Competition differentiates 137Cs concentrations in sympatric salmonids

Competition differentiates 137Cs concentrations in sympatric salmonids | Research Square window.SnipcartSettings = { analytics: { enabled: false } }; (function() { var accessVector = localStorage.getItem('access_vector') || ''; window.dataLayer = window.dataLayer || []; if (accessVector) { window.dataLayer.push({ user: { profile: { profileInfo: { snid: accessVector } } } }); } })(); (function(w,d,s,l,i){w[l]=w[l]||[];w[l].push({'gtm.start':new Date().getTime(),event:'gtm.js'});var f=d.getElementsByTagName(s)[0],j=d.createElement(s),dl=l!='dataLayer'?'&l='+l:'';j.async=true;j.src='https://www.googletagmanager.com/gtm.js?id='+i+dl;f.parentNode.insertBefore(j,f);})(window,document,'script','dataLayer','GTM-K279D39R'); Browse Preprints In Review Journals COVID-19 Preprints AJE Video Bytes Research Tools Research Promotion AJE Professional Editing AJE Rubriq About Preprint Platform In Review Editorial Policies Our Team Advisory Board Help Center Sign In Submit a Preprint Cite Share Download PDF Article Competition differentiates 137 Cs concentrations in sympatric salmonids Masaru Sakai, Yumiko Ishii, Seiji Hayashi, Kazuyoshi Takasaki, and 4 more This is a preprint; it has not been peer reviewed by a journal. https://doi.org/ 10.21203/rs.3.rs-8177095/v1 This work is licensed under a CC BY 4.0 License Status: Under Review Version 1 posted 11 You are reading this latest preprint version Abstract Sympatric headwater salmonids compete over food items that are the primary 137 Cs sources for fishes. This ecological process driven by competition is expected to be an important determinant of 137 Cs concentrations in headwater salmonids, but little is known regarding the process. Here, we investigated how a stronger competitor salmon ( Oncorhynchus masou ) affects 137 Cs concentrations in them and a sympatric weaker competitor charr ( Salvelinus leucomaenis ) in reaches of a highly contaminated headwater system in Fukushima, Japan. The results indicated that higher proportion of sympatric O. masou individuals within salmon communities lowers 137 Cs concentrations in both species, and the effect is more pronounced for S. leucomaenis . Because stronger salmonid competitors exploit falling and drifting terrestrial prey, which is generally more contaminated than aquatic one, O. masou could monopolize terrestrial prey, thereby be more contaminated than did S. leucomaenis as a consequence of interspecific competition. Although previous studies have reported the effects of abiotic environmental factors such as water quality and 137 Cs deposition on 137 Cs activity concentrations in freshwater salmonids, the present study is the first to discover that competition for food resources is an important determinant of 137 Cs concentrations in salmonids. The findings of this study open a new research area in which the ecological process of interspecific competition can affect radiocesium dynamics in headwater salmonids. Biological sciences/Ecology Earth and environmental sciences/Ecology Earth and environmental sciences/Environmental sciences Biological sciences/Zoology forest-stream ecosystem freshwater fish interspecific interaction radiocesium Figures Figure 1 Introduction Headwater salmonids are one of the most important inland fishery species, but nuclear accidents have caused persistent contaminations of them 1 , 2 because headwaters experience little human activities and are therefore generally not decontaminated 3 . Thus, factors affecting the radiocesium concentrations in headwater salmonids have been investigated from the viewpoints of radiocesium excretion and uptake by the fish for projecting and managing fish contaminations 1 , 2 , 4 – 8 . However, in the viewpoint of radiocesium uptake by fish, how ecological processes driven by interspecific competitions over food items influence the radiocesium contaminations in sympatric headwater salmonids is poorly understood, though the salmonids are of members in complex food webs in natural ecosystems 9 , 10 . Radiocesium excretion by fish is controlled by body size and temperature 2 , 11 – 13 . As specific metabolic rate of fish is determined negatively by its weight, many studies reported that positive size effects on radiocesium concentrations in salmonids 5 – 8 , 14 , 15 . Also, as specific metabolic rate changes with e –1/T , where T is the absolute temperature 11 , it shows clear seasonality (high in summer and low in winter in poikilothermic animals, including fishes). Thus, radiocesium concentrations in salmonid fish become high in winter and low in summer if radiocesium uptake is stable throughout the season 2 . While the relationship between radiocesium excretion and its concentrations in fishes is simple, radiocesium uptake by fish is relatively complicated 16 . Several previous studies reported evidence regarding the effects of radiocesium uptake through foraging on its concentrations in wild freshwater fishes from their conceptualized frameworks 2 , 16 . The studies suggested that consumptions of more contaminated prey can elevate the body burden of radiocesium in fishes, but the seasonality of radiocesium concentrations in fishes can also be affected by seasonal changes in radiocesium excretion through their metabolisms. The balance between radiocesium uptake and excretion is species-specific 16 , and thus seasonality of radiocesium concentration in fishes can exhibit various patterns. Meanwhile, Wada et al. 7,8 recently demonstrated that more contaminated terrestrial prey than aquatic ones, particularly supplied in summer and autumn seasons, elevate the radiocesium concentrations in headwater salmonids in highly contaminated watersheds in Fukushima. While the clear difference of radiocesium concentrations between terrestrial and aquatic preys had been known in highly contaminated headwater ecosystems 2 , 17 , the studies first clarified such concentration difference in preys actually influences the seasonal dynamics of radiocesium in headwater salmonids 7 , 8 . Headwater salmonids depend on energy flows from both terrestrial and aquatic preys 9 , 19 . Because headwater streams are often covered with forests, limitation of sunlight forms pronounced detrital food webs in both terrestrial and aquatic ecosystems 17 , 18 . Further, contaminated plant litter leaches radiocesium when submerged into streams 19 , and the difference of radiocesium concentrations between terrestrial and aquatic litters becomes greater in more contaminated sites 20 , 21 . Thus, radiocesium concentrations in terrestrial prey are generally higher than those in aquatic prey particularly in highly contaminated headwater systems owing to the pronounced detrital food webs 2 , 18 . These findings are supportive explaining that radiocesium in terrestrial prey is of most potent contaminants transferred to headwater salmonids in highly contaminated regions 7 , 8 . Falling and drifting terrestrial prey is a significant energy source for headwater salmonids 9 , 10 , and thus competitions over terrestrial prey induce biased consumption rate between sympatric salmonid species. For example, Oncorhynchus salmons often exploit terrestrial prey more than Salvelinus charrs under the competitions 22 , 23 . Meanwhile, such charrs actively consume terrestrial prey where the salmons are absent 23 , 24 . Therefore, such interspecific competition is expected to be an important determinant of radiocesium concentrations in headwater salmonids in highly contaminated sites because terrestrial prey is a major radiocesium transporter to the fish. Also, it is reported that radiocesium concentration is higher in O. masou than in sympatric S. leucomaenis in a highly contaminated headwater system in Fukushima 7 , implying possible effects of food competition between them. Here, we hypothesized that presence of stronger competitors ( Oncorhynchus salmon) influence 137 Cs concentrations in both stronger and weaker ( Salvelinus charr) headwater salmonids through intra- and inter-specific competitions. Testing the hypothesis is particularly important to unveil effects of ecological processes on radiocesium dynamics in fishes, thereby to project persistence of radiocesium contaminations in the important fishery species. Results and discussion 137 Cs Concentrations in Headwater Salmonids. Totally, 1,487 individuals of headwater salmonids (masu salmon: 809 individuals, white-spotted charr: 678 individuals) were sampled. The mean, maximum, and minimum 137 Cs activity concentrations were 1,200, 9,140, and 63.5 Bq/kg for masu salmon, and were 1,760, 8,300, and 69.3 Bq/kg for white-spotted charr. Mostly 100% of the individuals (804 masu salmon and 676 white-spotted charr) exceeded the Japanese standard of foodstuffs (100 Bq/kg). The model selection for the linear mixed model (LMM) that included 137 Cs activity concentrations in all the salmonid samples indicated that the full model yielded lowest AIC and the ΔAIC between the first- and second-best models was 70.8. The best model detected statistically significant effects of total length, season, and species on 137 Cs activity concentrations in fish (Table 1 and Fig. 1 ). As widely known from salmonid fishes 5 – 8 , the 137 Cs activity concentrations in masu salmon and white-spotted charr became greater with increase in body size. The positive body size effect on 137 Cs activity concentrations in headwater salmonids might be occurred through lower specific metabolic rates in larger fish 11 , 12 and shifts in food items with body size change 5 – 8 . As the studied salmonids are assumable to be exposed to severe competitions over food resources, larger individuals might exploit terrestrial prey 7 , 8 , thereby increase the 137 Cs concentrations in their body through both intra- and inter-specific interactions 7 , 8 . While we were primarily interested in effects of body size, seasonality, and species on 137 Cs concentrations in fishes rather than that of 137 Cs deposition (and thus this was considered as a random effect in this study), the 137 Cs activity concentrations in salmonids increased with increase in 137 Cs deposition (Fig. 1 b), similarly to the trends found in other taxa 25 – 28 . 137 Cs deposition is thus a prevailing factor determining the 137 Cs concentrations in organisms, and we could confirm that the salmonid fishes were more contaminated in more contaminated sites 7 . Table 1 Results of the best linear mixed model (full model) used to test for effects of total length, seasons, and species on 137 Cs activity concentrations in masu salmon and white-spotted charr. Estimate Standard error t P Total length 0.765 0.024 31.7 < 0.0001 Spring vs summer 0.377 0.037 10.2 < 0.0001 Spring vs autumn 0.673 0.029 23.5 < 0.0001 Spring vs winter 0.457 0.048 9.60 < 0.0001 Masu salmon vs white-spotted charr –0.253 0.028 –8.96 winter > summer > spring (Table 1 ). Because 137 Cs concentrations in fishes were determined based on the balance between 137 Cs excretion and uptake 2 , 16 , seasonality of 137 Cs concentration in fish can be inferred from this balance. A previous study 2 reported that seasonality of 137 Cs concentrations in white-spotted charr varies depending on amount of 137 Cs depositions, which change the balance of 137 Cs excretion and uptake, in streams lacking masu salmon populations. For example, at a less contaminated site (69.0 kBq/m 2 ), effect of 137 Cs leaching from litter on contrast of 137 Cs concentrations between terrestrial and aquatic preys is weak, and thus 137 Cs concentrations in potential preys (a mixture of terrestrial and aquatic preys) exhibit small seasonality. Consequently, seasonality of 137 Cs concentrations in white-spotted charr (i.e., high in winter and low in summer) follows negatively with seasonality of specific metabolic rate. Meanwhile, at a moderately contaminated site (311 kBq/m 2 ), stronger leaching effect enlarges the absolute difference of 137 Cs concentrations between terrestrial and aquatic preys, and those in potential preys become higher in summer when inputs of terrestrial prey into streams peak 9 , but lower in winter. In this situation, seasonality of 137 Cs concentrations in headwater charr is small because seasonal changes in 137 Cs excretion and uptake of headwater charr are seasonally similar. As the previous study discussed (2), in more contaminated sites like our study reaches, seasonality of 137 Cs concentrations in potential preys would show more