Radionuclides entering water bodies undergo various processes of distribution of substances between bottom sediments, water, and biological components of ecosystems, accumulating in aquatic organisms, particularly fish [1,2,3,4,5,6,7,8,9,10,11]. In water bodies that have been exposed to strong radionuclide impact as a result of the accident at the Chornobyl NPP and other man-made accidents at nuclear power plants, radionuclide accumulation in aquatic organisms can occur to very dangerous levels [12,13,14,15,16,17,18,19,20,21,22,23]. At the present stage, in the water bodies of the Chornobyl Exclusion Zone (ChEZ), the main dose-forming radionuclides for aquatic organisms are ⁹⁰Sr and ¹³⁷Cs [24,25,26,27,28,29,30,31,32,33,34,35,36]. In Ukraine, the State Hygienic Standards “Permissible Levels of ⁹⁰Sr and ¹³⁷Cs in Food and Drinking Water” were approved, in which the permissible specific activity of these radionuclides in fish was set at 35 and 150 Bq/kg, respectively [37].

The goal of our study was not only to analyze the levels of accumulation of long-lived radionuclides ⁹⁰Sr and ¹³⁷Cs in fish of various species from a dammed (conditionally non-flowing) reservoir, which is now the Yanivskyi backwater (Yaniv backwater) of the Pripyat River, but also to calculate the radiation dose rate to fish in this reservoir. Assessment of the dose rate to fish is also an important component of comprehensive radioecological and radiobiological studies of water bodies subjected to intense radionuclide contamination. The final task of the work was to assess the levels of absorbed irradiation dose rate (AIDR) of radiation received by representatives of the most common fish species from the Yanivskyi backwater of the Pripyat River in the ChEZ due to external and internal sources of ionizing radiation in the pre-war period (as of 2020-2025).

The Yanivskyi backwater, separated from the main channel of the Pripyat River by an alluvial dam after the Chornobyl accident, can be divided into two sections: a deep-water (with depths of 6-14 m) lake-like stretch with a water surface area of 0.84 km² and a spur extending from it (with a width of up to 100 m and depths of no more than 6 m) with an area of 0.10 km². Silty bottom sediments occupy an area of about 0.43 km², sandy sediments - about 0.54 km². The Yanivskyi backwater is located 2-3 km from the destroyed unit and in 1986, was one of the most radionuclide-contaminated water bodies in the ChEZ. During the construction of the Chornobyl NPP and the city of Pripyat, the backwater was expanded and deepened to accommodate the river port of the of Pripyat. By the autumn of 1986, the Yanivskyi backwater, like other backwaters in the near ChEZ, was covered with a bulk dam to contain radioactive fallout that had entered its waters. The volume of water masses in the backwater is about 3.7 million m³ with a water surface area of 0.94 km² [38].

The research was carried out mainly during the period 2020-2025 in the once-quite-contaminated-with-radionuclides Yanivskyi backwater of the Pripyat River, adjacent to the city of energy workers, Pripyat, which was evacuated in 1986. During the active phase of the Chornobyl accident, a large number of various ships and barges with cargo arrived at the city’s pier, which is located on the shores of the Yanivskyi backwater. It was used to overcome the consequences of the accident. Port cargo cranes still stand on the shores of the bay. Rusted port cargo cranes still stand on the shores of the backwater. Fish for research were selected in accordance with the regulations for radioecological monitoring of water bodies and in cooperation with the State Specialized Enterprise “Ecocentre” of the State Exclusion Zone Management Agency of Ukraine.

In total, 10 fish species were studied – 4 predatory and 6 prey fish. Among the predatory fish, the following were analyzed: pike Esox lucius L. (3-5 years), European perch Perca fluviatilis L. (3-5 years), common zander or pikeperch Sander lucioperca L. (2-4 years), European catfish Silurus glanis L. (5-6 years). Among the prey fish species, phytophagous species were studied - common rudd or redeye Scardinius erythrophthalmus L. (3-9 years); zooplanktonophagous species - bleak Alburnus alburnus (3-5 years) and zope or blue bream Ballerus ballerus L (3-6 years); benthophagous species - common bream Abramis brama L. (3-7 years), common tench Tinca tinca L. (3-7 years) and roach Rutilus rutilus L. (4-9 years). The classification of fish is based on the predominant feeding type for the indicated age groups, as described in [39,40]. More than 250 fish specimens were analyzed. The average number of fish in the sample for each species was at least 10. The exceptions were representatives of such predatory fish species as European catfish and common zander, of which there were fewer than 10. Fish sampling was carried out three times a year, according to the “Ecocentre” regulations, during expeditions to the ChEZ by employees of the Institute of Hydrobiology of the NAS of Ukraine. Expeditions are planned, usually at the same time of departure - this is the end of April-beginning of May, August, and November. Therefore, seasonal fluctuations in activity concentration in fish, if any, could not affect the average annual values of activity concentration in fish of the water body.

