Accumulation and excretion of cobalt-60 taken up from seawater by marine fishes

The accumulation of 60Co taken up from seawater in marine fishes and its excretion by the fishes were examined to elucidate the mechanism of the radioactive pollution of marine organisms. Five species of marine fishes, menhaden, sea bream, rook fish, yellowtail and flounder, were reared in the aquariums containing 60Co, and the radioactivity of the whole body and other parts or the whole body only of the dissected fishes were measured periodically. Some fishes were transferred from the radioactive to non-radioactive sea water to observe the excretion of the nuclides from the fishes. The blood was a good reserve pool of the nuclides in yellowtail and flounder, and more than 15% of the total radioactivity in the whole body was accumulated in the blood when they had been reared for about 90 days in the radioactive water. Among the tissues and organs examined, only livers and kidneys showed higher radioactivity per unit weight than the blood, but they contributed little to the total radioactivity of the whole body because of their small weight. On the other hand, muscles showed low radioactivity per unit weight, but contributed largely to the total radioactivity of the whole body because of their large weight. The biological half-lives in the muscles of sea bream, yellowtail and flounder at 15 deg C were calculated and were 38.5, 53.3 and 63.0 days, respectively. The ratio of the uptake rate to the excretion rate in the muscles varied from species to species, but was between 0.3 and 1.5, in the fishes examined. (Kaihara, S.)