Uptake and internal transfer of dietary 137Cs in rainbow trout (Oncorhynchus mykiss) assessed by a two-compartment model

We quantified the dynamics of dietary 137 Cs uptake, internal transfer, and elimination in rainbow trout by using a two-compartment model that separated the viscera and the muscle. During the contaminated feeding period, 137 Cs accumulated rapidly in the viscera and was subsequently transferred to the muscle with a distinct time lag, whereas model estimates suggested relatively limited back-transfer from the muscle to the viscera. Model estimates indicated that approximately 34% of dietary 137 Cs was incorporated into the two-compartment system (viscera and muscle), whereas the remainder was attributed to pathways outside the modeled system, including incomplete absorption and fecal loss as well as possible redistribution to other tissues. During the depuration period, 137 Cs declined rapidly in the viscera but persisted in the muscle, reflecting strong retention. By analyzing the absolute activity (Bq) rather than the concentration (Bq kg −1 ), the biological half-life of 137 Cs in the muscle was estimated independently of growth dilution. These findings provide quantitative parameters describing radiocesium kinetics in freshwater fish and improve our understanding of contamination persistence and recovery.