Measurements of activity concentrations of ¹³⁷Cs in water, bottom sediments, and fish were performed based on a γ-spectrometric complex consisting of a GC4018-DET semiconductor coaxial detector; a Lynx digital spectrum analyzer, S502C GENIE-2000 basic software, and LS06067 lead shielding (“Mirion Technologies – Canberra”, USA). Water samples were collected from a boat with a bathometer at the fish and sediment sampling sites. Depending on the water body, water samples were collected in 10-20 L sampling containers. The determination of ⁹⁰Sr activity concentrations was performed by radiochemical method using the oxalate method with measurement of the ⁹⁰Y daughter product using a low background setup (UMF-2000) [41]. Part of the samples was measured without a preliminary radiochemical procedure using a beta-radiation energy spectrometer (SEB-01-70, Ukraine). The values of the specific activity of radionuclides are given in the whole fish organism in Bq/kg mass at natural humidity. The measurement error was 10-25%. For the analysis of experimental values – obtaining standard deviations, average values, and correlation coefficients, the standard MS Excel set was used. The calculated standard deviation fully characterized the sample variations according to [42,43]. Depending on the bottom topography of the reservoir and the composition of bottom sediments in the places of fish catching, bottom sediments were collected in the amount of 1 to 3 samples using a device for selecting bottom sediments - bottom scoop SBS-100. In places where the bottom was homogeneous with the same depth along the net, 1 sample was collected in the central part. If the bottom sediments were heterogeneous (silted sand or silt), 2-3 samples were collected. In addition, bottom sediments were collected in shallow water, as well as in places where fish spawning was observed (in particular, egg laying on higher vegetation) and in pits. In the case of overlapping channels with different depths by nets, bottom sediments were collected at the edges of the net and in the center. Bottom sediment samples were taken, as a rule, from 0-35 cm well mixed.

The absorbed irradiation dose rate in fish was determined using a modified method described in [44,45], based on software [46], using the features of seasonal migratory behavior of different fish species [39,40].

The study of representatives of the ichthyofauna of the ChEZ water bodies poses certain difficulties. This is primarily due to the heterogeneity of radionuclide contamination in bottom sediments, which affects the diversity and differences in contamination of the fish organism itself. Often, in catches, when analyzing the activity concentration of radionuclides in fish caught in the same place in the reservoir and of the same size, it was observed that the activity concentration limits often differed greatly from each other. This, in our opinion, is related to the mosaic nature of radionuclide contamination of bottom sediments and ultimately fish food at a specific point on the bottom (vegetation that grows here, mollusks, benthic organisms, etc.). The activity concentrations across radionuclides in water and bottom sediments of various ecological zones of the studied Yanivskyi backwater are given in Table 1.

As our previous studies [25,27,38,44,45,47,48,49,50] show, the highest radionuclide activity concentrations are characterized by fish from lakes located in the western and southwestern traces of the Chornobyl NPP accident emissions. For a visual comparison, we give Table 2 with the content of radionuclides in several representatives of the studied ChEZ water bodies, such as Vershyna Lake, Glyboke Lake, and the North-Western part of the Chornobyl NPP Cooling Pond (CP). The North-Western part, which became independent, is one of three water bodies that formed on the site of the former CP, after the water level dropped since 2014.

As can be seen from Table 2, representatives of the ichthyofauna of the Yanivskyi backwater at this stage have comparable levels of radionuclide contamination of ⁹⁰Sr and ¹³⁷Cs with fish from the CP and significantly lower radionuclide content than fish from Vershyna Lake and Glyboke Lake.

In general, for all studied representatives of the ichthyofauna of the Yanivskyi backwater of the Pripyat River, as of 2020-2025, the activity concentration of ⁹⁰Sr was within 462-3900 (1331 ± 383), and ¹³⁷Cs – 217-2313 (667 ± 184) Bq/kg. Thus, the activity concentration of ⁹⁰Sr and ¹³⁷Cs in prey fish was, respectively, 709-3900 (1659 ± 467) and 217-2227 (493 ± 121) Bq/kg, and for predatory fish, 462-1500 (787 ± 149) and 483-2313 (954 ± 205) Bq/kg, respectively.

Among the studied fish species of the Yanivskyi backwater that have a high ability to accumulate ⁹⁰Sr and ¹³⁷Cs, it is worth noting the rudd, which is characterized by very high levels of activity concentration of the radionuclides. The rather high levels of radionuclide accumulation in rudd are associated with plant food and, in particular, algae covered with phytoepiphyton, which are characterized by an extremely high ability to accumulate radionuclides [51].

