INTRODUCTION

The rainbow trout Parasalmo mykiss (Walbaum, 1792) on the Asian coast of the Pacific Ocean occurs mainly in water bodies of the Kamchatka Peninsula (Pavlov et al., 2001). The species structure is quite complex, as there are individuals with either migratory (anadromous) or resident (landlock, non-anadromous) life strategy in populations (Pavlov et al., 1999, 2001; Pavlov and Savvaitova, 2008; Kuzishchin, 2011). The ratio of resident and anadromous fish in local populations is determined by the geomorphology of the river system (Pavlov et al., 1999, 2001, 2008; Kuzishchin, 2011). The anadromous rainbow trout of Kamchatka is a specially protected object of biodiversity and has been included in the Red Data Book of the Russian Federation since 1983 (Savvaitova, 2001; Krasnaya Kniga…, 2021). In the rivers of Northwestern Kamchatka, the rainbow trout populations inhabit undisturbed ecosystems and are just slightly affected by anthropogenic impact; they retained their original structure and anadromous fish predominate in local populations (Pavlov et al., 2001, 2009b, 2016; Kuzishchin et al., 2020). Thus, the rainbow trout populations of the rivers of Northwestern Kamchatka are unique in terms of biodiversity and serve as natural standards that are of great importance for assessing the state and conservation of the specially protected object (Pavlov et al., 2007).

The state of the rainbow trout populations in the rivers of Northwestern Kamchatka (Snatolvayam, Kvachina, and Utkholok) has been monitoring since 1970. During this period, populations underwent constant short-term and long-term changes in their main parameters (Savvaitova et al., 2002; Pavlov et al., 2016). At the same time, estimates of such an important parameter as the number of migratory spawners are associated with a number of objective difficulties due to the geomorphology of rivers and the behavioral characteristics of fish after their migration into the river. The spawners counting by the visual method from the shore and during underwater observations is ineffective. In this regard, the usage of hydroacoustic methods seems promising in the study of migration and accounting of migratory fish. Such methods are actively introduced into the practice of ichthyological studies of anadromous salmon fish (Cheng et al., 1991; Levy et al., 1991; Pavlov et al., 2009, 2009a; Borisenko et al., 2013, 2019; Degtev et al., 2012; Conrad et al., 2019).

The first observations of the autumn spawning migration of the Kamchatka rainbow trout using a two-frequency identification sonar DIDSON (SMC, USA) were carried out on the Utkholok River in 2006–2007; preliminary data on the abundance of the species in the river were obtained (Pavlov et al., 2009). However, on other rivers that differ from the Utkholok River in size, water content, and other structural elements, no works on the fish abundance assessment have been carried out. Thus, currently, there are no reliable data on the number of specially protected species for other rivers, except the Utkholok. This hampers the development and implementation of effective conservation measures. For example, the Kvachina River basin is recommended for the creation of salmon fishery protected areas of the third type, a zone for preserving the gene pool of especially valuable species of salmon fish in rivers, which ecosystems and the salmon populations inhabiting them are not disturbed or slightly disturbed by human activities (Pavlov and Bukvareva, 2010). Currently the Kvachina is inhabited by one of the most significant populations of anadromous rainbow trout for the conservation of natural species diversity in Kamchatka (Pavlov et al., 2007).

It is known that even under fundamentally similar conditions, the distribution of fish in watercourses is unstable and dynamically changing under the influence of external and internal factors (Borisenko et al., 2013; Pavlov and Skorobogatov, 2014). Thus, it is necessary to develop special approaches and methods for correct assessment the number of spawners of anadromous rainbow trout, one of the parameters of integrated monitoring of the state of the species listed in the Red Data Book.

The aim of our work is to identify patterns of autumn spawning migration of the anadromous rainbow trout into the Kvachina River using a hydroacoustic system, to determine its abundance and size composition of the local population, and to describe the daily and multi-day dynamics of the movement of spawners.

MATERIALS AND METHODS

Research on the Kvachina River was carried out in the first half of October in 2010 and 2011. Additional data on the migration patterns and the distribution of fish in the river were obtained through fishing and direct observations from the shore in 2014–2021 with no use of any hydroacoustic equipment.

