13 Islands, which ranged from 0-16 d/m/g. however, Activity in the water, immediately south of Guam was about three times high- er at the time of the Marsh survey than during Operation Troll. The distribution of plankton activity is and Table l. from Table 1. shown in Figure 3 Figure 4 is a graphic presentation of the data There is a sharp decrease in activity east of Bikini and a gradual but irregular decrease west of Eniwetok. Activity levels in plankton samples from immediately east of Bikini are about the same as those in the vicinity of Guam. The plankton tows were made to a depth of about 200 meters, which assured complete sampling of the stirred layer. It is gen- erally accepted that the stirred layer exists only above the thermocline, a region in which temperature decreases rapidly with increase in depth. Bathythermograph casts were made at each station to determine temperature changes with depth. Un- fortunately the only bathythermograph which was operable «t stations 1 ~ 30 was only able to measure temperatures to a depth of 200 feet. This situation wes remedied at Quam and from that point on temperature measurements were avellable to 400 feet. From the later data and the results of the Walton and Troll surveys it is evident that in general the upper level of the thermocline was just below 100 meters. The average of the ratios of plankton activity to water activity was 2500; possible reasons for variations in this value are discussed on pages 27-33. Water The activity of the water from all depths is presented in