dynamic seasonality (very high in summer, but low in winter). Notably, rhaphidophorid crickets, which are strongly contaminated terrestrial prey 2 , 18 specifically fall into headwater streams during summer to autumn 29 , 30 . This input may have driven the peak 137 Cs activity concentrations detected in headwater salmonids in autumn in our study, because these fishes consume a substantial mass of fallen crickets in autumn, although they also feed on various other terrestrial and aquatic insects at our study site 8 . Meanwhile, lowest metabolisms might secondarily elevate the 137 Cs concentrations in winter, but greater inputs of terrestrial preys in summer than in spring 2 , 7 resulted higher 137 Cs concentrations in headwater salmonids in summer than in spring. As previously reported elsewhere 7 , the LMM indicated that masu salmon was statistically more contaminated than sympatric white-spotted charr. These results supported our hypothesis that masu salmon exploits more contaminated prey (i.e., terrestrial prey), thereby possesses more 137 Cs in their body because excretion rate is very similar between masu salmon and white-spotted charr 1 . To examine how such unfair competition affects the 137 Cs concentrations in each species, discussion based on the additional two LMMs was extended from the next paragraph. Effects of Food Resource Competition. The full LMMs for each species exhibited lowest AICs. For masu salmon, ΔAIC with the second-best model that lacks masu salmon dominance in the explanatory variables was 1.09, suggesting that smaller effect of masu salmon dominance on the 137 Cs activity concentrations in masu salmon compared to those of total length and seasons. For white-spotted charr, ΔAIC with the second-best model was larger (26.07), similarly to the LMM with pooled data on all the individuals of masu salmon and white-spotted charr. Relationship between 137 Cs activity concentrations and total length was mostly identical between the species (Table 2 and Fig. 1 ). The result suggests that physiological features (i.e., excretion rate of 137 Cs) of the two species are substantially similar 1 , and thus the two species represented the common responses of 137 Cs activity concentrations to body size. This is particularly an ideal condition to test how 137 Cs uptake by foraging induces interspecific difference of 137 Cs concentrations in the two fish species. Table 2 Results of the best linear mixed models (full models) used to test for the effects of 137 Cs deposition, total length, seasons, and masu salmon dominance on 137 Cs activity concentrations in each salmonid species. Estimate Standard error t P Masu salmon Total length 0.751 0.0301 25.0 < 0.0001 Spring vs summer 0.561 0.0555 10.1 < 0.0001 Spring vs autumn 0.829 0.0371 22.4 < 0.0001 Spring vs winter 0.652 0.0621 10.5 < 0.0001 Masu salmon dominance –0.233 0.0890 -2.62 < 0.05 White-spotted charr Total length 0.751 0.0357 21.1 < 0.0001 Spring vs summer 0.555 0.0726 7.65 < 0.0001 Spring vs autumn 0.542 0.0419 12.9 < 0.0001 Spring vs winter 0.670 0.0977 6.86 < 0.0001 Masu salmon dominance –1.58 0.287 -5.50 < 0.0001 Interestingly, the masu salmon dominance negatively affected the 137 Cs activity concentrations in both masu salmon and white-spotted charr, but the effect was stronger in white-spotted charr (Table 2 ). These results firmly supported our hypothesis that exploitation of more contaminated terrestrial prey by stronger competitors influence 137 Cs concentrations particularly in weaker competitors. Although several previous studies have reported the effects of abiotic environmental factors such as water quality and 137 Cs deposition on 137 Cs activity concentrations in freshwater salmonids 5 , 7 , 31 , the present study is the first to discover that competition for food resources is an important determinant of 137 Cs concentrations in salmonids. We infer that this effect is more prominent in highly contaminated regions and sooner after contamination events, as these conditions would increase the absolute difference in 137 Cs concentrations between terrestrial and aquatic preys 2 , 7 , 8 , 20 , 21 . The difference in seasonality of 137 Cs activity concentrations between species was also influenced by food competition. 137 Cs activity concentrations in masu salmon peaked in autumn, when the input of more highly contaminated terrestrial prey reached a maximum 2 , 7 , 8 ; in contrast, 137 Cs activity concentrations in white-spotted charr peaked in winter, when fish metabolic rates were lowest (Table 2 ). These results suggest that exploitation of terrestrial prey by masu salmon increases their 137 Cs concentrations, reducing opportunities for the consumption of terrestrial prey by white-spotted charr and thereby decreasing their 137 Cs uptake in autumn. Consequently, seasonality of 137 Cs concentrations in white-spotted charr is strongly affected by 137 Cs excretion through their metabolisms. For adequate management of 137 Cs contaminations in fishery species, the findings of this study emphasize the importance of interspecific interactions among living organisms on the determinants of contamination levels of focal species. The present study provided important insights into the 137 Cs transfers to animals that interact sympatric organisms. First, stronger competitors can influence both intra- and inter-specific variations in 137 Cs concentrations in fishes through prey exploitations when preys have variable 137 Cs concentrations. Second, habitat segregation formed by such competitions can influence 137 Cs concentrations in a focal fish species. In the case of this study, 137 Cs activity concentrations in white-spotted charr decreased in lower reaches not only by the lower initial 137 Cs deposition, but also by the greater masu salmon dominance. Third, such competition effects are potent enough as is the cases with known factors including effects of 137 Cs deposition, body size, and season on 137 Cs concentrations in headwater salmonids. These insights suggest that ecological processes need to be considered to unravel the 137 Cs dynamics in organisms together with abiotic and biotic processes. The present study successfully tackled into a black box of ecological processes in 137 Cs dynamics of headwater salmonids using the sufficient number of samples of physiologically similar competing species and well-known foraging ecology of the species associated with 137 Cs uptake 2 , 7 , 8 , 16 . The findings of this study open a new research area in which the ecological process of interspecific competition can actually affect 137 Cs dynamics in headwater salmonids. Methods Study Site. This study was conducted in the Ota River, approximately 20 km north of the Fukushima Daiichi nuclear power plant. We set four study reaches (Station 1–4) as our sampling sites (Table 3 ), corresponding to part of the sites investigated in the previous study 32 . Length of the study reaches was ranging from 500 to 1,000 m to obtain representative data with sufficient number of fishes in each study reach. The upmost reach (Station 4) was classified as second-order stream, whereas Station 3 was third-order stream, and Stations 1 and 2 were fourth-order stream according to the Strahler’s rule 33 . Channel morphology of Station 1 was primarily characterized by riffle-pool structure whereas that of Station 4 was primarily step-pool structure, and those of Station 2 and 3 were categorized as mixture of the geomorphologies 34 . All the study reaches were forested with broad-leaved deciduous and evergreen coniferous trees, and the dominant surface geology of the area is the granite of the Mesozoic 16 , 35 . The mean annual precipitation, measured at the nearby Tsushima AmeDAS automated weather station (37.56 N, 140.75 E), from 2017 to 2022 was 1,292 mm. The mean annual air temperature, measured at the nearby Iitate AmeDAS automated weather station (37.67 N, 140.73 E), from 2017 to 2022 was 10.8℃. The 137 Cs deposition after the Fukushima accident was greatest in Station 4 (2,280 kBq/m 2 ), decreased to the downstream, and was lowest in Station 1 (1,169 kBq/m 2 ), estimated by an airborne monitoring survey on July 2 in 2011 (Table 3 ) 25 . Stations 2, 3, and 4 were inside the difficult-to-return zone, which had the highest ambient dose rates among the three government-designated evacuation zones. There was no artificial obstacle among each station, and thus it was assumable that fishes can migrate among the study reaches. The relative abundance of masu salmon to white-spotted charr typically becomes greater in lower reaches in the study site 34 . Table 3 Latitude, longitude, altitude, Strahler stream order, 137 Cs deposition after the Fukushima accident, and ratio of number of the captured masu salmon individuals to that of all the captured salmonid individuals in each study reach. Study reach Latitude (°) Longitude (°) Altitude (m) Strahler stream order 137 Cs deposition (kBq/m 2 ) Ratio of captured masu salmon to total salmonids (%) Station 1 37.57897 140.87520 194 4 1169 69.61 Station 2 37.58950 140.85667 303 4 1632 77.88 Station 3 37.59314 140.83373 380 3 2274 62.68 Station 4 37.59882 140.81549 446 2 2280 9.115 Sampling. At each station, salmonid fishes were collected by angling and electrofishing from 2017 to 2022 under the permission of Fukushima Prefecture (permission numbers: 29 − 5, 30 − 6, 1–20, 2–5, 3–16, and 4–14). Each station contained typical multiple habitat structures (riffle-pool and step-pool), and thus the obtained fish samples were assumable to represent the salmonid assemblages in each station. The samplings were performed in August and October 2017; April, May, August, October, and December 2018; March, June, August, and December 2019; May, June, and December 2020; and June and December in both 2021 and 2022. We defined spring (April–June), summer (July–September), autumn (October–December), and winter (January–March) according to month ranges. Two salmonid species; masu salmon ( Oncorhynchus masou ) and white-spotted charr ( Salvelinus leucomaenis ) inhabited the study site 8 , 34 , and the two species are well known to exhibit longitudinally segregated distribution in headwater channels 29 , similarly to a ubiquity of habitat segregations found in other headwater salmonids 30 . White-spotted charr generally inhabits upmost streams whereas masu salmon inhabits downstream, and intermediate streams often harbor both species 22 , 23 , 34 , 36 . In our study site, all the stations harbored both species, but masu salmon was generally dominated in lower reach whereas white-spotted charr was dominated in upper reach (Table 3 ). The longitudinal habitat segregation is affected by competitions over food resources between the species. Masu salmon generally monopolizes fallen and drifting terrestrial prey, whereas the terrestrial prey becomes significant food items for white-spotted charr where masu salmon is absent 22 – 24 . Thus, abundance ratio of the two species is considered to be a good indicator for consumption ratio of terrestrial and aquatic prey which is an important determinant of 137 Cs concentrations in these salmonids 2 , 7 , 8 , particularly for weaker competitors. All the collected fishes were stored at − 20℃ prior to radioactivity measurements. Radioactivity Measurements. Head and internal organs were removed from each fish individual after defrosting and measurements of total lengths. Dissected remaining tissues were minced and packed into 100 mL plastic containers. The tissues mainly consisted of muscles and their 137 Cs concentrations are empirically evidenced 89% of those in muscle-only samples of the two salmonid species 7 , 8 . The sample weights and densities were measured prior to radioactivity measurement. The 137 Cs activity concentrations in the samples were determined by gamma-ray spectroscopy. Gamma-ray emissions were measured at an energy of 661.6 keV using high-purity germanium semiconductor detector systems (GC3018, GC4018 and GC4020, Mirion Technologies, Inc.) installed at Fukushima University. The accuracy of 137 Cs activity was within 10% (error counts/net area counts). The 137 Cs activity concentrations in each sample were corrected for radioactive decay since the collection dates, and all the concentrations were based on wet sample weights. The counting efficiency of these semiconductor detectors was calibrated using volume standard sources (MX033U8PP for U8 container, The Japan Radioisotope Association). Statistical