The Kvachina River originates in the high tundra of the Primorsky Plateau and flows into the Kvachina Creek of the Shelikhov Gulf of the Sea of Okhotsk. The total length of the river is approx. 95 km. At the source, the width of the river is 1–2 m, in the lower reaches, 25–30 m, and at the mouth, ~40–50 m (at low tide); the water discharge at the mouth is up to 3.2 m3/s, the flow velocity in the middle and lower reaches is 0.2–0.4 m/s, the catchment area is ~750 km2. The tide acts 15–20 km up from the mouth. The water has a brown color, in the lower reaches the bottom is formed by fine gravel and sand. In the upper and middle reaches, the river strongly meanders, there is practically no woody vegetation on the banks; however, occasionally there are thickets of alder. A characteristic feature of the river, as well as other rivers of Western Kamchatka, is the alternation of deep pits and shallow areas, i.e. riffles and reaches. Most of the pits are located in the middle and lower reaches of the river, below the confluence of the Kvachina and Pukhl rivers (Pavlov et al., 2001).

The counting of the number of passing fish, determination of their size composition and direction of movement were carried out with the NetCor computer-aided hydroacoustic system (hereinafter referred to as the system) manufactured by PromHydroacoustics OOO (Russia). The system consists of a hydroacoustic high-frequency multi-beam station connected via a radio channel with a coastal computerized control and measuring system. The main tactical and technical characteristics of the system are the following: acoustic operating frequency, 455 kHz; electric power on the hydroacoustic antenna, 40–80 W; repetition rate of hydroacoustic radiation bursts, up to 12 Hz; width of the directivity of one beam at the level of –6 dB, 10°; width of the multibeam sector at one station, up to 70° in the plane of the beam fan; the distance of stable radio communication, up to 300 m. The antenna of the system horizontal sounding of the water space, and when the fish passes through the plane of the beam fan, it is possible to determine the direction of its movement up/downstream by successively reading the echo signals from the fish by the numbered acoustic beams of the multibeam sector. In this mode, the system provides registration of fish at a depth of at least 0.3 m and with a maximum range of registration of a single fish with a target strength of –50 dB, 20 m.

The mathematical support of the system includes a Windows-based software package developed by PromHydroacoustics OOO, an expeditionary program for managing the system and collecting data from a hydroacoustic station in real time and a program for office data processing. The software package is installed on a computer on the x86 platform.

The fish abundance in the watercourse was estimated by measuring the parameters of the density of their flow in time when they cross the fixed zone of hydroacoustic registration with the subsequent calculation of the number of individuals. The mathematical software of the NetCor system provides quantitative interpretation of the obtained hydroacoustic recordings of passing fish in two ways, echo counting and echo integration (Yudanov et al., 1984; Simmonds and MacLennan, 2005). On the Kvachina River, the abundance of anadromous rainbow trout was estimated by echo counting method, which makes it possible to register individual fish (resolved single targets during echolocation) passing through the controlled section of the river. Further processing of such registrations was carried out by digital noise filtering (low-pass filter) and using the procedure for combining fish tracks using elements of cluster analysis based on the proximity of instantaneous registrations in each burst of radiation. The traces of registration of passing fish individuals, identified in this way, yielded to echo counting with the restoration of the distribution of the target strength and the direction of movement of the fish up and downstream relative to the immobile recorder. When identifying and tracking each target (fish), all amplitudes of echo signals from it were determined and measured in each hydroacoustic radiation message, and only the maximum amplitude of the echo signal was taken into account for calculating the strength of the target. Thus, the possibility of multiple counting of the same individual that passed through the target is eliminated for the total number calculating. During the processing, the time of registration of the fish that passed through the river section, the direction of its movement, the location in the river section, and the acoustic power of the target were determined, and the length of the fish was calculated. The end-to-end calibration procedure of the multibeam sonar was carried out on a standard reference sphere made of electrolytic copper with a diameter of 39.1 mm, the target strength (TS) value of which at an operating frequency of 455 kHz was −39.6 dB. Measurements were made for each beam of a multibeam antenna at distances of 5, 10, 15, and 20 m. The hydrodynamic noise level was measured in the passive mode using a built-in oscilloscope. According to the results of electroacoustic measurements, the optimal operating modes of the equipment and correction factors were set, which were used in the process of cameral processing of echo signals.

