habitat conditions in the summer-autumn low-flow period, that contribute to the conservation of energy and fat reserves to successfully overcome the conditions during the subsequent winter period.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/d17010068/s1, The video files S1 and S2 show fragments of echograms with recorded depths, coordinates and targets (fish), obtained during hydroacoustic surveys during the flood period on 30 May 2022. S3 provides a scheme of the fishing gear and methodology of control fishing.

Author Contributions

Conceptualization, A.A.C. and M.S.; methodology, A.A.C. and E.I.P.; formal analysis, A.A.C. and E.I.P.; investigation, A.A.C. and E.I.P.; resources, A.A.C.; writing—original draft preparation, A.A.C., E.I.P., and M.S.; project administration, A.A.C.; funding acquisition, A.A.C. All authors have read and agreed to the published version of the manuscript.

Funding

This work was carried out with the support of the fundamental scientific research topic “Patterns of distribution and migration of fish in lotic and limnic water bodies of the Ob-Irtysh basin” (Ministry of Science and Higher Education of the Russian Federation, № 1022040700418-3-1.6.21).

Data Availability Statement

Raw data related to this study are available upon request from A.A.C.

Acknowledgments

The authors express their gratitude to the management of the Tobolsk Complex Scientific Station of the Ural Branch of the Russian Academy of Sciences (TCSS UrB RAS) for the technical support provided, as well as to the staff of the Laboratory of Ecology of Hydrobionts (TCSS UrB RAS) for the technical and physical assistance provided in collecting material for conducting the study and preparing the article. And we thank two anonymous reviewers for their valuable comments and constructive suggestions.

Conflicts of Interest

The authors declare no conflicts of interest.