Analysis. First, a linear mixed model (LMM) was constructed to evaluate the factors affecting 137 Cs activity concentrations in fish in the study stations. The response variable for the LMM was log e -transformed 137 Cs activity concentrations in all the fish samples that included both masu salmon and white-spotted charr, and the explanatory variables were total length, season, and species. The collection year for each fish individual was included in the LMM as a random intercept, because 137 Cs concentrations in fish can temporally be attenuated. Also, the station number was included in the LMM as a random intercept to consider the variability in 137 Cs deposition among the stations. A Gaussian error structure was used for the response variables. Models including all combinations of the explanatory variables were constructed, and models that yielded the lowest Akaike information criterion (AIC, ΔAIC < 2) were used for descriptive purpose 38 . Second, to evaluate the factors affecting 137 Cs activity concentrations in each fish species, we constructed a separate LMM for each species. Because masu salmon exploit terrestrial prey, which are more highly contaminated than aquatic prey 2 , 7 , 8 , the dominance of masu salmon in focal salmonid communities can be a strong determinant of fish 137 Cs concentrations under competitive foraging conditions 22 – 24 . Therefore, we included the mean ratios of individual masu salmon numbers to total salmonid numbers at each station (masu salmon dominance) as an explanatory variable for both LMMs. Masu salmon dominance was calculated based on the total numbers of individuals of the two species in each season at each station because the explanatory variable can change seasonally in response to behaviors such as spawning 39 , 40 . Other explanatory variables included total length of fish and season, and sampling year and station number were included as random intercepts in both the LMMs to consider variabilities associated with time passage and 137 Cs deposition. The best LMMs were also selected using the same method as described above. We tentatively tested variance factors (VIFs) for the explanatory variables in the three LMMs, and all the VIFs were less than 3, indicating that the collinearity of the explanatory variables was negligible for the LMM results. To determine the relative strength of the parameter estimates, the input explanatory variables were standardized to a mean of 0 and a standard deviation of 0.5 before the model was analyzed 41 , which allows a direct comparison of the resulting coefficients between the numerical and categorical predictors. The LMMs were constructed using the lme4 package 42 , model selection was performed using the MuMIn package 43 , standardization of the explanatory variables was performed using the arm package 44 , and VIFs were calculated using the car package 45 of R 4.2.1 46 . Declarations Ethical approval. The studied species, masu salmon ( Oncorhynchus masou ) and white-spotted charr ( Salvelinus leucomaenis ) are not endangered. All fish sample collections were approved by Fukushima Prefecture (permission numbers: 29 − 5, 30 − 6, 1–20, 2–5, 3–16, and 4–14) to comply with relevant legislation regarding the sample collections, and the samples are stored in Fukushima University. We euthanized fish using cold shock in accordance with guidelines for fish research 47 to minimize stresses on the fish. Voucher specimens of fish materials in the present study do not exist. All methods in this study are reported in accordance with ARRIVE guidelines 48 . Competing interests The authors declare that they have no competing interests. Funding Part of this study was financially supported by the commissioned research fund by F-REI (JPFR23050301 and JPFR25050501). Author Contribution Masaru Sakai: Conceptualization, Data curation, Formal analysis, Visualization, Writing – Original Draft. Yumiko Ishii: Conceptualization, Date curation, Formal analysis, Investigation, Resources, Writing – Review & Editing. Seiji Hayashi: Funding acquisition, Investigation, Resources, Writing – Review & Editing. Kazuyoshi Takasaki: Investigation, Resources, Writing – Review & Editing. Wataru Teramoto: Investigation, Resources, Writing – Review & Editing. Yuto Funaki: Investigation, Resources, Writing – Review & Editing. Tsutomu Kanasashi: Data curation, Investigation, Resources, Writing – Review & Editing. Toshihiro Wada: Conceptualization, Data curation, Funding acquisition, Investigation, Resources, Writing – Review & Editing. All authors have given approval to the final version of the manuscript. Acknowledgement We deeply appreciate the tremendous efforts by the staffs of Fukushima Prefecture and the students at Fukushima University to collect fishes and measure radioactivity. We are also grateful to Professor Kenji Nanba of Fukushima University for his valuable support. These efforts were indispensable for testing the hypothesis of this study. Data Availability The datasets generated during and/or analyzed during the current study are available from the corresponding author on reasonable request. References Matsuda, K., Yamamoto, S. & Miyamoto, K. 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Radiocesium-bearing microparticles cause a large variation in 137 Cs activity concentration in the aquatic insect Stenopsyche marmorata (Trichoptera: Stenopsychidae) in the Ota River, Fukushima, Japan. PLOS ONE . 17 , e0268629 (2022). Strahler, A. N. Quantitative analysis of watershed geomorphology. EOS 38 , 913–920 (1957). Wada, T. et al. Collection records of a cyprinid fish, Pseudaspius sachalinensis , from rivers in eastern Fukushima Prefecture after the nuclear power plant accident. J. Cent. Reg. Affs Fukushima Univ. 32 , 233–239 (2021). (in Japanese). Funaki, H. et al. Remobilisation of radiocaesium from bottom sediments to water column in reservoirs in Fukushima, Japan. Sci. Total Environ. 812 , 15234 (2022). Morita, K., Sahashi, G. & Tsuboi, J. Altitudinal niche partitioning between white-spotted charr ( Salvelinus leucomaenis ) and masu salmon ( Oncorhynchus masou ) in a Japanese river. Hydrobiologia 783 , 93–103 (2016). Hearn, W. E. Interspecific competition and habitat segregation among stream-dwelling trout and salmon: a review. Fisheries 12 , 24–31 (1987). Burnham, K. P. & Anderson, D. R. Model selection and mutimodel inference: A practical information-theoretic approach 2nd edn (Springer-Verlag Inc., 2002). Masuda, H., Amaoka, K., Araga, C., Uyano, T. & Yoshino, T. The fishes of the Japanese Archipelago (Tokai University, 1984). He, L. M. & Marcinkevage, C. Incorporating thermal requirements into flow regime development for multiple Pacific salmonid species in regulated rivers. Ecol. Eng. 99 , 141–158 (2017). Gelman, A. Scaling regression inputs by dividing by two standard deviations. Stat. Med. 27 , 2865–2873 (2008). Bates, D. et al. lme4: Linear mixed-effects models using ‘Eigen’ and S4. Available at https://cran.r-project.org/web/packages/lme4/index.html Bartoń, K. MuMIn: Multi-model inference. Available at https://cran.r-project.org/web/packages/MuMIn/index.html Gelman, A. et al. arm: Data analysis using regression and multilevel/hierarchical models. Available at https://cran.r-project.org/web/packages/arm/index.html Fox, J. et al. car: Companion to applied regression. Available at https://cran.r-project.org/web/packages/car/index.html R Core Team. R: a language and environment for statistical computing. R Foundation for Statistical Computing. Available at https://www.R-project.org/ UFR (Use of Fishes in Research) Committee. Guidelines for the use of fishes in research American Fisheries Society, (2013). Available at https://static1.squarespace.com/static/618bf11a71fcdf5398996eda/t/618fbed1f40e6c713dfa71ee/1636810449675/asf-guidelines-use-of-fishes-in-research-2013.pdf Percie du Sert, N. et al. The ARRIVE guidelines 2.0: updated guidelines for reporting animal research. PLOS Biol. 18, e3000410 (2020). (2020). Additional Declarations No competing interests reported. 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Sakai","email":"data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAAZAAAAAyAQMAAABI0h/eAAAABlBMVEX///8AAABVwtN+AAAACXBIWXMAAA7EAAAOxAGVKw4bAAAA/UlEQVRIiWNgGAWjYFACNgaGBBj7A4MElMVDpBbGGURrgQFmvAphQL79WOKHBwzb5OXbzz6Ttt1hISffwH7xAYPMHZxaDM6kHZZIYLhtuOFMupl07hkJY4MDPMUGDDzPcGthSG8AaWHcwJDGJp3bJpG4gYEnTYKB5zBuh/U/b/4B1GI/v/8Zm7Rlm0T9/AYCWhhupB0D2ZLYcANoC2MbkH2A/RheLQY3nqVZJBjcTt5w4xmzZW+bhOGGwzzMBgl4/CLfn2Z880fFbdv5/WmMN3621QGDrv3hg489uEMMFghIgJnHgCGx5wABLaiA/QEDww/StIyCUTAKRsGwBgA7t1Fq3GGvSgAAAABJRU5ErkJggg==","orcid":"","institution":"National Institute for Environmental Studies","correspondingAuthor":true,"prefix":"","firstName":"Masaru","middleName":"","lastName":"Sakai","suffix":""},{"id":571962472,"identity":"34740e9c-ccbe-4465-8f4e-f68a42f1efe6","order_by":1,"name":"Yumiko Ishii","email":"","orcid":"","institution":"National Institute for Environmental Studies","correspondingAuthor":false,"prefix":"","firstName":"Yumiko","middleName":"","lastName":"Ishii","suffix":""},{"id":571962473,"identity":"903368fe-a3e0-473f-941f-048977ee7d4e","order_by":2,"name":"Seiji Hayashi","email":"","orcid":"","institution":"National Institute for Environmental Studies","correspondingAuthor":false,"prefix":"","firstName":"Seiji","middleName":"","lastName":"Hayashi","suffix":""},{"id":571962474,"identity":"3dea1b81-074c-46ac-be8a-d009356c7cf7","order_by":3,"name":"Kazuyoshi Takasaki","email":"","orcid":"","institution":"Fukushima Prefectural Fisheries and Marine Science Research Center","correspondingAuthor":false,"prefix":"","firstName":"Kazuyoshi","middleName":"","lastName":"Takasaki","suffix":""},{"id":571962475,"identity":"84fb6bfb-5648-4570-b915-475ec6b4ee0e","order_by":4,"name":"Wataru Teramoto","email":"","orcid":"","institution":"Fukushima Prefectural Government","correspondingAuthor":false,"prefix":"","firstName":"Wataru","middleName":"","lastName":"Teramoto","suffix":""},{"id":571962476,"identity":"00b89c8c-7676-4534-818f-c5ac847f7b05","order_by":5,"name":"Yuto Funaki","email":"","orcid":"","institution":"Fukushima Prefectural Government","correspondingAuthor":false,"prefix":"","firstName":"Yuto","middleName":"","lastName":"Funaki","suffix":""},{"id":571962478,"identity":"dcae6c80-72fe-40aa-b792-4cc07fbe5dc8","order_by":6,"name":"Tsutomu Kanasashi","email":"","orcid":"","institution":"Fukushima University","correspondingAuthor":false,"prefix":"","firstName":"Tsutomu","middleName":"","lastName":"Kanasashi","suffix":""},{"id":571962483,"identity":"ca111d4a-e27e-419b-9a25-44485e5f9274","order_by":7,"name":"Toshihiro Wada","email":"","orcid":"","institution":"Fukushima University","correspondingAuthor":false,"prefix":"","firstName":"Toshihiro","middleName":"","lastName":"Wada","suffix":""}],"badges":[],"createdAt":"2025-11-22 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12:10:39","extension":"xml","order_by":5,"title":"","display":"","copyAsset":false,"role":"acdc-reference","size":117834,"visible":true,"origin":"","legend":"","description":"","filename":"8f4086efe8544212a05ecc88e626dc2b1structuring.xml","url":"https://assets-eu.researchsquare.com/files/rs-8177095/v1/07e0876b3a21f57395b7d38b.xml"},{"id":100586919,"identity":"4bef7b85-59d9-4b19-82cd-31a6d40b75a3","added_by":"auto","created_at":"2026-01-19 12:10:45","extension":"html","order_by":6,"title":"","display":"","copyAsset":false,"role":"acdc-reference","size":127271,"visible":true,"origin":"","legend":"","description":"","filename":"earlyproof.html","url":"https://assets-eu.researchsquare.com/files/rs-8177095/v1/c5e5fe1af7a3fe37db7f8876.html"},{"id":100586925,"identity":"2e39740f-9bab-477f-b703-561c02ba821d","added_by":"auto","created_at":"2026-01-19 12:11:08","extension":"png","order_by":1,"title":"Figure 1","display":"","copyAsset":false,"role":"figure","size":340584,"visible":true,"origin":"","legend":"\u003cp\u003ePatterns in \u003csup\u003e137\u003c/sup\u003eCs activity concentration data for masu salmon (pink) and white-spotted charr (blue) sampled at four stations along the Ota River. (a–c) Distributions of \u003csup\u003e137\u003c/sup\u003eCs activity concentrations in masu salmon and white-spotted charr (a) for pooled data, (b) by station (St.), and (c) by season. (d) Relationship between \u003csup\u003e137\u003c/sup\u003eCs activity concentrations and total length by species.