Predatory fish are traditionally considered to be the group that intensively accumulates ¹³⁷Cs. In the Yanivskyi backwater, they were represented mainly by perch and pike. High levels of ¹³⁷Cs accumulation in ichthyophagous fish are a well-known phenomenon in radioecology, which is associated with the effect of trophic levels in the assimilation of the radionuclide from the muscle tissues of prey fish species (which are mainly fed by predatory fish) and its increased accumulation in the body of predators. ⁹⁰Sr in fish is mainly found in tissues that are poorly digestible – bones and scales, head, and fins. Its accumulation by predatory species is less efficient than by “peaceful” species [25,38,44,52].

The activity concentrations of radionuclides in fish of the Yanivskyi backwater in 2020-2025, in all cases, exceeded the permissible levels set by the standards adopted in Ukraine for fish products [37] – 13-111 times for ⁹⁰Sr and 2-15 times for ¹³⁷Cs.

Figure 1 and Figure 2 show the dynamics of the activity concentration of radionuclides in fish from the Yanivskyi backwater of the Pripyat River using the example of two fish species - rudd and perch, which dominated catches during 2008-2025 and are the most common in the water bodies of the ChEZ.

Analysis of the data obtained for the studied period shows that the activity concentration of ¹³⁷Cs in all representatives of the ichthyofauna from the Yanivskyi backwater continues to decrease with certain fluctuations, as shown in Figure 1 and Figure 2 using the example of rudd and perch. The content of ⁹⁰Sr in representatives of different species of fish in this water body decreases rather smoothly or, practically, remains at the same level, with certain deviations in both directions. This is primarily due to the increase in the activity concentration of ⁹⁰Sr in groundwater of the surrounding areas, and accordingly in the water of the Yaniv backwater, which has been observed more noticeably for the lakes of the left bank floodplain of the Pripyat River (Vershyna Lake, Glyboke Lake, etc.) and Azbuchyn Lake (located on the right bank of the Pripyat River) since the late 1990s [27,38,48]. Subsequently, such jumps in the ⁹⁰Sr content were recorded in terrestrial plants and hydrobionts – higher aquatic plants, mollusks, and fish [24,25,35,36,49]. But, if in closed water bodies such as Azbuchyn Lake, Vershyna Lake, or Glyboke Lake, the activity concentration of ⁹⁰Sr increases in fish, then there is probably a certain outflow of this radionuclide with the water masses through the embankment dam that separates the backwater from the Pripyat River. In connection with which, we classified the Yanivsky backwater to a conditionally non-flowing water body.

The fish of the studied water body differ significantly not only in the total specific activity of radionuclides, but also in their ratio in the body of various representatives of the ichthyofauna. As of 2020-2025, the activity concentration of ⁹⁰Sr in prey fish of the Yanivskyi backwater exceeds the activity concentration of ¹³⁷Cs by 1.5-11.9 times, and in predatory fish, this indicator is 0.3-2.6.

Thus, the activity concentration of radionuclide ¹³⁷Cs in predatory fish of the studied backwater, in the vast majority of cases, exceeds the activity concentration of ⁹⁰Sr in their body. Prey fish accumulate ⁹⁰Sr more effectively, whereas predatory (ichthyophagous) fish accumulate higher levels of ¹³⁷Cs. This is because the radionuclide ¹³⁷Cs is mainly found in the muscle tissues of fish, which the predator feeds on, and is absorbed by the predator’s body. At the same time, ⁹⁰Sr is mainly concentrated in inedible calcium-containing organs and tissues (scales, bones, head, fins) that transit through the body of the ichthyophagous organism.

Therefore, knowing the activity concentrations in the abiotic components of the ecosystem of a water body and the activity concentration of radionuclides in fish, it is possible to calculate the dose rate experienced by representatives of the ichthyofauna of water bodies and the consequences associated with it [28,29,44]. According to our calculations, the average total radiation dose rate to the body of various fish species in 2021 ranged from 2.0 to 8.7 μGy/h. The highest absorbed dose rate was recorded in bottom-dwelling fish species, as shown in Figure 3. And the smallest is in the pelagic bleak.

As is known, the total dose rate to fish consists of the dose from external radiation exposure (from radionuclides in the shores of the water body, bottom sediments and water column) and the dose received from internal sources of radiation, i.e. from radionuclides incorporated into fish (internal radiation dose).