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Figure 1. Location of the studied areas of wintering riverbed depressions in the Lower Irtysh: (a) Ukinskaya; (b) Gornoslinkinskaya. The red outline schematically shows the boundaries of the studied water areas; arrows schematically indicate the direction of the current. (c) Relief map of the bottom surface of the deep-water part of the Gornoslinkinskaya (left) and Ukinskaya (right) wintering riverbed depressions during the flood period.
Figure 1. Location of the studied areas of wintering riverbed depressions in the Lower Irtysh: (a) Ukinskaya; (b) Gornoslinkinskaya. The red outline schematically shows the boundaries of the studied water areas; arrows schematically indicate the direction of the current. (c) Relief map of the bottom surface of the deep-water part of the Gornoslinkinskaya (left) and Ukinskaya (right) wintering riverbed depressions during the flood period.
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Figure 2. Installation of the antenna-emitter of the “AsCor” hydroacoustic complex on a small craft. (a) Location of the antenna-emitter with vertical view in a fairing in the bow of the small craft for conducting echo survey. (b) Small craft with installed fairing containing the antenna-emitter for transportation. (c) Small craft with installed fairing containing the antenna-emitter for echo survey.
Figure 2. Installation of the antenna-emitter of the “AsCor” hydroacoustic complex on a small craft. (a) Location of the antenna-emitter with vertical view in a fairing in the bow of the small craft for conducting echo survey. (b) Small craft with installed fairing containing the antenna-emitter for transportation. (c) Small craft with installed fairing containing the antenna-emitter for echo survey.
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Figure 3. Survey schemes in the area of wintering riverbed depressions: (a) Gornoslinkinskaya; (b) Ukinskaya. Dotted line is a schematic representation of the zigzag tacks of a mobile sonar survey (the course of a small crafter); points are places of stationary hydrometric survey.
Figure 3. Survey schemes in the area of wintering riverbed depressions: (a) Gornoslinkinskaya; (b) Ukinskaya. Dotted line is a schematic representation of the zigzag tacks of a mobile sonar survey (the course of a small crafter); points are places of stationary hydrometric survey.
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Figure 4. Diagram of installing a flow velocity meter on a small craft and the process of conducting a hydrometric survey. (a) Scheme of installing a metal rotary-extendable device in the bow of the small craft for transporting the flow velocity meter and conducting hydrometric measurements. (b) Small vessel with an installed flow velocity meter for transportation. (c) Scheme of conducting a hydrometric survey (L1—part of the metal rod above the small craft ’s side; L2—part of the metal rod below the small craft ’s side; V1—river flow velocity; V2—small craft speed). (d) Changing the position of the flow velocity meter for conducting a hydrometric survey.
Figure 4. Diagram of installing a flow velocity meter on a small craft and the process of conducting a hydrometric survey. (a) Scheme of installing a metal rotary-extendable device in the bow of the small craft for transporting the flow velocity meter and conducting hydrometric measurements. (b) Small vessel with an installed flow velocity meter for transportation. (c) Scheme of conducting a hydrometric survey (L1—part of the metal rod above the small craft ’s side; L2—part of the metal rod below the small craft ’s side; V1—river flow velocity; V2—small craft speed). (d) Changing the position of the flow velocity meter for conducting a hydrometric survey.
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Figure 5. Size structure of fish in the areas of riverbed depressions: (a) Ukinskaya; (b) Gornoslinkinskaya. Graph bars 1 (in White)—Flood (30 May 2022) and 2 (Hatched)—low water period (31 July 2022).
Figure 5. Size structure of fish in the areas of riverbed depressions: (a) Ukinskaya; (b) Gornoslinkinskaya. Graph bars 1 (in White)—Flood (30 May 2022) and 2 (Hatched)—low water period (31 July 2022).
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Figure 6. Dynamics of areas with different flow velocities in (a) Ukinskaya riverbed depression and (b) Gornoslinkinskaya riverbed depression during the flood period (white, dotted line) and low water period (gray, solid line).