\u003c/p\u003e","description":"","filename":"floatimage1.png","url":"https://assets-eu.researchsquare.com/files/rs-8177095/v1/bf86289353a8833131edf0ed.png"},{"id":100587678,"identity":"fa34344c-f52e-4295-ad4b-c4ee3b896ea9","added_by":"auto","created_at":"2026-01-19 12:17:56","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":1109735,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-8177095/v1/eaa5cce3-5c0f-4828-b923-f280667d38d2.pdf"}],"financialInterests":"No competing interests reported.","formattedTitle":"\u003cp\u003eCompetition differentiates \u003csup\u003e137\u003c/sup\u003eCs concentrations in sympatric salmonids\u003c/p\u003e","fulltext":[{"header":"Introduction","content":"\u003cp\u003eHeadwater salmonids are one of the most important inland fishery species, but nuclear accidents have caused persistent contaminations of them\u003csup\u003e\u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e1\u003c/span\u003e,\u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2\u003c/span\u003e\u003c/sup\u003e because headwaters experience little human activities and are therefore generally not decontaminated\u003csup\u003e\u003cspan citationid=\"CR3\" class=\"CitationRef\"\u003e3\u003c/span\u003e\u003c/sup\u003e. Thus, factors affecting the radiocesium concentrations in headwater salmonids have been investigated from the viewpoints of radiocesium excretion and uptake by the fish for projecting and managing fish contaminations\u003csup\u003e\u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e1\u003c/span\u003e,\u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2\u003c/span\u003e,\u003cspan additionalcitationids=\"CR5 CR6 CR7\" citationid=\"CR4\" class=\"CitationRef\"\u003e4\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR8\" class=\"CitationRef\"\u003e8\u003c/span\u003e\u003c/sup\u003e. However, in the viewpoint of radiocesium uptake by fish, how ecological processes driven by interspecific competitions over food items influence the radiocesium contaminations in sympatric headwater salmonids is poorly understood, though the salmonids are of members in complex food webs in natural ecosystems\u003csup\u003e\u003cspan citationid=\"CR9\" class=\"CitationRef\"\u003e9\u003c/span\u003e,\u003cspan citationid=\"CR10\" class=\"CitationRef\"\u003e10\u003c/span\u003e\u003c/sup\u003e.\u003c/p\u003e \u003cp\u003eRadiocesium excretion by fish is controlled by body size and temperature\u003csup\u003e\u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2\u003c/span\u003e,\u003cspan additionalcitationids=\"CR12\" citationid=\"CR11\" class=\"CitationRef\"\u003e11\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR13\" class=\"CitationRef\"\u003e13\u003c/span\u003e\u003c/sup\u003e. As specific metabolic rate of fish is determined negatively by its weight, many studies reported that positive size effects on radiocesium concentrations in salmonids\u003csup\u003e\u003cspan additionalcitationids=\"CR6 CR7\" citationid=\"CR5\" class=\"CitationRef\"\u003e5\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR8\" class=\"CitationRef\"\u003e8\u003c/span\u003e,\u003cspan citationid=\"CR14\" class=\"CitationRef\"\u003e14\u003c/span\u003e,\u003cspan citationid=\"CR15\" class=\"CitationRef\"\u003e15\u003c/span\u003e\u003c/sup\u003e. Also, as specific metabolic rate changes with \u003cem\u003ee\u003c/em\u003e\u003csup\u003e\u003cem\u003e\u0026ndash;1/T\u003c/em\u003e\u003c/sup\u003e, where \u003cem\u003eT\u003c/em\u003e is the absolute temperature\u003csup\u003e\u003cspan citationid=\"CR11\" class=\"CitationRef\"\u003e11\u003c/span\u003e\u003c/sup\u003e, it shows clear seasonality (high in summer and low in winter in poikilothermic animals, including fishes). Thus, radiocesium concentrations in salmonid fish become high in winter and low in summer if radiocesium uptake is stable throughout the season\u003csup\u003e\u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2\u003c/span\u003e\u003c/sup\u003e.\u003c/p\u003e \u003cp\u003eWhile the relationship between radiocesium excretion and its concentrations in fishes is simple, radiocesium uptake by fish is relatively complicated\u003csup\u003e\u003cspan citationid=\"CR16\" class=\"CitationRef\"\u003e16\u003c/span\u003e\u003c/sup\u003e. Several previous studies reported evidence regarding the effects of radiocesium uptake through foraging on its concentrations in wild freshwater fishes from their conceptualized frameworks\u003csup\u003e\u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2\u003c/span\u003e,\u003cspan citationid=\"CR16\" class=\"CitationRef\"\u003e16\u003c/span\u003e\u003c/sup\u003e. The studies suggested that consumptions of more contaminated prey can elevate the body burden of radiocesium in fishes, but the seasonality of radiocesium concentrations in fishes can also be affected by seasonal changes in radiocesium excretion through their metabolisms. The balance between radiocesium uptake and excretion is species-specific\u003csup\u003e\u003cspan citationid=\"CR16\" class=\"CitationRef\"\u003e16\u003c/span\u003e\u003c/sup\u003e, and thus seasonality of radiocesium concentration in fishes can exhibit various patterns. Meanwhile, Wada et al.\u003csup\u003e7,8\u003c/sup\u003e recently demonstrated that more contaminated terrestrial prey than aquatic ones, particularly supplied in summer and autumn seasons, elevate the radiocesium concentrations in headwater salmonids in highly contaminated watersheds in Fukushima. While the clear difference of radiocesium concentrations between terrestrial and aquatic preys had been known in highly contaminated headwater ecosystems\u003csup\u003e\u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2\u003c/span\u003e,\u003cspan citationid=\"CR17\" class=\"CitationRef\"\u003e17\u003c/span\u003e\u003c/sup\u003e, the studies first clarified such concentration difference in preys actually influences the seasonal dynamics of radiocesium in headwater salmonids\u003csup\u003e\u003cspan citationid=\"CR7\" class=\"CitationRef\"\u003e7\u003c/span\u003e,\u003cspan citationid=\"CR8\" class=\"CitationRef\"\u003e8\u003c/span\u003e\u003c/sup\u003e.\u003c/p\u003e \u003cp\u003eHeadwater salmonids depend on energy flows from both terrestrial and aquatic preys\u003csup\u003e\u003cspan citationid=\"CR9\" class=\"CitationRef\"\u003e9\u003c/span\u003e,\u003cspan citationid=\"CR19\" class=\"CitationRef\"\u003e19\u003c/span\u003e\u003c/sup\u003e. Because headwater streams are often covered with forests, limitation of sunlight forms pronounced detrital food webs in both terrestrial and aquatic ecosystems\u003csup\u003e\u003cspan citationid=\"CR17\" class=\"CitationRef\"\u003e17\u003c/span\u003e,\u003cspan citationid=\"CR18\" class=\"CitationRef\"\u003e18\u003c/span\u003e\u003c/sup\u003e. Further, contaminated plant litter leaches radiocesium when submerged into streams\u003csup\u003e\u003cspan citationid=\"CR19\" class=\"CitationRef\"\u003e19\u003c/span\u003e\u003c/sup\u003e, and the difference of radiocesium concentrations between terrestrial and aquatic litters becomes greater in more contaminated sites\u003csup\u003e\u003cspan citationid=\"CR20\" class=\"CitationRef\"\u003e20\u003c/span\u003e,\u003cspan citationid=\"CR21\" class=\"CitationRef\"\u003e21\u003c/span\u003e\u003c/sup\u003e. Thus, radiocesium concentrations in terrestrial prey are generally higher than those in aquatic prey particularly in highly contaminated headwater systems owing to the pronounced detrital food webs\u003csup\u003e\u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2\u003c/span\u003e,\u003cspan citationid=\"CR18\" class=\"CitationRef\"\u003e18\u003c/span\u003e\u003c/sup\u003e. These findings are supportive explaining that radiocesium in terrestrial prey is of most potent contaminants transferred to headwater salmonids in highly contaminated regions\u003csup\u003e\u003cspan citationid=\"CR7\" class=\"CitationRef\"\u003e7\u003c/span\u003e,\u003cspan citationid=\"CR8\" class=\"CitationRef\"\u003e8\u003c/span\u003e\u003c/sup\u003e.\u003c/p\u003e \u003cp\u003eFalling and drifting terrestrial prey is a significant energy source for headwater salmonids\u003csup\u003e\u003cspan citationid=\"CR9\" class=\"CitationRef\"\u003e9\u003c/span\u003e,\u003cspan citationid=\"CR10\" class=\"CitationRef\"\u003e10\u003c/span\u003e\u003c/sup\u003e, and thus competitions over terrestrial prey induce biased consumption rate between sympatric salmonid species. For example, \u003cem\u003eOncorhynchus\u003c/em\u003e salmons often exploit terrestrial prey more than \u003cem\u003eSalvelinus\u003c/em\u003e charrs under the competitions\u003csup\u003e\u003cspan citationid=\"CR22\" class=\"CitationRef\"\u003e22\u003c/span\u003e,\u003cspan citationid=\"CR23\" class=\"CitationRef\"\u003e23\u003c/span\u003e\u003c/sup\u003e. Meanwhile, such charrs actively consume terrestrial prey where the salmons are absent\u003csup\u003e\u003cspan citationid=\"CR23\" class=\"CitationRef\"\u003e23\u003c/span\u003e,\u003cspan citationid=\"CR24\" class=\"CitationRef\"\u003e24\u003c/span\u003e\u003c/sup\u003e. Therefore, such interspecific competition is expected to be an important determinant of radiocesium concentrations in headwater salmonids in highly contaminated sites because terrestrial prey is a major radiocesium transporter to the fish. Also, it is reported that radiocesium concentration is higher in \u003cem\u003eO. masou\u003c/em\u003e than in sympatric \u003cem\u003eS. leucomaenis\u003c/em\u003e in a highly contaminated headwater system in Fukushima\u003csup\u003e\u003cspan citationid=\"CR7\" class=\"CitationRef\"\u003e7\u003c/span\u003e\u003c/sup\u003e, implying possible effects of food competition between them. Here, we hypothesized that presence of stronger competitors (\u003cem\u003eOncorhynchus\u003c/em\u003e salmon) influence \u003csup\u003e137\u003c/sup\u003eCs concentrations in both stronger and weaker (\u003cem\u003eSalvelinus\u003c/em\u003e charr) headwater salmonids through intra- and inter-specific competitions. Testing the hypothesis is particularly important to unveil effects of ecological processes on radiocesium dynamics in fishes, thereby to project persistence of radiocesium contaminations in the important fishery species.\u003c/p\u003e"},{"header":"Results and discussion","content":"\u003cp\u003e \u003csup\u003e \u003cb\u003e137\u003c/b\u003e \u003c/sup\u003e \u003cb\u003eCs Concentrations in Headwater Salmonids.\u003c/b\u003e Totally, 1,487 individuals of headwater salmonids (masu salmon: 809 individuals, white-spotted charr: 678 individuals) were sampled. The mean, maximum, and minimum \u003csup\u003e137\u003c/sup\u003eCs activity concentrations were 1,200, 9,140, and 63.5 Bq/kg for masu salmon, and were 1,760, 8,300, and 69.3 Bq/kg for white-spotted charr. Mostly 100% of the individuals (804 masu salmon and 676 white-spotted charr) exceeded the Japanese standard of foodstuffs (100 Bq/kg). The model selection for the linear mixed model (LMM) that included \u003csup\u003e137\u003c/sup\u003eCs activity concentrations in all the salmonid samples indicated that the full model yielded lowest AIC and the ΔAIC between the first- and second-best models was 70.8. The best model detected statistically significant effects of total length, season, and species on \u003csup\u003e137\u003c/sup\u003eCs activity concentrations in fish (Table\u0026nbsp;\u003cspan refid=\"Tab1\" class=\"InternalRef\"\u003e1\u003c/span\u003e and Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e). As widely known from salmonid fishes\u003csup\u003e\u003cspan additionalcitationids=\"CR6 CR7\" citationid=\"CR5\" class=\"CitationRef\"\u003e5\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR8\" class=\"CitationRef\"\u003e8\u003c/span\u003e\u003c/sup\u003e, the \u003csup\u003e137\u003c/sup\u003eCs activity concentrations in masu salmon and white-spotted charr became greater with increase in body size. The positive body size effect on \u003csup\u003e137\u003c/sup\u003eCs activity concentrations in headwater salmonids might be occurred through lower specific metabolic rates in larger fish\u003csup\u003e\u003cspan citationid=\"CR11\" class=\"CitationRef\"\u003e11\u003c/span\u003e,\u003cspan citationid=\"CR12\" class=\"CitationRef\"\u003e12\u003c/span\u003e\u003c/sup\u003e and shifts in food items with body size change\u003csup\u003e\u003cspan additionalcitationids=\"CR6 CR7\" citationid=\"CR5\" class=\"CitationRef\"\u003e5\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR8\" class=\"CitationRef\"\u003e8\u003c/span\u003e\u003c/sup\u003e. As the studied salmonids are assumable to be exposed to severe competitions over food resources, larger individuals might exploit terrestrial prey\u003csup\u003e\u003cspan citationid=\"CR7\" class=\"CitationRef\"\u003e7\u003c/span\u003e,\u003cspan citationid=\"CR8\" class=\"CitationRef\"\u003e8\u003c/span\u003e\u003c/sup\u003e, thereby increase the \u003csup\u003e137\u003c/sup\u003eCs concentrations in their body through both intra- and inter-specific interactions\u003csup\u003e\u003cspan citationid=\"CR7\" class=\"CitationRef\"\u003e7\u003c/span\u003e,\u003cspan citationid=\"CR8\" class=\"CitationRef\"\u003e8\u003c/span\u003e\u003c/sup\u003e. While we were primarily interested in effects of body size, seasonality, and species on \u003csup\u003e137\u003c/sup\u003eCs concentrations in fishes rather than that of \u003csup\u003e137\u003c/sup\u003eCs deposition (and thus this was considered as a random effect in this study), the \u003csup\u003e137\u003c/sup\u003eCs activity concentrations in salmonids increased with increase in \u003csup\u003e137\u003c/sup\u003eCs deposition (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003eb), similarly to the trends found in other taxa\u003csup\u003e\u003cspan additionalcitationids=\"CR26 CR27\" citationid=\"CR25\" class=\"CitationRef\"\u003e25\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR28\" class=\"CitationRef\"\u003e28\u003c/span\u003e\u003c/sup\u003e. \u003csup\u003e137\u003c/sup\u003eCs deposition is thus a prevailing factor determining the \u003csup\u003e137\u003c/sup\u003eCs concentrations in organisms, and we could confirm that the salmonid fishes were more contaminated in more contaminated sites\u003csup\u003e\u003cspan citationid=\"CR7\" class=\"CitationRef\"\u003e7\u003c/span\u003e\u003c/sup\u003e.