The hydroacoustic system was installed on the shallow right bank of the river, 16 km from its mouth, at the point with coordinates 57°41′48″ N, 157°13′44″ E (Fig. 1). The control of operating modes and the transmission of information from the coastal monitoring and measuring system installed in the laboratory at a distance of 70 m from the antenna were carried out via a radio channel. Acoustic sounding covered the entire water column from surface to the bottom and to the opposite bank of the river, which made it possible to register with a high probability all the fish passing through the controlled section of the river with a width of ~20 m (Fig. 2).

Fig. 1.
figure 1

Location of the scientific hydroacoustic system NetCor on the Kvachina River: 1, hydroacoustic multibeam antenna (coordinates 57°41′48″ N, 157°13′44″ E); 2, laboratory; (→), river flow direction. Scale: 35 m.

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Fig. 2.
figure 2

Cross section of the Kvachina River at the observation site (57°41′48″ N, 157°13′44″ E): 1, river bottom; 2, the antenna coverage area of by depth; 3, antenna of the NetCor system.

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As an example, Figure 3 shows a typical echogram of registration of two individuals of the rainbow trout after noise filtering and cluster analysis, as well as the results of the final processing of echo signals. As can be seen from the echogram, the complex clearly registers anadromous rainbow trout in the probed sector of the antenna pattern from the beginning to the end of echo contacts with fish, the target strength of which was −18.0 and −21.4 dB.

Fig. 3.
figure 3

Results of the laboratory processing of the echogram recording of two specimens of the anadromous rainbow trout Parasalmo mykiss, which passed upstream through the section of the Kvachina River in the zone of action of the multibeam sonar of the NetCor system: 1, registered fish; 2, results of the processing; (→), direction of the river flow.

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All processing data (date, time of registration, acoustic strength of the target of fish and direction of their movement), as well as the main values of the electroacoustic parameters of the system, which were used during the work, were recorded in separate files created every hour.

The characteristics of the daily and seasonal (over the observation period) dynamics of the passage of fish in the controlled section of the river and the size composition of individuals were obtained. The linear dimensions of the registered spawners of the rainbow trout were calculated based on the in situ measured values of the target strength of the fish using the known regression equation obtained for this fish species at a frequency of 420 kHz for the lateral aspect of irradiation (Kubecka and Duncan, 1998), which, taking into account the correction for the operating frequency of the NetCor sonar has the form: \(TS = 27.48\,\log \,FL - 97.42,\) where FL is the Smith’s length of the fish (mm), from the tip of the snout to the fork of the caudal fin.

Despite the fact that the above equation was obtained for cultivated rainbow trout up to 425 mm long (Kubecka and Duncan, 1998), it can be used to calculate the body length of larger individuals. This assumption was confirmed by our measurements of caught live specimens of the rainbow trout with a body length of 705 and 650 mm by their repeated passage through a controlled section of the river. So, taking into account the correction of 4.47 dB for the gain of the sonar receiving path, obtained by end-to-end calibration of the system using an exemplary sphere, the target strength of these fish was –19.2 and –20.1 dB, respectively.

To compare the data on the size composition of fish obtained by the methods of hydroacoustic and biological analyses, catching and measurements of individuals were performed. The fish were caught with fishing rods according to the catch-and-release principle in accordance with the permits of Rosprirodnadzor No. 41 dated May 13, 2010 and No. 65 dated May 30, 2011. The Smith’s body length was measured from the top of the snout to the fork of the caudal fin; the girth of the body in front of the dorsal fin was also determined. The body weight of the fish was determined by direct weighing: the fish were placed in a soft mesh bag and weighed without injury. Sex was determined by the shape of the head, the location of the upper jaw relative to the posterior margin of the eye, and the shape of the body. The places of fish capture were recorded using a portable GPS navigator Garmin eTrex Vista (Garmin, USA). Thereby, we determined the main types of its temporary habitats during its upstream movements. In total, 149 fish specimens were caught in 2010 and 61 specimens in 2011.