Figure 6. Dynamics of areas with different flow velocities in (a) Ukinskaya riverbed depression and (b) Gornoslinkinskaya riverbed depression during the flood period (white, dotted line) and low water period (gray, solid line).
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Figure 7. Distribution of flow velocity fields and fish fry (<10 cm) in the Ukinskaya riverbed depressions during the spring–summer flood and summer–autumn low water period. (a) Flow velocities on 31 May 2022 and (b) 30 July 2022. Distribution of cyprinids on (c) 31 May 2022 and (d) 30 July 2022. Distribution of percids on (e) 31 May 2022 and (f) 30 July 2022. Additionally, a correlation analysis was performed to assess the presence and magnitude of relationships (r) between absolute (S_abs) and relative (S_rel) area indicators occupied by young carp and perch with values of these same indicators for zones with recorded flow velocities during the flood and low water period. The statistical analysis revealed a strong direct correlation for carp between the area occupied and the relative area of zones with flow velocities of 0.70 m s−1 (r = 0.952, p < 0.05) (Table 3).
Figure 7. Distribution of flow velocity fields and fish fry (<10 cm) in the Ukinskaya riverbed depressions during the spring–summer flood and summer–autumn low water period. (a) Flow velocities on 31 May 2022 and (b) 30 July 2022. Distribution of cyprinids on (c) 31 May 2022 and (d) 30 July 2022. Distribution of percids on (e) 31 May 2022 and (f) 30 July 2022. Additionally, a correlation analysis was performed to assess the presence and magnitude of relationships (r) between absolute (S_abs) and relative (S_rel) area indicators occupied by young carp and perch with values of these same indicators for zones with recorded flow velocities during the flood and low water period. The statistical analysis revealed a strong direct correlation for carp between the area occupied and the relative area of zones with flow velocities of 0.70 m s−1 (r = 0.952, p < 0.05) (Table 3).
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Figure 8. Distribution of flow velocity fields and fish fry (<10 cm) in the Gornoslinkinskaya riverbed depression during the spring–summer flood and summer–autumn low water period. (a) Flow velocities on 31 May 2022 and (b) 30 July 2022. Distribution of cyprinids on (c) 31 May 2022 and (d) 30 July 2022. Distribution of percids on (e) 31 May 2022 and (f) 30 July 2022.
Figure 8. Distribution of flow velocity fields and fish fry (<10 cm) in the Gornoslinkinskaya riverbed depression during the spring–summer flood and summer–autumn low water period. (a) Flow velocities on 31 May 2022 and (b) 30 July 2022. Distribution of cyprinids on (c) 31 May 2022 and (d) 30 July 2022. Distribution of percids on (e) 31 May 2022 and (f) 30 July 2022.
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Table 1. Distribution of fish (fish density, thousand specimens/ha and share, %) in the area of riverbed depressions of the Lower Irtysh (Ukinskaya, Gornoslinkinskaya) during different hydrological phases.
Table 1. Distribution of fish (fish density, thousand specimens/ha and share, %) in the area of riverbed depressions of the Lower Irtysh (Ukinskaya, Gornoslinkinskaya) during different hydrological phases.
LocationUkinskayaGornoslinkinskayaUkinskayaGornoslinkinskaya
Hydrological PhaseFloodLow Water
Water level (at the nearest hydrological post “Uvat”)437203
Fish density, (thousand specimens/ha)4.833.985.614.37
Cyprinids, (%)50.9151.3348.0157.33
Percids, (%)33.0833.8530.8130.94
Whitefish, pike (%)9.188.3312.347.14
Unrecognized fish, (sturgeons, burbots, %)6.836.498.844.59
Area, (ha)81418141
Percentage of utilized area, (cyprinids, %)20.036048.2242.73
Utilized area, (cyprinids, ha)15.2024.6036.6017.52
Percentage of utilized area, (percids, %)14.0028.6840.6038.83
Utilized area, (percids, ha)10.6311.7630.8115.92
Table 2. Distribution of the area of aquatic zones with different flow velocities in the Ukinsky and Gornoslinkinsky riverbed depressions during different periods of the water level regime of the Irtysh River.
Table 2. Distribution of the area of aquatic zones with different flow velocities in the Ukinsky and Gornoslinkinsky riverbed depressions during different periods of the water level regime of the Irtysh River.
