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Fig. 3. Distributions of 137 Cs activity in skin, intestine, muscle, gill, and gonad in the fishes. Colors represent individual fish species.
activity range for each species, except for the gills of catfish and mackerel. 137 Cs activity in the livers of hairtail, catfish, and conger was below the detection limit.
4 Discussion
4.1 Bioaccumulation of 137 Cs in fishes
Previous studies have shown that 137 Cs activity in fish tends to increase with body length, due to the direct uptake of 137 Cs from seawater and the ingestion of already contaminated
organisms, combined with slower clearance rates( Harmelin- Vivien et al., 2012; Hirose, 2016; Hiyama, 1964; Smith et al., 2002). Specifically, in the medium-sized armored weaselfish 137 Cs activity was approximately three times higher than that
observed in the small fish of this species( Fig. 2). Similarly, large-sized catfish showed 137 Cs activity that was approximately two times higher than that of medium-sized catfish. However, the 137 Cs activity in the large-sized armored weaselfish was 33 % lower than that in the medium-sized weaselfish. Additionally, small-sized croaker, catfish, and conger showed 1.9-, 2.6-, and 3.6-times higher 137 Cs activities, respectively, than that observed in medium-sized fish of the same species. The medium-sized croaker also displayed 137 Cs activity that was 1.8 times higher than that observed in the large-sized croaker( Fig. 2). Kim et al.( 2019) reported an increase in 137 Cs with increasing body length in fishes in Korean Seas; however, in the species studied here( such as flounder, hairtail, and mackerel), this trend was not observed because there was no clear correlation between body size and 137 Cs activity. According to a previous study, reverse
proportionality of fish mass, as well as body length, against
137 Cs activity concentrations in muscles could be the direct
result of the dilution effect related to the increase of fish body weight( Zalewska and Suplińska, 2013).
Dissected parts such as the skin, organs, and gonads generally showed similar or lower 137 Cs activity than that observed in the muscle. However, in catfish, the gills showed 137 Cs activity at an approximately two-fold higher level than
that observed in the muscle, which could be due to the efficient absorption of 137 Cs during respiration( Fig. 3). These results indicated that 137 Cs within the body can vary depending on the organ analyzed. Therefore, assessing 137 Cs bioaccumulation based on muscle may not adequately reflect the overall distribution of the radionuclide across different tissues and species. The mechanisms underlying these variations may include factors such as metabolic rate, feeding behavior, and habitat preferences, which affect the exposure to and uptake of 137 Cs from seawater and grazing( Doi et al., 2012; Matsuda
et al., 2020; Okada et al., 2021). Further studies related to trophic levels are necessary to determine the 137 Cs accumulation in various fish species according to body length. Additionally, obtaining large amounts of data through continuous monitoring is necessary to clarify the biological accumulation of 137 Cs.
4.2 Concentration factors and annual effective dose rates of 137 Cs in Fishes
The calculated CFs in all species ranged from 28 to 166, with an average of 81 ± 36( Fig. 4). These results are similar to those observed for various marine products from the seas surrounding South Korea( 21 to 143; Kim et al., 2019).