The radionuclide content of bird species presents a sharp contrast, both qualitatively and quantitatively, associated with feeding habit (Table 17). The fairy terns and noddy terns feed mostly at sea outside the lagoon and contain small amounts of fallout radionuclides, less than the amount of naturally occurring "Ok. They contain barely detectable amounts of 137¢g, The curlew, on the other hand, feeds on the reef and on Scaevola sp. seeds, and consequently contains relatively large amounts of 137¢s, as much as 2,300 pCi/g dry in muscle. The turnstones also feed along the beaches and on the reef, and contain both ®°Co and 137cs. The source of 137cg for the turnstones is not known, although it could be by direct ingestion of sand particles. The yellowfin tuna, which are feeding on essentially the same organisms as the terns, contain about the same levels of ©%Co as the fairy terns. The ©9Co levels in the noddy terns are somewhat higher but still are of the same order of magnitude. Thus, the area in which an animal is feeding is a factor affecting its radionuclide content, as expected, in relation to the distance from the source of the radionuclide. The source of ©%Co for the tuna must be Bikini Atoll and not worldwide fallout, because we analyzed tissues from 214 tuna, including 75 yellowfin tuna, taken from the Japanese tuna fishery during 1968 and 1969, and found no 69C¢9 (NVO-269-7, Annual Report). In contrast, the >> Fe concentrations in the dark muscle of the tuna from the Japanese fishery ranged from 3.3 to 1600 pCi/g dry; most of the values fell in the range of 101 to 500 pCi/g dry, similar to the values for yellowfin tuna from Bikini (Table 18). It appears, therefore, that a major amount of the °°Fe in the Bikini tuna is from worldwide fallout. 4.4 CRATER SEDIMENTS One of the principal sources of radionuclides at Bikini is Bravo Crater in the reef adjacent to and southwest of Nam Ll. Figure 3 shows a gamma-ray spectrum of sediment taken from a depth of 160 feet. Clearly, °Co and 297Bi predominate among the gamma emitters. The most abundant radionuclide in soils is 137¢s, and an intermediate condition exists at the southwestern end of Eneman I., where a low area is occasionally overwashed by seawater, and the !37’cg is being leached from the soil. The retention of ®Co and 297Bi by the sediments is reflected in the fact that the bottom-feeding goatfish in the vicinity of the craters contain ten times more ©°Co than the herbivorous convict surgeonfish and detritus— feeding mullet. However, some 6%Co is being transported eastward by the bottom current in the lagoon either in solution or associated with fine (colloidal?) particles, because the difference in 60c¢o9 content between convict surgeon fish and mullet in the vicinity of the Bravo Crater and 16 miles eastward near Bikini I. is only by a factor of less than two. At the same time, the ©°Co concentrations in goatfish from near the crater and those at Bikini I. differ by a factor of about ten. 11