ro OFFICIAL USE ONLY RADIOACTIVITY OF OPEN-SEA PLANKTON SAMPLES ; Immediately following Shot 5 of Operation Castle in 1954 the Fleet Tug U.S.S. SIOUX manned by scientific personnel from NRDL and the Scripps Institution of Oceanography made a four-day long cruise through the oceanic area adjacent to Bikini. The extensive measurements of the pattern of gamma activity in the sea water were made and are the subject of a comprehensive report, Reference 1; and during the cruise, at twa different stations, a net was lowered and a sample of the zooplankton population was taken. These zooplankton samples exhibited an intense concentration of gamma activity over that of the surrounding water. This was immediately apparent from the effect that their pregence in the specimen. jars had upon a portable gammaindicator, in spite of relatively high back- ground aboard the shin. The two bottles of plankton were immediately sent to NRDL and SIO for classification anc analysis by biologists and radicchemists. The outcomeis the subject of this report. PROCEDURE ! The saynples were collected with a standard silk zooplankton net, having a diameter of one meter, using the technique customary in biological cceanography. The net was lowered inte the water at 50 meters per minute until 200 meters of wire had run out. The wire was then hauled in at 20 meters per minute. This technique collects the organisms from roughly 509 cubic meters of water, including all depths between 0 and about 140 meters. The samples were received at SIO about one week after collection and ware then further preserved with formalin; most of the organisms were in good condition. Biological identification of the organisms was madeat SIO. Objectives of the Laboratory Studies. Howfission products are distributed in the ocean after a fallout is of importance to those planning weapons tests and disposal of atomic wastes at sea. The distribution within the marine biosphereis of special importance, because (1) certain marine zooplankton are known to migrate vertically and therefore could be significant vectors of fallout activity through the stable layers where water transport is much reduced; (2) the activity in organisms is in a critical material, potential foodstuffs. Among other things, it was decided to investigate the possibility that an organism ’s activity was influenced by its feeding habits. General Character of Biological Samples. phytoplankton and very smallest zooplankters. Nets of the type used, pass most of tha Most of what is caught is of vigible size. Many of the small animals displav their ability for movement by darting about the collea- tion jar. Certain large transparent passive gellatenous animals can be seen to coniain smaller organisms, alive cr dead. Since it is known that zooplankton depend ultimately 9 OFFICIAL USE ONLY sy we BACKGROUND AND OBJECTIVES