In general, the absorbed dose rate of radiation to fish in the studied reservoir from external sources depends on the radionuclide ¹³⁷Cs by 94-99%. While the absorbed radiation dose rate to fish from radionuclides incorporated into fish (internal radiation dose) is 58-93% due to ⁹⁰Sr. Figure 3 also shows that the total radiation dose to representatives of various ichthyofauna species in the Yanivskyi backwater is 58-85% due to the absorbed dose received by fish from irradiation with ¹³⁷Cs. Basically, this is the fraction received by fish due to external exposure to ¹³⁷Cs from radionuclide contamination of the bottom sediments, shores, water, etc. (because the fraction of internal absorbed dose from incorporated ¹³⁷Cs is generally negligible, as shown in Figure 3).

For fish in this water body, the total dose rate consists of 59-87% from external exposure and only 13-41% from the internal absorbed dose of exposure from radionuclides incorporated into the body. Our research also showed that the total absorbed dose rate (external and internal) received by the fish of the Yanivskyi backwater in 2025 from ⁹⁰Sr exposure was only 12-37%. It should be noted that the role of ⁹⁰Sr in the formation of the total radiation dose to fish is significantly higher for closed water bodies - lakes of the left-bank floodplain of the Pripyat River and the right-bank Azbuchyn Lake [25,45,50].

Thus, the recorded levels of the average annual absorbed dose rate for practically all studied fish of the Yanivskyi backwater exceed the screening dose of 2 μGy/h and are lower than the safe level of 10 μGy/h recommended within the framework of the European Commission’s “PROTECT” Project [53,54].

Based on our research on the ichthyofauna of the conditionally impermeable, dammed Yanivskyi backwater of the Pripyat River, the following has been established:

1. The activity concentration of ⁹⁰Sr in representatives of the ichthyofauna of the Yanivskyi backwater was within the range of 0.46-3.90 kBq/kg, and ¹³⁷Cs – 0.22-2.31 kBq/kg. The activity concentration of radionuclides in fish from the water body during the research period often exceeded the permissible levels, according to the standards adopted in Ukraine for fish products – 13-111 (on average 39) times for ⁹⁰Sr and 2-15 (on average 4) times for ¹³⁷Cs.

2. The activity concentration of ⁹⁰Sr in prey (“peaceful”) fish species of the Yanivskyi backwater was on average 2.0 times higher than in predatory fish (except for the pelagic bleak). At the same time, ¹³⁷Cs was, on average, 2.8 times lower. The activity concentration of ⁹⁰Sr in prey fish species as of 2021-2025 was in the range of 0.71-3.90, and in predatory fish, within 0.46-1.50 kBq/kg, while the activity concentration of ¹³⁷Cs in prey and predatory fish species was, respectively, 0.22-2.23 and 0.48-2.31 kBq/kg. As of 2020-2025, the activity concentration of ⁹⁰Sr in prey fish species of the Yanivskyi backwater exceeds the activity concentration of ¹³⁷Cs by 1.5-11.9 times, and in predatory fish, this indicator is 0.3-2.6. Among the studied fish of the water body, the highest activity concentration of ⁹⁰Sr was noted in the rudd.

3. The activity concentration of ¹³⁷Cs in fish of the studied water body during the research period continued to decrease naturally with fluctuations within the limits of variation for different samples. The ⁹⁰Sr activity concentration in representatives of the ichthyofauna of this water body remained at a practically constant level or showed a barely noticeable tendency to decrease.

4. The average total absorbed dose rate to various fish species in the studied water body as of 2020-2021 was within the range of 2.0-8.7 μGy/h. The share of the total dose rate from exposure to only ¹³⁷Cs received by fish from the Yanivskyi backwater is more than half of the total dose to fish with radionuclide exposure, namely 58-85%. It arose mainly from external exposure to fish (via radionuclides in bottom sediments, water, shores, etc.). Although the main contribution of ¹³⁷Cs to the fish’s entire body may be due to external irradiation, the internal dose to individual organs, such as bones, spinal cord, and brain, is significantly higher and is caused by ⁹⁰Sr (which accumulates most in calcium-containing organs and tissues of fish - the head, spine, bones, and scales).

5. The calculated levels of the average annual absorbed dose rate for practically all studied fish from the Yanivskyi backwater exceed the screening dose of 2 μGy/h, but do not exceed the safe level of 10 μGy/h, recommended within the framework of the European Commission’s “PROTECT” Project and some other documents [53,54] (unlike the lakes, which are also located in the 10 km ChEZ).

This study was supported by the National Research Foundation of Ukraine (project No. 2023.03/0156), by the National Academy of Sciences of Ukraine, as well as in cooperation with the State Specialized Enterprise “Ecocentre” of the State Exclusion Zone Management Agency of Ukraine and the Chornobyl Radiation and Ecological Biosphere Reserve.