For continuous full sounding of the river section during the observation period, the depth of the sonar antenna was adjusted in accordance with changes in the water level in the river.

RESULTS

During the study period, the controlled section of the Kvachina River, according to the results of direct echo counting, was overflowed by 3081 and 637 individuals of anadromous rainbow trout in 2010 and 2011, respectively. Only fish which size exceeded 500 mm and which was oriented upstream of the river were taken into account. Figure 4 shows the dynamics of migration of spawners of anadromous rainbow trout upstream the river from October 2 to October 15 in 2010 and from October 5 to October 16 in 2011. The maximum per day number of individuals of anadromous rainbow trout that passed through the river cross section was recorded on October 11, 2010 (324 spec.), slightly less specimens per day passed a week earlier, on October 04, 2010 (318 spec.). The number of migrating fish on these dates was almost twice as high as on the previous and subsequent days. Thus, on October 3 and October 13, 2010, 170 and 161 specimens of the rainbow trout were registered. A similar pattern of the migration of the rainbow trout spawners was also observed in 2011.

Fig. 4.
figure 4

Dynamics of the movements through the section of the Kvachina River by Parasalmo mykiss producers in the first half of October and the corresponding polynomial trend lines. Here and in Figs. 5, 6: (―◼―), 2010; (―△―), 2011.

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The polynomial approximation of the abundance of the rainbow trout over the periods of the study resulted in the corresponding equations and values of its reliability: (1) for 2010: y = –1.2191x2 +17.131x + 182.35, R2 = 0.565; (2) for 2011: y = – 0.3275x2 + 2.6678x + 63.7, R2 = 0.564. The number of fish in the Kvachina River calculated according to these equations in the first half of October 2010 and 2011 amounted to 3114 and 658 individuals, respectively. The correspondence of the calculated data to the results of direct echo counting allows extrapolation of the obtained data for the entire expected period of entry from the sea, from the beginning of September to the first days of November (Fig. 5).

Fig. 5.
figure 5

Graphs of polynomial extrapolation of the obtained values of the fish abundance during the periods of hydroacoustic observations for the expected period of migration of winter spawners of Parasalmo mykiss in the Kvachina River.

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Extrapolation of the data for the period from September 1 to October 30 for the neighboring years (2010–2011) provided the corresponding equations with high reliability values: (1) 2010: y = – 0.288x2 + 17.504x – 15.385, R2 = 0.781; (2) 2011: y = – 0.079x2 + 4.6934x – 2.1976, R2 = 0.8267. According to these equations, the number of fish in the Kvachina River was 9832 and 2855 individuals in 2010 and 2011, respectively.

The daily dynamics of spawning migration in 2010 was characterized by two periods of increased intensity of fish movement upstream the river (Fig. 6). The first period began with 3.0 ind./h at dawn (07:00) and lasted until 10:30 with a peak of the movement intensity (13.9 ind./h) at 08:00. The second period began in the evening hours (from 16:30), the maximum value (19.7 ind./h) was recorded at 20:00. In 2011, there also were two peaks of the movement intensity, but they were much less pronounced. The intensity of fish movements in the morning (from 06:00) increased from ~1.0 to 4.2 ind./h by 07:00, then gradually decreased to 1 ind./h by 15:00. Starting from 16:00, the migration intensity slowly increased to 4.6 ind./h by 18:00, and the fish moved with a similar intensity until 22:00. After this time and until the morning, the intensity of the rainbow trout migration was minimal. It should be noted that during periods with a low intensity of spawning migration, the controlled section of the river was mainly crossed by single individuals, while during periods of increased intensity of migration we noted the simultaneous passages of small groups of fish numbering from 2 to 6 individuals.

Fig. 6.
figure 6

Daily dynamics of migration of the anadromous rainbow trout Parasalmo mykiss, which passed through the controlled section of the Kvachina River (averaged data).

