Comparison of ¹³⁷Cs uptake, depuration and continuous uptake, originating from feed, in five salmonid fish species

The origin of ¹³⁷Cs contamination, contamination levels, and its effective ecological half-life can differ among fish species, but until recently it was unknown whether interspecific differences in radiocaesium metabolism existed. We compare the trophic transfer of ¹³⁷Cs in five salmonid species under controlled conditions. Fish were fed a diet containing 200 Bq kg^-1 wet wt concentration of ¹³⁷Cs for a fixed period of time. While there were almost no differences in contamination levels among individual fish within a species during fixed periods, the rate of ¹³⁷Cs uptake was statistically different among the species examined, for which the effective half-life of ¹³⁷Cs ranged 49-84 days. No significant difference in rate of decline in the quantity of ¹³⁷Cs was apparent among species in two experiments after correcting for differences in growth. The effective half-life was shortest in Oncorhynchus mykiss and O. masou-two species exhibiting the best growth rate in both experiments-indicating an influence of the dilution effect associated with growth. When continuously fed 200 Bq kg^-1 wet wt concentration of ¹³⁷Cs for a period of 209-294 days, contamination levels in the five species ranged 216-240 Bq kg^-1 wet wt, and reached a steady state value. Our results indicate metabolic rate, although accompanying a change of body size, did not affect ¹³⁷Cs contamination levels in fish, which implies that contamination levels in the diet strongly determined contamination levels in fish.