No.Flow Velocity Class,
(m s−1)		Area During FloodArea During Low WaterArea Changes (+/−)Area During FloodArea During Low WaterArea Changes (+/−)
UkinskayaGornoslinkinskaya
hahaha%hahaha%
1.1.30.960−−0.960−100.00−−−−
2.1.25.595−−5.595−100.00−0.273−0.273−100.00
3.1.12.477−−2.477−100.00−2.470−2.470−100.00
4.12.477−−2.477−100.00−8.221−8.221−100.00
5.0.94.765−−4.765−100.00−8.028−8.028−100.00
6.0.83.674−−3.674−100.000.9617.188−6.227−86.63
7.0.754.1370.355−3.782−91.424.5963.6550.94225.76
8.0.72.6792.397−0.282−10.5114.9693.05711.911389.57
9.0.651.7663.1711.40579.5611.3863.5437.842221.32
10.0.61.3283.4942.166163.179.6645.1134.55289.03
11.0.551.1623.1932.031174.854.1942.6931.50155.75
12.0.51.5292.9991.47096.153.0232.7440.28010.20
13.0.451.1143.6332.519226.114.7103.6951.01527.46
14.0.40.9483.7412.793294.514.6404.1910.44910.70
15.0.350.8183.5902.773339.005.0244.2220.80319.01
16.0.30.7593.2792.520332.205.1646.247−1.082−17.33
17.0.250.6992.2141.515216.653.1541.6101.54595.96
18.0.20.6872.4721.785259.643.2773.1490.1284.07
19.0.150.8423.6442.803333.020.5163.027−2.512−82.97
20.0.11.1851.6450.45938.760.3501.731−1.382−79.81
21.01.3991.172−0.227−16.230.2711.043−0.772−74.02
Table 3. Magnitude of correlation relationships between distributions of carp (CPR) and perch (PRC) abundance and flow velocity zones in riverbed depressions, taking into account the dynamics of values during the flood–low water period.
Table 3. Magnitude of correlation relationships between distributions of carp (CPR) and perch (PRC) abundance and flow velocity zones in riverbed depressions, taking into account the dynamics of values during the flood–low water period.
No.Pair of Variables **Fish (S_abs) × Current (S_rel)Fish (S_rel) × Current (S_abs)Fish (S_rel) × Current (S_rel)Fish (S_abs) ×
Current (S_abs)
Velocity ClassCPRPRCCPRPRCCPRPRCCPRPRC
1.0−0.495−0.6950.078−0.4110.422−0.033−0.743−0.921
2.0.10−0.782−0.733−0.534−0.5880.007−0.093−0.999 *−0.882
3.0.15−0.722−0.407−0.656−0.282−0.2860.083−0.868−0.523
4.0.20−0.2280.267−0.6960.005−0.4630.3340.0860.488
5.0.250.1330.523−0.1400.5890.0450.7620.5250.871
6.0.30−0.2850.195−0.831−0.289−0.786−0.041−0.0380.293
7.0.35−0.1120.376−0.5910.136−0.3930.4230.1990.598
8.0.40−0.2150.267−0.6400.111−0.3720.4090.1030.530
9.0.45−0.1170.342−0.5060.258−0.2310.5370.2440.657
10.0.50−0.333−0.014−0.5450.2770.0440.5260.0390.513
11.0.55−0.0020.402−0.2860.477−0.0140.6930.4110.796
12.0.600.5630.889−0.2270.304−0.1390.5650.6390.858
13.0.650.8170.994 *0.0560.4970.2020.7170.8220.960 *
14.0.700.952 *0.975 *0.1900.5130.3560.6390.9030.959 *
15.0.750.417−0.0660.139−0.2670.512−0.2160.5860.271
16.0.80−0.360−0.685−0.584−0.982 *−0.214−0.878−0.460−0.613
17.0.90−0.402−0.750−0.507−0.984 *−0.123−0.835−0.540−0.727
18.1.00−0.548−0.719−0.738−0.989 *−0.537−0.989 *−0.584−0.632
19.1.10−0.240−0.676−0.186−0.8680.207−0.608−0.429−0.755
20.1.200.063−0.4120.652−0.1470.668−0.1260.050−0.425
21.1.300.078−0.3960.686−0.1010.686−0.1010.078−0.396
*—Bold indicate significant correlation relationships (at p < 0.05); ** FISH (S_abs)—absolute index of the area of riverbed depressions occupied by carp and perch groups, ha; CURRENT (S_abs)—absolute index of the area of water zones with a certain current speed; FISH (S_rel)—relative index of the area of riverbed depressions occupied by carp and perch groups, %; CURRENT (S_rel)—absolute index of the area of water zones with a certain current speed.
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Chemagin, A.A.; Popova, E.I.; Schletterer, M. Effects of Flow Velocity on the Dynamics of Juvenile Fish Habitats in River Meanders of the Irtysh River. Diversity 2025, 17, 68. https://doi.org/10.3390/d17010068

AMA Style

Chemagin AA, Popova EI, Schletterer M. Effects of Flow Velocity on the Dynamics of Juvenile Fish Habitats in River Meanders of the Irtysh River. Diversity. 2025; 17(1):68. https://doi.org/10.3390/d17010068

Chicago/Turabian Style

Chemagin, Andrey A., Elena I. Popova, and Martin Schletterer. 2025. "Effects of Flow Velocity on the Dynamics of Juvenile Fish Habitats in River Meanders of the Irtysh River" Diversity 17, no. 1: 68. https://doi.org/10.3390/d17010068

APA Style

Chemagin, A. A., Popova, E. I., & Schletterer, M. (2025). Effects of Flow Velocity on the Dynamics of Juvenile Fish Habitats in River Meanders of the Irtysh River. Diversity, 17(1), 68. https://doi.org/10.3390/d17010068

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