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A comparative analysis of the size composition of fish showed that the calculations based on the data of hydroacoustic observations, showed a high level of similarity with the data obtained by direct measurements of the length and body weight of fish in catches (Table 1; Fig. 7). This indicates the high accuracy of the hydroacoustic estimations of the size composition of migratory spawners of anadromous rainbow trout. Thus, according to different methods of fish size analysis, the majority of spawners were individuals with a body length from 700 to 850 mm and modal size classes of 750 mm and 800 mm in 2010 and 2011, respectively.

Table 1. Dimensional and weight indicators of the rainbow trout Parasalmo mykiss specimens from the Kvachina River population
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Fig. 7.
figure 7

Size composition of Parasalmo mykiss spawners in the Kvachina River in 2010 (a) and 2011 (b), estimated by catches (◼) or according to hydroacoustic observations (◻).

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DISCUSSON

The local population of the rainbow trout in the Kvachina River includes individuals with three types of life strategy: typical migratory type, migratory-B type (including the half-pounder stage), and half-pounders (Pavlov et al., 2001, 2007). At the beginning of the XXI century, single individuals with a river type of life strategy appeared (Kuzishchin et al., 2020). The typical anadromous rainbow trout sharply dominates in the composition of the population (Pavlov et al., 2001; Kuzishchin et al., 2020). This ratio of fish can be explained by the geomorphological structure of the Kvachina River, which belongs to the canal-type rivers with no estuary and liman. The main factors that determine the type of life strategy of the rainbow trout in the Kvachin river include the reproduction conditions of the species and the productivity of the river, the availability of sufficient areas for spawning of large fish and wintering pits (Pavlov et al., 2008), as well as the availability of feeding areas for juveniles in the riverbed. The main wintering pits for the rainbow trout are located in the river channel at a distance of 18–40 km from the mouth. So, all spawners that come from the sea have to pass through the site controlled by hydroacoustic equipment. Thus, the counting of the anadromous migrants in the lower reaches of the river is a quite accurate estimate of the number of spawners of anadromous rainbow trout of the local population of the Kvachina River.

The anadromous migration of the rainbow trout from the sea to the river begins from the last days of August, and its intensity gradually increases during September (Savvaitova et al., 1973; Maksimov, 1976; Pavlov et al., 2001). The peak of migration is observed in the first weeks of October. Then, in the third week of October, the intensity of the movement decreases and sometimes drops down to a complete stop. Under certain weather conditions (the absence of a prolonged drought and a decrease in the river water level), in late October–early November, the intensive movement resumes again and, possibly, continues under the ice (Pavlov et al., 2001). The intensity of migration varies from year to year and is closely related to the temperature and water level in the river. According to our observations, at a low water level and ice formation on shallow riffles, the intensity of fish movement decreases sharply and fish mainly accumulate in the lower reaches of the river (18–25 km) in wintering pits. This was induced by a small amount of precipitation and, as a result, a low water level on the riffles.

The study of anadromous fish migrations via hydroacoustic equipment was started in 1960–1970. At that period the studies were focused on the direction and rhythm of fish migrations in the area of operation of hydrotechnical facilities (Tesch, 1964; Braithwaite, 1971, 1975; Kristinsson and Alexandersdottir, 1978) and were associated with hydroconstruction (Poddubnyi, 1971; Pavlov, 1979, 1989). The accounting systems used were based on various physical principles, but the observations were made mainly in anthropogenically-transformed environments. In these cases, the partly modified behavior of fish was studied, although the general patterns of their migrations— rhythm, speed, direction of movement, and composition of the fish aggregations—did not change compared to natural conditions (Pavlov, 1979, 1989; Pavlov et al., 2009a; Pavlov and Skorobogatov, 2014). Thus, the use of the NetCor hydroacoustic system provides the identification of the migration patterns of spawners of the Atlantic salmon Salmo salar affected by the fish counting barriers on the Shuya River (Karelia) (Pavlov et al., 2011) and the sockeye salmon Oncorhynchus nerka in the Ozernaya River (Kamchatka) (Degtev et al., 2012). It has been established that fish counting barriers has a significant impact on the spawning migration of these fish. The net obstacle disrupted the migration regime of spawners, which accumulated in front of the barrier and chaotically moved within the river water area adjacent to the fish counting barriers; some individuals rolled downstream (Pavlov et al., 2011; Degtev et al., 2012).