\u003c/p\u003e \u003cp\u003e \u003cdiv class=\"gridtable\"\u003e\u003ctable float=\"Yes\" id=\"Tab1\" border=\"1\"\u003e \u003ccaption language=\"En\"\u003e \u003cdiv class=\"CaptionNumber\"\u003eTable 1\u003c/div\u003e \u003cdiv class=\"CaptionContent\"\u003e \u003cp\u003eResults of the best linear mixed model (full model) used to test for effects of total length, seasons, and species on \u003csup\u003e137\u003c/sup\u003eCs activity concentrations in masu salmon and white-spotted charr.\u003c/p\u003e \u003c/div\u003e \u003c/caption\u003e \u003ccolgroup cols=\"5\"\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c1\" colnum=\"1\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c2\" colnum=\"2\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c3\" colnum=\"3\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c4\" colnum=\"4\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c5\" colnum=\"5\"\u003e\u003c/div\u003e \u003cthead\u003e \u003ctr\u003e \u003cth align=\"left\" colname=\"c1\"\u003e\u0026nbsp;\u003c/th\u003e \u003cth align=\"left\" colname=\"c2\"\u003e \u003cp\u003eEstimate\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c3\"\u003e \u003cp\u003eStandard error\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c4\"\u003e \u003cp\u003e\u003cem\u003et\u003c/em\u003e\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c5\"\u003e \u003cp\u003e\u003cem\u003eP\u003c/em\u003e\u003c/p\u003e \u003c/th\u003e \u003c/tr\u003e \u003c/thead\u003e \u003ctbody\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eTotal length\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e0.765\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e0.024\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e31.7\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e\u0026lt;\u0026thinsp;0.0001\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eSpring vs summer\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e0.377\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e0.037\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e10.2\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e\u0026lt;\u0026thinsp;0.0001\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eSpring vs autumn\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e0.673\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e0.029\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e23.5\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e\u0026lt;\u0026thinsp;0.0001\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eSpring vs winter\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e0.457\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e0.048\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e9.60\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e\u0026lt;\u0026thinsp;0.0001\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eMasu salmon vs white-spotted charr\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e\u0026ndash;0.253\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e0.028\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e\u0026ndash;8.96\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e\u0026lt;\u0026thinsp;0.0001\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003c/tbody\u003e \u003c/colgroup\u003e \u003c/table\u003e\u003c/div\u003e \u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eThe \u003csup\u003e137\u003c/sup\u003eCs activity concentrations in salmonids were changed seasonally, and the concentrations were autumn\u0026thinsp;\u0026gt;\u0026thinsp;winter\u0026thinsp;\u0026gt;\u0026thinsp;summer\u0026thinsp;\u0026gt;\u0026thinsp;spring (Table\u0026nbsp;\u003cspan refid=\"Tab1\" class=\"InternalRef\"\u003e1\u003c/span\u003e). Because \u003csup\u003e137\u003c/sup\u003eCs concentrations in fishes were determined based on the balance between \u003csup\u003e137\u003c/sup\u003eCs excretion and uptake\u003csup\u003e\u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2\u003c/span\u003e,\u003cspan citationid=\"CR16\" class=\"CitationRef\"\u003e16\u003c/span\u003e\u003c/sup\u003e, seasonality of \u003csup\u003e137\u003c/sup\u003eCs concentration in fish can be inferred from this balance. A previous study\u003csup\u003e\u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2\u003c/span\u003e\u003c/sup\u003e reported that seasonality of \u003csup\u003e137\u003c/sup\u003eCs concentrations in white-spotted charr varies depending on amount of \u003csup\u003e137\u003c/sup\u003eCs depositions, which change the balance of \u003csup\u003e137\u003c/sup\u003eCs excretion and uptake, in streams lacking masu salmon populations. For example, at a less contaminated site (69.0 kBq/m\u003csup\u003e2\u003c/sup\u003e), effect of \u003csup\u003e137\u003c/sup\u003eCs leaching from litter on contrast of \u003csup\u003e137\u003c/sup\u003eCs concentrations between terrestrial and aquatic preys is weak, and thus \u003csup\u003e137\u003c/sup\u003eCs concentrations in potential preys (a mixture of terrestrial and aquatic preys) exhibit small seasonality. Consequently, seasonality of \u003csup\u003e137\u003c/sup\u003eCs concentrations in white-spotted charr (i.e., high in winter and low in summer) follows negatively with seasonality of specific metabolic rate. Meanwhile, at a moderately contaminated site (311 kBq/m\u003csup\u003e2\u003c/sup\u003e), stronger leaching effect enlarges the absolute difference of \u003csup\u003e137\u003c/sup\u003eCs concentrations between terrestrial and aquatic preys, and those in potential preys become higher in summer when inputs of terrestrial prey into streams peak\u003csup\u003e\u003cspan citationid=\"CR9\" class=\"CitationRef\"\u003e9\u003c/span\u003e\u003c/sup\u003e, but lower in winter. In this situation, seasonality of \u003csup\u003e137\u003c/sup\u003eCs concentrations in headwater charr is small because seasonal changes in \u003csup\u003e137\u003c/sup\u003eCs excretion and uptake of headwater charr are seasonally similar. As the previous study discussed (2), in more contaminated sites like our study reaches, seasonality of \u003csup\u003e137\u003c/sup\u003eCs concentrations in potential preys would show more dynamic seasonality (very high in summer, but low in winter). Notably, rhaphidophorid crickets, which are strongly contaminated terrestrial prey\u003csup\u003e\u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2\u003c/span\u003e,\u003cspan citationid=\"CR18\" class=\"CitationRef\"\u003e18\u003c/span\u003e\u003c/sup\u003e specifically fall into headwater streams during summer to autumn\u003csup\u003e\u003cspan citationid=\"CR29\" class=\"CitationRef\"\u003e29\u003c/span\u003e,\u003cspan citationid=\"CR30\" class=\"CitationRef\"\u003e30\u003c/span\u003e\u003c/sup\u003e. This input may have driven the peak \u003csup\u003e137\u003c/sup\u003eCs activity concentrations detected in headwater salmonids in autumn in our study, because these fishes consume a substantial mass of fallen crickets in autumn, although they also feed on various other terrestrial and aquatic insects at our study site\u003csup\u003e\u003cspan citationid=\"CR8\" class=\"CitationRef\"\u003e8\u003c/span\u003e\u003c/sup\u003e. Meanwhile, lowest metabolisms might secondarily elevate the \u003csup\u003e137\u003c/sup\u003eCs concentrations in winter, but greater inputs of terrestrial preys in summer than in spring\u003csup\u003e\u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2\u003c/span\u003e,\u003cspan citationid=\"CR7\" class=\"CitationRef\"\u003e7\u003c/span\u003e\u003c/sup\u003e resulted higher \u003csup\u003e137\u003c/sup\u003eCs concentrations in headwater salmonids in summer than in spring.\u003c/p\u003e \u003cp\u003eAs previously reported elsewhere\u003csup\u003e\u003cspan citationid=\"CR7\" class=\"CitationRef\"\u003e7\u003c/span\u003e\u003c/sup\u003e, the LMM indicated that masu salmon was statistically more contaminated than sympatric white-spotted charr. These results supported our hypothesis that masu salmon exploits more contaminated prey (i.e., terrestrial prey), thereby possesses more \u003csup\u003e137\u003c/sup\u003eCs in their body because excretion rate is very similar between masu salmon and white-spotted charr\u003csup\u003e\u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e1\u003c/span\u003e\u003c/sup\u003e. To examine how such unfair competition affects the \u003csup\u003e137\u003c/sup\u003eCs concentrations in each species, discussion based on the additional two LMMs was extended from the next paragraph.\u003c/p\u003e \u003cp\u003e \u003cb\u003eEffects of Food Resource Competition.\u003c/b\u003e The full LMMs for each species exhibited lowest AICs. For masu salmon, ΔAIC with the second-best model that lacks masu salmon dominance in the explanatory variables was 1.09, suggesting that smaller effect of masu salmon dominance on the \u003csup\u003e137\u003c/sup\u003eCs activity concentrations in masu salmon compared to those of total length and seasons. For white-spotted charr, ΔAIC with the second-best model was larger (26.07), similarly to the LMM with pooled data on all the individuals of masu salmon and white-spotted charr. Relationship between \u003csup\u003e137\u003c/sup\u003eCs activity concentrations and total length was mostly identical between the species (Table\u0026nbsp;\u003cspan refid=\"Tab2\" class=\"InternalRef\"\u003e2\u003c/span\u003e and Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e). The result suggests that physiological features (i.e., excretion rate of \u003csup\u003e137\u003c/sup\u003eCs) of the two species are substantially similar\u003csup\u003e\u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e1\u003c/span\u003e\u003c/sup\u003e, and thus the two species represented the common responses of \u003csup\u003e137\u003c/sup\u003eCs activity concentrations to body size. This is particularly an ideal condition to test how \u003csup\u003e137\u003c/sup\u003eCs uptake by foraging induces interspecific difference of \u003csup\u003e137\u003c/sup\u003eCs concentrations in the two fish species.\u003c/p\u003e \u003cp\u003e \u003cdiv class=\"gridtable\"\u003e\u003ctable float=\"Yes\" id=\"Tab2\" border=\"1\"\u003e \u003ccaption language=\"En\"\u003e \u003cdiv class=\"CaptionNumber\"\u003eTable 2\u003c/div\u003e \u003cdiv class=\"CaptionContent\"\u003e \u003cp\u003eResults of the best linear mixed models (full models) used to test for the effects of \u003csup\u003e137\u003c/sup\u003eCs deposition, total length, seasons, and masu salmon dominance on \u003csup\u003e137\u003c/sup\u003eCs activity concentrations in each salmonid species.