60Co Transfer from Water to the Rainbow Trout (Oncorhynchus mikiss Walbaum)

⁶⁰Co uptake from natural water, release and tissue distribution were investigated in the rainbow trout (Oncorhynchus mikiss), which is representative of the third order consumers of the freshwater trophic chains. The experiments were carried out on two groups of fingerling trout placed in 0.45 μm filtered river water, maintained at 12 ± 0.5°C, contaminated with about 30 Bq ml⁻¹ of ⁶⁰Co and renewed three times a week. After a 8-week exposure phase, the fish of one group were dissected to determine the contamination of the tissues and organs and the distribution of the accumulated ⁶⁰Co. The fish of the other group were placed in non-contaminated water, renewed daily, to monitor radionuclide release. After a 42-day depuration phase, the specimens were dissected to study the tissue distribution of the residual ⁶⁰Co. ⁶⁰Co accumulation from water by trout can be described by a one-compartment exponential model. The concentration factor, calculated from the ratio of the radionuclide concentration in filtered water and in the fish, reached a maximum value of 4.6 (w.w.) after 30 days exposure. After the 42-day depuration phase, the fish retained about 29% of the accumulated radionuclide. A single-compartment exponential model was fitted to the ⁶⁰Co elimination data, and the corresponding radionuclide half-life was 21 days. At the end of the exposure phase, tissue contamination study showed ⁶⁰Co accumulation by the gills, viscera (air bladder, heart and spleen) and kidneys, to be the highest. At the end of the depuration phase, the kidney was the most contaminated organ, followed by the viscera, head, gills, and liver. In both cases, ⁶⁰Co concentration was by far the lowest in the muscle, which accounted for about 45% of the total body weight and only 20% of the total radionuclide body load.