Research conducted using DIDSON sonar on the Utkholok River on September 24–October 15, 2007, evaluated the abundance of anadromous rainbow trout and studied the dynamics of its migration and daily activity. Data on the number of rainbow trout were obtained during round-the-clock stationary observations of the river section (2563 ± 374 ind.) and in mobile mode (echo surveys of the river water area, 6580 ± 640 ind.). Approximation of the obtained data resulted in the calculation of the total abundance of the rainbow trout, which amounted to 10  800 ± 1060 individuals (Pavlov et al., 2009). Stationary observations provided estimation of the abundance only for a 20‑meter section of the channel (distance from the sonar antenna), while the width of the Utkholok River in this place reached 40 m. Such location of the device was limited by the technical characteristics of the sonar (Pavlov et al., 2009). Considering the foregoing, it is permissible to assume that the actual number of anadromous rainbow trout in the Utkholok River can exceed the received estimate by two or more times.

The use of the NetCor system on the Kvachina River showed that the abundance of anadromous rainbow trout differed significantly even in the neighboring years. Thus, for the entire period of autumn migration in 2010, the number of fish (taking into account extrapolation) amounted to 9832 individuals, while in 2011, to 2855 individuals. The calculations of the abundance included only those fish that were oriented against the river current and which size exceeded 500 mm. This approach allows us to take into account only the anadromous rainbow trout, the minimal size of which in the Kvachina River is 535 mm (Pavlov et al., 2001). At the same time, in contrast to studies on the Utkholok River, the stationary observations on the Kvachina River consist of the sounding over the entire area of the controlled section of the river. Thus, all passing fish was registered and, thereby, we obtain representative data on the abundance of anadromous rainbow trout.

Such a significant difference in the number of the anadromous rainbow trout in the neighboring years can be explained by a number of factors. One of them is the geomorphological features and hydrological regime of the Kvachina River. The Kvachina is small (the width of the river is ~25–30 m in parts of the lower reaches, and the depth at riffles in low water is 25–30 cm). Its hydrological regime highly depends on precipitation. In some years (2014, 2018, 2020, and 2021), due to a prolonged drought in September, the river level at the riffles dropped to 15–20 cm, which prevented the upstream migration of large fish.

A sharp increase in the upstream movement of the rainbow trout in certain dates (October 4 and October 11, 2010; October 6 and October 11, 2011), in our opinion, resulted from the intensive entry of spawners from the sea. The coastal area of Northwestern Kamchatka has a complex rhythm of tidal phenomena, when the so-called “double low waters” (semidiurnal tide) are replaced by “large single waters” (diurnal tide) (Chernyavskii, 1981; Luchin, 1998; Lotsyya Okhotskogo morya, 2007; our observations). During a short period (~4–5 days) of large single waters, the water level rises in the lower reaches of the river with an increase in the absolute level by 1.0–1.2 m. The influence of large single waters extends up to 13–14 km from the river mouth. It is very likely that during this period favorable water level conditions promote the migration of anadromous rainbow trout from the sea into the river. In addition to the increase in the water level in the river in the first decade of October, the water temperature drops below 5°C. Earlier, it was noted that the anadromous migration of the trout is activated by the decrease of the river water temperature (Savvaitova et al., 1973; Maksimov, 1976). The established fact of a sharp increase in the migration of anadromous rainbow trout on similar dates of the neighboring years (2010 and 2011), indicates the cyclical nature of the migration of the species associated with tidal cycles, as well as changes in temperature and water level in the river.