\u003c/p\u003e \u003c/div\u003e \u003c/caption\u003e \u003ccolgroup cols=\"5\"\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c1\" colnum=\"1\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c2\" colnum=\"2\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c3\" colnum=\"3\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c4\" colnum=\"4\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c5\" colnum=\"5\"\u003e\u003c/div\u003e \u003cthead\u003e \u003ctr\u003e \u003cth align=\"left\" colname=\"c1\"\u003e\u0026nbsp;\u003c/th\u003e \u003cth align=\"left\" colname=\"c2\"\u003e \u003cp\u003eEstimate\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c3\"\u003e \u003cp\u003eStandard error\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c4\"\u003e \u003cp\u003e\u003cem\u003et\u003c/em\u003e\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c5\"\u003e \u003cp\u003e\u003cem\u003eP\u003c/em\u003e\u003c/p\u003e \u003c/th\u003e \u003c/tr\u003e \u003c/thead\u003e \u003ctbody\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eMasu salmon\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e\u0026nbsp;\u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e\u0026nbsp;\u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e\u0026nbsp;\u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e\u0026nbsp;\u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eTotal length\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e0.751\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e0.0301\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e25.0\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e\u0026lt;\u0026thinsp;0.0001\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eSpring vs summer\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e0.561\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e0.0555\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e10.1\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e\u0026lt;\u0026thinsp;0.0001\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eSpring vs autumn\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e0.829\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e0.0371\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e22.4\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e\u0026lt;\u0026thinsp;0.0001\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eSpring vs winter\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e0.652\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e0.0621\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e10.5\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e\u0026lt;\u0026thinsp;0.0001\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eMasu salmon dominance\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e\u0026ndash;0.233\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e0.0890\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e-2.62\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e\u0026lt;\u0026thinsp;0.05\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eWhite-spotted charr\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e\u0026nbsp;\u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e\u0026nbsp;\u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e\u0026nbsp;\u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e\u0026nbsp;\u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eTotal length\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e0.751\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e0.0357\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e21.1\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e\u0026lt;\u0026thinsp;0.0001\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eSpring vs summer\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e0.555\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e0.0726\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e7.65\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e\u0026lt;\u0026thinsp;0.0001\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eSpring vs autumn\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e0.542\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e0.0419\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e12.9\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e\u0026lt;\u0026thinsp;0.0001\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eSpring vs winter\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e0.670\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e0.0977\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e6.86\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e\u0026lt;\u0026thinsp;0.0001\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eMasu salmon dominance\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e\u0026ndash;1.58\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e0.287\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e-5.50\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e\u0026lt;\u0026thinsp;0.0001\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003c/tbody\u003e \u003c/colgroup\u003e \u003c/table\u003e\u003c/div\u003e \u003c/p\u003e \u003cp\u003eInterestingly, the masu salmon dominance negatively affected the \u003csup\u003e137\u003c/sup\u003eCs activity concentrations in both masu salmon and white-spotted charr, but the effect was stronger in white-spotted charr (Table\u0026nbsp;\u003cspan refid=\"Tab2\" class=\"InternalRef\"\u003e2\u003c/span\u003e). These results firmly supported our hypothesis that exploitation of more contaminated terrestrial prey by stronger competitors influence \u003csup\u003e137\u003c/sup\u003eCs concentrations particularly in weaker competitors. Although several previous studies have reported the effects of abiotic environmental factors such as water quality and \u003csup\u003e137\u003c/sup\u003eCs deposition on \u003csup\u003e137\u003c/sup\u003eCs activity concentrations in freshwater salmonids\u003csup\u003e\u003cspan citationid=\"CR5\" class=\"CitationRef\"\u003e5\u003c/span\u003e,\u003cspan citationid=\"CR7\" class=\"CitationRef\"\u003e7\u003c/span\u003e,\u003cspan citationid=\"CR31\" class=\"CitationRef\"\u003e31\u003c/span\u003e\u003c/sup\u003e, the present study is the first to discover that competition for food resources is an important determinant of \u003csup\u003e137\u003c/sup\u003eCs concentrations in salmonids. We infer that this effect is more prominent in highly contaminated regions and sooner after contamination events, as these conditions would increase the absolute difference in \u003csup\u003e137\u003c/sup\u003eCs concentrations between terrestrial and aquatic preys\u003csup\u003e\u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2\u003c/span\u003e,\u003cspan citationid=\"CR7\" class=\"CitationRef\"\u003e7\u003c/span\u003e,\u003cspan citationid=\"CR8\" class=\"CitationRef\"\u003e8\u003c/span\u003e,\u003cspan citationid=\"CR20\" class=\"CitationRef\"\u003e20\u003c/span\u003e,\u003cspan citationid=\"CR21\" class=\"CitationRef\"\u003e21\u003c/span\u003e\u003c/sup\u003e.\u003c/p\u003e \u003cp\u003eThe difference in seasonality of \u003csup\u003e137\u003c/sup\u003eCs activity concentrations between species was also influenced by food competition. \u003csup\u003e137\u003c/sup\u003eCs activity concentrations in masu salmon peaked in autumn, when the input of more highly contaminated terrestrial prey reached a maximum\u003csup\u003e\u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2\u003c/span\u003e,\u003cspan citationid=\"CR7\" class=\"CitationRef\"\u003e7\u003c/span\u003e,\u003cspan citationid=\"CR8\" class=\"CitationRef\"\u003e8\u003c/span\u003e\u003c/sup\u003e; in contrast, \u003csup\u003e137\u003c/sup\u003eCs activity concentrations in white-spotted charr peaked in winter, when fish metabolic rates were lowest (Table\u0026nbsp;\u003cspan refid=\"Tab2\" class=\"InternalRef\"\u003e2\u003c/span\u003e). These results suggest that exploitation of terrestrial prey by masu salmon increases their \u003csup\u003e137\u003c/sup\u003eCs concentrations, reducing opportunities for the consumption of terrestrial prey by white-spotted charr and thereby decreasing their \u003csup\u003e137\u003c/sup\u003eCs uptake in autumn. Consequently, seasonality of \u003csup\u003e137\u003c/sup\u003eCs concentrations in white-spotted charr is strongly affected by \u003csup\u003e137\u003c/sup\u003eCs excretion through their metabolisms. For adequate management of \u003csup\u003e137\u003c/sup\u003eCs contaminations in fishery species, the findings of this study emphasize the importance of interspecific interactions among living organisms on the determinants of contamination levels of focal species.\u003c/p\u003e \u003cp\u003eThe present study provided important insights into the \u003csup\u003e137\u003c/sup\u003eCs transfers to animals that interact sympatric organisms. First, stronger competitors can influence both intra- and inter-specific variations in \u003csup\u003e137\u003c/sup\u003eCs concentrations in fishes through prey exploitations when preys have variable \u003csup\u003e137\u003c/sup\u003eCs concentrations. Second, habitat segregation formed by such competitions can influence \u003csup\u003e137\u003c/sup\u003eCs concentrations in a focal fish species. In the case of this study, \u003csup\u003e137\u003c/sup\u003eCs activity concentrations in white-spotted charr decreased in lower reaches not only by the lower initial \u003csup\u003e137\u003c/sup\u003eCs deposition, but also by the greater masu salmon dominance. Third, such competition effects are potent enough as is the cases with known factors including effects of \u003csup\u003e137\u003c/sup\u003eCs deposition, body size, and season on \u003csup\u003e137\u003c/sup\u003eCs concentrations in headwater salmonids. These insights suggest that ecological processes need to be considered to unravel the \u003csup\u003e137\u003c/sup\u003eCs dynamics in organisms together with abiotic and biotic processes. The present study successfully tackled into a black box of ecological processes in \u003csup\u003e137\u003c/sup\u003eCs dynamics of headwater salmonids using the sufficient number of samples of physiologically similar competing species and well-known foraging ecology of the species associated with \u003csup\u003e137\u003c/sup\u003eCs uptake\u003csup\u003e\u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2\u003c/span\u003e,\u003cspan citationid=\"CR7\" class=\"CitationRef\"\u003e7\u003c/span\u003e,\u003cspan citationid=\"CR8\" class=\"CitationRef\"\u003e8\u003c/span\u003e,\u003cspan citationid=\"CR16\" class=\"CitationRef\"\u003e16\u003c/span\u003e\u003c/sup\u003e. The findings of this study open a new research area in which the ecological process of interspecific competition can actually affect \u003csup\u003e137\u003c/sup\u003eCs dynamics in headwater salmonids.\u003c/p\u003e"},{"header":"Methods","content":" \u003cdiv id=\"Sec3\" class=\"Section2\"\u003e \u003cp\u003e \u003cb\u003eStudy Site.\u003c/b\u003e This study was conducted in the Ota River, approximately 20 km north of the Fukushima Daiichi nuclear power plant. We set four study reaches (Station 1\u0026ndash;4) as our sampling sites (Table\u0026nbsp;\u003cspan refid=\"Tab3\" class=\"InternalRef\"\u003e3\u003c/span\u003e), corresponding to part of the sites investigated in the previous study\u003csup\u003e\u003cspan citationid=\"CR32\" class=\"CitationRef\"\u003e32\u003c/span\u003e\u003c/sup\u003e. Length of the study reaches was ranging from 500 to 1,000 m to obtain representative data with sufficient number of fishes in each study reach. The upmost reach (Station 4) was classified as second-order stream, whereas Station 3 was third-order stream, and Stations 1 and 2 were fourth-order stream according to the Strahler\u0026rsquo;s rule\u003csup\u003e\u003cspan citationid=\"CR33\" class=\"CitationRef\"\u003e33\u003c/span\u003e\u003c/sup\u003e. Channel morphology of Station 1 was primarily characterized by riffle-pool structure whereas that of Station 4 was primarily step-pool structure, and those of Station 2 and 3 were categorized as mixture of the geomorphologies\u003csup\u003e\u003cspan citationid=\"CR34\" class=\"CitationRef\"\u003e34\u003c/span\u003e\u003c/sup\u003e. All the study reaches were forested with broad-leaved deciduous and evergreen coniferous trees, and the dominant surface geology of the area is the granite of the Mesozoic\u003csup\u003e\u003cspan citationid=\"CR16\" class=\"CitationRef\"\u003e16\u003c/span\u003e,\u003cspan citationid=\"CR35\" class=\"CitationRef\"\u003e35\u003c/span\u003e\u003c/sup\u003e. The mean annual precipitation, measured at the nearby Tsushima AmeDAS automated weather station (37.56 N, 140.75 E), from 2017 to 2022 was 1,292 mm. The mean annual air temperature, measured at the nearby Iitate AmeDAS automated weather station (37.67 N, 140.73 E), from 2017 to 2022 was 10.8℃. The \u003csup\u003e137\u003c/sup\u003eCs deposition after the Fukushima accident was greatest in Station 4 (2,280 kBq/m\u003csup\u003e2\u003c/sup\u003e), decreased to the downstream, and was lowest in Station 1 (1,169 kBq/m\u003csup\u003e2\u003c/sup\u003e), estimated by an airborne monitoring survey on July 2 in 2011 (Table\u0026nbsp;\u003cspan refid=\"Tab3\" class=\"InternalRef\"\u003e3\u003c/span\u003e)\u003csup\u003e\u003cspan citationid=\"CR25\" class=\"CitationRef\"\u003e25\u003c/span\u003e\u003c/sup\u003e. Stations 2, 3, and 4 were inside the difficult-to-return zone, which had the highest ambient dose rates among the three government-designated evacuation zones. There was no artificial obstacle among each station, and thus it was assumable that fishes can migrate among the study reaches. The relative abundance of masu salmon to white-spotted charr typically becomes greater in lower reaches in the study site\u003csup\u003e\u003cspan citationid=\"CR34\" class=\"CitationRef\"\u003e34\u003c/span\u003e\u003c/sup\u003e.