The dynamics of the anadromous migration of the rainbow trout in 2020 differed sharply from both the long-term average and anomalous (observed in the previous year, 2019) ones. Throughout September, the water level was abnormally low: –41 cm compared to the long-term average in the low season of September, 63 cm of the conventional standard. As a result, the flow velocity in the river was only 0.1 m/s, the width of the river decreased to 4–5 m at the riffles, and the water level over the riffles fell to 10–12 cm. Due to the low water level on the riffles, the entry of migratory fish into the river was significantly hampered. Single spawners of anadromous rainbow trout were noted mainly in the lower reaches of the river (in the zone of action of the sea tide), when the water level on the riffles rose by 40–50 cm. According to our observations, until October 6, 2020, the anadromous rainbow trout did not migrate upstream than the areas in only 5–6 km from the mouth. Thus, there is a reason to say that by the end of the first week of September, not a single individual had migrated to areas with deep pits, suitable for wintering for large migratory fish. The active upstream migration of the anadromous rainbow trout in 2020 began only after October 14–15, after heavy rains and a significant rise in the water level by 80–100 cm.

During the years of hydroacoustic studies, the weather conditions and the level regime of the river varied significantly. In 2010, the autumn was rainy, the water level in the Kvachina river was elevated (~+50–55 cm in relation to the average long-term values for September and +55–60 cm, for October). Under these conditions, the anadromous migration of the rainbow trout in September was leveled, and its intensity increased only in early October. But in 2011, September was dry, the water level in the river was extremely low (comparable to 2020), and the anadromous rainbow trout practically did not migrate upstream. In early October 2011, there were light rains, due to which the water level in the river rose by about 30 cm, which led to an intensification of the migration, however, by mid-October, due to clear and frosty weather, the water level again fell by 20 cm. Heavy rains in the area were recorded only after October 17, 2011. Thus, in 2011, apparently, there was a shift in anadromous migration to a later date, when hydroacoustic surveys have been already completed.

The population estimates were carried out on the small Kvachina River for the first time. Previously, hydroacoustic surveys were carried out in more or less large rivers, where water level changes do not lead to dramatic shallowing of riffles and sharp modifications in the dynamics of anadromous migration. In this regard, the results of these studies can be considered as the basis for further improvement of the methods of hydroacoustic assessment of the abundance of anadromous fish. In particular, more complete and prolonged studies over the anadromous migration period and simultaneous collection of data on the water level and temperature regime of the river is needed to obtain more accurate estimates. Also hydroacoustic research should be considered as an integral element of integrated monitoring of the state of local populations of the species, included in the Red Data Book of Russia, the anadromous rainbow trout of Kamchatka.

CONCLUSIONS

The use of the modern hydroacoustic multibeam research system NetCor provides opportunity to study the patterns of spawning migration of the anadromous rainbow trout of the Kvachina River in the first half of October in 2010 and 2011 and to obtain representative data on its abundance during the autumn migration period.

The dynamics of migration of the anadromous rainbow trout that passed through the river section during the observation period is characterized by an increase in the intensity of fish movement in similar periods of the neighboring years (2010–2011), which indicates the cyclic migration of the species associated with changes in temperature and water level in the river.

Also there are two periods of a significant increase in the intensity of the movement in the daily dynamics of spawning migration of fish: the beginning of the first was coincide with dawn, the second began in the evening hours. The maximum values of the movement intensity are noted from 18:00 to 22:00. Moreover, during periods of increased intensity of movements, simultaneous passages of small groups of fish from 2 to 6 specimens were recorded, while in the rest of the time only single individuals passed through the surveyed river section.

The number of spawners of the autumn rainbow trout obtained as a result of direct counting was 3081 and 637 individuals in 2010 and 2011, respectively. Close values of the results of counting and their approximation for the period of observation made it possible to extrapolate the data obtained for the entire expected period of autumn migration of the trout spawners from September 1 to October 30. The extrapolation of the results suggested that the estimated number of rainbow trout in 2010 was 9832 and in 2011, 2855 individuals.

The difference in the abundance of anadromous rainbow trout in the neighboring years is probably determined by the very low water level in the river in 2011, which resulted from the geomorphological peculiarities and the hydrological regime of the Kvachina River that, in turn, depends on the amount of precipitation that determines the depth of water on the riffles.