\u003c/p\u003e \u003cp\u003e \u003cdiv class=\"gridtable\"\u003e\u003ctable float=\"Yes\" id=\"Tab3\" border=\"1\"\u003e \u003ccaption language=\"En\"\u003e \u003cdiv class=\"CaptionNumber\"\u003eTable 3\u003c/div\u003e \u003cdiv class=\"CaptionContent\"\u003e \u003cp\u003eLatitude, longitude, altitude, Strahler stream order, \u003csup\u003e137\u003c/sup\u003eCs deposition after the Fukushima accident, and ratio of number of the captured masu salmon individuals to that of all the captured salmonid individuals in each study reach.\u003c/p\u003e \u003c/div\u003e \u003c/caption\u003e \u003ccolgroup cols=\"7\"\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c1\" colnum=\"1\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c2\" colnum=\"2\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c3\" colnum=\"3\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c4\" colnum=\"4\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c5\" colnum=\"5\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c6\" colnum=\"6\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c7\" colnum=\"7\"\u003e\u003c/div\u003e \u003cthead\u003e \u003ctr\u003e \u003cth align=\"left\" colname=\"c1\"\u003e \u003cp\u003eStudy reach\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c2\"\u003e \u003cp\u003eLatitude (\u0026deg;)\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c3\"\u003e \u003cp\u003eLongitude (\u0026deg;)\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c4\"\u003e \u003cp\u003eAltitude (m)\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c5\"\u003e \u003cp\u003eStrahler stream order\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c6\"\u003e \u003cp\u003e\u003csup\u003e137\u003c/sup\u003eCs deposition (kBq/m\u003csup\u003e2\u003c/sup\u003e)\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c7\"\u003e \u003cp\u003eRatio of captured masu salmon to total salmonids (%)\u003c/p\u003e \u003c/th\u003e \u003c/tr\u003e \u003c/thead\u003e \u003ctbody\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eStation 1\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e37.57897\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e140.87520\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e194\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e4\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c6\"\u003e \u003cp\u003e1169\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c7\"\u003e \u003cp\u003e69.61\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eStation 2\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e37.58950\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e140.85667\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e303\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e4\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c6\"\u003e \u003cp\u003e1632\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c7\"\u003e \u003cp\u003e77.88\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eStation 3\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e37.59314\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e140.83373\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e380\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e3\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c6\"\u003e \u003cp\u003e2274\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c7\"\u003e \u003cp\u003e62.68\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eStation 4\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e37.59882\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e140.81549\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e446\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e2\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c6\"\u003e \u003cp\u003e2280\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c7\"\u003e \u003cp\u003e9.115\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003c/tbody\u003e \u003c/colgroup\u003e \u003c/table\u003e\u003c/div\u003e \u003c/p\u003e \u003cp\u003e \u003cb\u003eSampling.\u003c/b\u003e At each station, salmonid fishes were collected by angling and electrofishing from 2017 to 2022 under the permission of Fukushima Prefecture (permission numbers: 29\u0026thinsp;\u0026minus;\u0026thinsp;5, 30\u0026thinsp;\u0026minus;\u0026thinsp;6, 1\u0026ndash;20, 2\u0026ndash;5, 3\u0026ndash;16, and 4\u0026ndash;14). Each station contained typical multiple habitat structures (riffle-pool and step-pool), and thus the obtained fish samples were assumable to represent the salmonid assemblages in each station. The samplings were performed in August and October 2017; April, May, August, October, and December 2018; March, June, August, and December 2019; May, June, and December 2020; and June and December in both 2021 and 2022. We defined spring (April\u0026ndash;June), summer (July\u0026ndash;September), autumn (October\u0026ndash;December), and winter (January\u0026ndash;March) according to month ranges. Two salmonid species; masu salmon (\u003cem\u003eOncorhynchus masou\u003c/em\u003e) and white-spotted charr (\u003cem\u003eSalvelinus leucomaenis\u003c/em\u003e) inhabited the study site\u003csup\u003e\u003cspan citationid=\"CR8\" class=\"CitationRef\"\u003e8\u003c/span\u003e,\u003cspan citationid=\"CR34\" class=\"CitationRef\"\u003e34\u003c/span\u003e\u003c/sup\u003e, and the two species are well known to exhibit longitudinally segregated distribution in headwater channels\u003csup\u003e\u003cspan citationid=\"CR29\" class=\"CitationRef\"\u003e29\u003c/span\u003e\u003c/sup\u003e, similarly to a ubiquity of habitat segregations found in other headwater salmonids\u003csup\u003e\u003cspan citationid=\"CR30\" class=\"CitationRef\"\u003e30\u003c/span\u003e\u003c/sup\u003e. White-spotted charr generally inhabits upmost streams whereas masu salmon inhabits downstream, and intermediate streams often harbor both species\u003csup\u003e\u003cspan citationid=\"CR22\" class=\"CitationRef\"\u003e22\u003c/span\u003e,\u003cspan citationid=\"CR23\" class=\"CitationRef\"\u003e23\u003c/span\u003e,\u003cspan citationid=\"CR34\" class=\"CitationRef\"\u003e34\u003c/span\u003e,\u003cspan citationid=\"CR36\" class=\"CitationRef\"\u003e36\u003c/span\u003e\u003c/sup\u003e. In our study site, all the stations harbored both species, but masu salmon was generally dominated in lower reach whereas white-spotted charr was dominated in upper reach (Table\u0026nbsp;\u003cspan refid=\"Tab3\" class=\"InternalRef\"\u003e3\u003c/span\u003e). The longitudinal habitat segregation is affected by competitions over food resources between the species. Masu salmon generally monopolizes fallen and drifting terrestrial prey, whereas the terrestrial prey becomes significant food items for white-spotted charr where masu salmon is absent\u003csup\u003e\u003cspan additionalcitationids=\"CR23\" citationid=\"CR22\" class=\"CitationRef\"\u003e22\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR24\" class=\"CitationRef\"\u003e24\u003c/span\u003e\u003c/sup\u003e. Thus, abundance ratio of the two species is considered to be a good indicator for consumption ratio of terrestrial and aquatic prey which is an important determinant of \u003csup\u003e137\u003c/sup\u003eCs concentrations in these salmonids\u003csup\u003e\u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2\u003c/span\u003e,\u003cspan citationid=\"CR7\" class=\"CitationRef\"\u003e7\u003c/span\u003e,\u003cspan citationid=\"CR8\" class=\"CitationRef\"\u003e8\u003c/span\u003e\u003c/sup\u003e, particularly for weaker competitors. All the collected fishes were stored at \u0026minus;\u0026thinsp;20℃ prior to radioactivity measurements.\u003c/p\u003e \u003cp\u003e \u003cb\u003eRadioactivity Measurements.\u003c/b\u003e Head and internal organs were removed from each fish individual after defrosting and measurements of total lengths. Dissected remaining tissues were minced and packed into 100 mL plastic containers. The tissues mainly consisted of muscles and their \u003csup\u003e137\u003c/sup\u003eCs concentrations are empirically evidenced 89% of those in muscle-only samples of the two salmonid species\u003csup\u003e\u003cspan citationid=\"CR7\" class=\"CitationRef\"\u003e7\u003c/span\u003e,\u003cspan citationid=\"CR8\" class=\"CitationRef\"\u003e8\u003c/span\u003e\u003c/sup\u003e. The sample weights and densities were measured prior to radioactivity measurement. The \u003csup\u003e137\u003c/sup\u003eCs activity concentrations in the samples were determined by gamma-ray spectroscopy. Gamma-ray emissions were measured at an energy of 661.6 keV using high-purity germanium semiconductor detector systems (GC3018, GC4018 and GC4020, Mirion Technologies, Inc.) installed at Fukushima University. The accuracy of \u003csup\u003e137\u003c/sup\u003eCs activity was within 10% (error counts/net area counts). The \u003csup\u003e137\u003c/sup\u003eCs activity concentrations in each sample were corrected for radioactive decay since the collection dates, and all the concentrations were based on wet sample weights. The counting efficiency of these semiconductor detectors was calibrated using volume standard sources (MX033U8PP for U8 container, The Japan Radioisotope Association).\u003c/p\u003e \u003cp\u003e \u003cb\u003eStatistical Analysis.\u003c/b\u003e First, a linear mixed model (LMM) was constructed to evaluate the factors affecting \u003csup\u003e137\u003c/sup\u003eCs activity concentrations in fish in the study stations. The response variable for the LMM was log\u003csub\u003ee\u003c/sub\u003e-transformed \u003csup\u003e137\u003c/sup\u003eCs activity concentrations in all the fish samples that included both masu salmon and white-spotted charr, and the explanatory variables were total length, season, and species. The collection year for each fish individual was included in the LMM as a random intercept, because \u003csup\u003e137\u003c/sup\u003eCs concentrations in fish can temporally be attenuated. Also, the station number was included in the LMM as a random intercept to consider the variability in \u003csup\u003e137\u003c/sup\u003eCs deposition among the stations. A Gaussian error structure was used for the response variables. Models including all combinations of the explanatory variables were constructed, and models that yielded the lowest Akaike information criterion (AIC, ΔAIC\u0026thinsp;\u0026lt;\u0026thinsp;2) were used for descriptive purpose\u003csup\u003e\u003cspan citationid=\"CR38\" class=\"CitationRef\"\u003e38\u003c/span\u003e\u003c/sup\u003e.\u003c/p\u003e \u003cp\u003eSecond, to evaluate the factors affecting \u003csup\u003e137\u003c/sup\u003eCs activity concentrations in each fish species, we constructed a separate LMM for each species. Because masu salmon exploit terrestrial prey, which are more highly contaminated than aquatic prey\u003csup\u003e\u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2\u003c/span\u003e,\u003cspan citationid=\"CR7\" class=\"CitationRef\"\u003e7\u003c/span\u003e,\u003cspan citationid=\"CR8\" class=\"CitationRef\"\u003e8\u003c/span\u003e\u003c/sup\u003e, the dominance of masu salmon in focal salmonid communities can be a strong determinant of fish \u003csup\u003e137\u003c/sup\u003eCs concentrations under competitive foraging conditions\u003csup\u003e\u003cspan additionalcitationids=\"CR23\" citationid=\"CR22\" class=\"CitationRef\"\u003e22\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR24\" class=\"CitationRef\"\u003e24\u003c/span\u003e\u003c/sup\u003e. Therefore, we included the mean ratios of individual masu salmon numbers to total salmonid numbers at each station (masu salmon dominance) as an explanatory variable for both LMMs. Masu salmon dominance was calculated based on the total numbers of individuals of the two species in each season at each station because the explanatory variable can change seasonally in response to behaviors such as spawning\u003csup\u003e\u003cspan citationid=\"CR39\" class=\"CitationRef\"\u003e39\u003c/span\u003e,\u003cspan citationid=\"CR40\" class=\"CitationRef\"\u003e40\u003c/span\u003e\u003c/sup\u003e. Other explanatory variables included total length of fish and season, and sampling year and station number were included as random intercepts in both the LMMs to consider variabilities associated with time passage and \u003csup\u003e137\u003c/sup\u003eCs deposition. The best LMMs were also selected using the same method as described above. We tentatively tested variance factors (VIFs) for the explanatory variables in the three LMMs, and all the VIFs were less than 3, indicating that the collinearity of the explanatory variables was negligible for the LMM results. To determine the relative strength of the parameter estimates, the input explanatory variables were standardized to a mean of 0 and a standard deviation of 0.5 before the model was analyzed\u003csup\u003e\u003cspan citationid=\"CR41\" class=\"CitationRef\"\u003e41\u003c/span\u003e\u003c/sup\u003e, which allows a direct comparison of the resulting coefficients between the numerical and categorical predictors. The LMMs were constructed using the lme4 package\u003csup\u003e\u003cspan citationid=\"CR42\" class=\"CitationRef\"\u003e42\u003c/span\u003e\u003c/sup\u003e, model selection was performed using the MuMIn package\u003csup\u003e\u003cspan citationid=\"CR43\" class=\"CitationRef\"\u003e43\u003c/span\u003e\u003c/sup\u003e, standardization of the explanatory variables was performed using the arm package\u003csup\u003e\u003cspan citationid=\"CR44\" class=\"CitationRef\"\u003e44\u003c/span\u003e\u003c/sup\u003e, and VIFs were calculated using the car package\u003csup\u003e\u003cspan citationid=\"CR45\" class=\"CitationRef\"\u003e45\u003c/span\u003e\u003c/sup\u003e of R 4.2.1\u003csup\u003e46\u003c/sup\u003e.\u003c/p\u003e \u003c/div\u003e"},{"header":"Declarations","content":"\u003cp\u003e \u003ch2\u003eEthical approval.\u003c/h2\u003e \u003cp\u003eThe studied species, masu salmon (\u003cem\u003eOncorhynchus masou\u003c/em\u003e) and white-spotted charr (\u003cem\u003eSalvelinus leucomaenis\u003c/em\u003e) are not endangered. All fish sample collections were approved by Fukushima Prefecture (permission numbers: 29\u0026thinsp;\u0026minus;\u0026thinsp;5, 30\u0026thinsp;\u0026minus;\u0026thinsp;6, 1\u0026ndash;20, 2\u0026ndash;5, 3\u0026ndash;16, and 4\u0026ndash;14) to comply with relevant legislation regarding the sample collections, and the samples are stored in Fukushima University. We euthanized fish using cold shock in accordance with guidelines for fish research\u003csup\u003e\u003cspan citationid=\"CR47\" class=\"CitationRef\"\u003e47\u003c/span\u003e\u003c/sup\u003e to minimize stresses on the fish. Voucher specimens of fish materials in the present study do not exist. All methods in this study are reported in accordance with ARRIVE guidelines\u003csup\u003e\u003cspan citationid=\"CR48\" class=\"CitationRef\"\u003e48\u003c/span\u003e\u003c/sup\u003e.\u003c/p\u003e \u003c/p\u003e\u003cp\u003e \u003ch2\u003eCompeting interests\u003c/h2\u003e \u003cp\u003eThe authors declare that they have no competing interests.\u003c/p\u003e \u003c/p\u003e\u003ch2\u003eFunding\u003c/h2\u003e \u003cp\u003ePart of this study was financially supported by the commissioned research fund by F-REI (JPFR23050301 and JPFR25050501).\u003c/p\u003e\u003ch2\u003eAuthor Contribution\u003c/h2\u003e\u003cp\u003eMasaru Sakai: Conceptualization, Data curation, Formal analysis, Visualization, Writing \u0026ndash; Original Draft. Yumiko Ishii: Conceptualization, Date curation, Formal analysis, Investigation, Resources, Writing \u0026ndash; Review \u0026amp; Editing. Seiji Hayashi: Funding acquisition, Investigation, Resources, Writing \u0026ndash; Review \u0026amp; Editing. Kazuyoshi Takasaki: Investigation, Resources, Writing \u0026ndash; Review \u0026amp; Editing. Wataru Teramoto: Investigation, Resources, Writing \u0026ndash; Review \u0026amp; Editing. Yuto Funaki: Investigation, Resources, Writing \u0026ndash; Review \u0026amp; Editing. Tsutomu Kanasashi: Data curation, Investigation, Resources, Writing \u0026ndash; Review \u0026amp; Editing. Toshihiro Wada: Conceptualization, Data curation, Funding acquisition, Investigation, Resources, Writing \u0026ndash; Review \u0026amp; Editing. All authors have given approval to the final version of the manuscript.\u003c/p\u003e\u003ch2\u003eAcknowledgement\u003c/h2\u003e\u003cp\u003eWe deeply appreciate the tremendous efforts by the staffs of Fukushima Prefecture and the students at Fukushima University to collect fishes and measure radioactivity. We are also grateful to Professor Kenji Nanba of Fukushima University for his valuable support. These efforts were indispensable for testing the hypothesis of this study.\u003c/p\u003e\u003ch2\u003eData Availability\u003c/h2\u003e\u003cp\u003eThe datasets generated during and/or analyzed during the current study are available from the corresponding author on reasonable request.\u003c/p\u003e"},{"header":"References","content":"\u003col\u003e\u003cli\u003e\u003cspan\u003eMatsuda, K., Yamamoto, S. \u0026amp; Miyamoto, K. Comparison of \u003csup\u003e137\u003c/sup\u003eCs uptake, depuration and continuous uptake, originating from feed, in five salmonid fish species. \u003cem\u003eJ. Environ. Radioact\u003c/em\u003e. \u003cb\u003e222\u003c/b\u003e, 106350 (2020).\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eOkada, K. et al. Seasonal variations of \u003csup\u003e137\u003c/sup\u003eCs concentration in freshwater charr through uptake and metabolism in 1\u0026ndash;2 years after the Fukushima accident. \u003cem\u003eEcol. 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(2020).\u003c/span\u003e\u003c/li\u003e\u003c/ol\u003e"}],"fulltextSource":"","fullText":"","funders":[],"hasAdminPriorityOnWorkflow":true,"hasManuscriptDocX":true,"hasOptedInToPreprint":true,"hasPassedJournalQc":"","hasAnyPriority":false,"hideJournal":false,"highlight":"","institution":"","isAcceptedByJournal":false,"isAuthorSuppliedPdf":false,"isDeskRejected":"","isHiddenFromSearch":false,"isInQc":false,"isInWorkflow":false,"isPdf":false,"isPdfUpToDate":true,"isWithdrawnOrRetracted":false,"journal":{"display":true,"email":"[email protected]","identity":"scientific-reports","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":false,"externalIdentity":"scirep","sideBox":"Learn more about [Scientific Reports](http://www.nature.com/srep/)","snPcode":"","submissionUrl":"","title":"Scientific Reports","twitterHandle":"","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"stoa","reportingPortfolio":"Scientific Reports","inReviewEnabled":true,"inReviewRevisionsEnabled":true},"keywords":"forest-stream ecosystem, freshwater fish, interspecific interaction, radiocesium","lastPublishedDoi":"10.21203/rs.3.rs-8177095/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-8177095/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"\u003cp\u003eSympatric headwater salmonids compete over food items that are the primary \u003csup\u003e137\u003c/sup\u003eCs sources for fishes. This ecological process driven by competition is expected to be an important determinant of \u003csup\u003e137\u003c/sup\u003eCs concentrations in headwater salmonids, but little is known regarding the process. Here, we investigated how a stronger competitor salmon (\u003cem\u003eOncorhynchus masou\u003c/em\u003e) affects \u003csup\u003e137\u003c/sup\u003eCs concentrations in them and a sympatric weaker competitor charr (\u003cem\u003eSalvelinus leucomaenis\u003c/em\u003e) in reaches of a highly contaminated headwater system in Fukushima, Japan. The results indicated that higher proportion of sympatric \u003cem\u003eO. masou\u003c/em\u003e individuals within salmon communities lowers \u003csup\u003e137\u003c/sup\u003eCs concentrations in both species, and the effect is more pronounced for \u003cem\u003eS. leucomaenis\u003c/em\u003e. Because stronger salmonid competitors exploit falling and drifting terrestrial prey, which is generally more contaminated than aquatic one, \u003cem\u003eO. masou\u003c/em\u003e could monopolize terrestrial prey, thereby be more contaminated than did \u003cem\u003eS. leucomaenis\u003c/em\u003e as a consequence of interspecific competition. Although previous studies have reported the effects of abiotic environmental factors such as water quality and \u003csup\u003e137\u003c/sup\u003eCs deposition on \u003csup\u003e137\u003c/sup\u003eCs activity concentrations in freshwater salmonids, the present study is the first to discover that competition for food resources is an important determinant of \u003csup\u003e137\u003c/sup\u003eCs concentrations in salmonids. The findings of this study open a new research area in which the ecological process of interspecific competition can affect radiocesium dynamics in headwater salmonids.\u003c/p\u003e","manuscriptTitle":"Competition differentiates 137Cs concentrations in sympatric salmonids","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2026-01-19 12:04:35","doi":"10.21203/rs.3.rs-8177095/v1","editorialEvents":[{"type":"communityComments","content":0},{"type":"decision","content":"Revision requested","date":"2026-03-04T07:21:29+00:00","index":"","fulltext":""},{"type":"editorInvitedReview","content":"","date":"2026-02-25T05:08:45+00:00","index":"hide","fulltext":""},{"type":"reviewerAgreed","content":"110966705642533926913712414954396519201","date":"2026-01-29T00:20:47+00:00","index":"hide","fulltext":""},{"type":"reviewerAgreed","content":"121026926317885636321491239286440133585","date":"2026-01-24T16:44:39+00:00","index":"hide","fulltext":""},{"type":"editorInvitedReview","content":"","date":"2025-12-16T08:42:39+00:00","index":"hide","fulltext":""},{"type":"reviewerAgreed","content":"13718107503473965392702988867291364405","date":"2025-12-15T08:02:21+00:00","index":"hide","fulltext":""},{"type":"reviewersInvited","content":"","date":"2025-12-15T07:52:12+00:00","index":"","fulltext":""},{"type":"editorInvited","content":"","date":"2025-11-28T15:55:19+00:00","index":"","fulltext":""},{"type":"editorAssigned","content":"","date":"2025-11-27T09:22:13+00:00","index":"","fulltext":""},{"type":"checksComplete","content":"","date":"2025-11-26T21:23:20+00:00","index":"","fulltext":""},{"type":"submitted","content":"Scientific Reports","date":"2025-11-26T21:18:27+00:00","index":"","fulltext":""}],"status":"published","journal":{"display":true,"email":"[email protected]","identity":"scientific-reports","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":false,"externalIdentity":"scirep","sideBox":"Learn more about [Scientific Reports](http://www.nature.com/srep/)","snPcode":"","submissionUrl":"","title":"Scientific Reports","twitterHandle":"","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"stoa","reportingPortfolio":"Scientific Reports","inReviewEnabled":true,"inReviewRevisionsEnabled":true}}],"origin":"","ownerIdentity":"a67d2441-7678-42d0-9018-c250138e0e6f","owner":[],"postedDate":"January 19th, 2026","published":true,"recentEditorialEvents":[],"rejectedJournal":[],"revision":"","amendment":"","status":"under-review","subjectAreas":[{"id":61367844,"name":"Biological sciences/Ecology"},{"id":61367845,"name":"Earth and environmental sciences/Ecology"},{"id":61367846,"name":"Earth and environmental sciences/Environmental sciences"},{"id":61367847,"name":"Biological sciences/Zoology"}],"tags":[],"updatedAt":"2026-04-01T16:23:20+00:00","versionOfRecord":[],"versionCreatedAt":"2026-01-19 12:04:35","video":"","vorDoi":"","vorDoiUrl":"","workflowStages":[]},"version":"v1","identity":"rs-8177095","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-8177095","identity":"rs-8177095","version":["v1"]},"buildId":"XKTyCvWXoU3ODBz1xrDgd","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}