180 



FISHERY BULLETIN OF THE FISH AND WILDLIFE SERVICE 



in the average weight of the flounder. During 

 the follomng 14-da\' period the amount of food 

 given was reduced by approximately one-half. 

 This resulted in a leveUng off of the weight cui-ve 

 with a corresponding increase in Cs'" accumula- 

 tion to a concentration factor of 11. When regular 

 feeding was resumed and the average weight 

 increased from the 64th to the 77th day, the 

 Cs"^ concentration in the fish actually decreased. 

 Results during the final 14 days of the experiment 

 were similar to those found during the period from 

 the 50th to the 6;kl day. 



The reduced rate of accumulation of Cs'" per 

 imit weight during periods of rapid weight in- 

 crease probably was the result of the fish increasing 

 in mass more rapidly than Cs'" was accunmlated. 

 That is, the amount of Cs'" accumulated by new 

 tissue was so small that the increase of radio- 

 activity due to growth was not detectable, as 

 indicated by the middle cm-ve in figure 1, produced 

 by plotting the radioactivity per fish rather than 

 per unit weight. The result was, in effect, a 

 "biological dilution" of the isotope. During 

 periods of slow weight increase the opposite effect 

 was evident apparently because the rate of 

 accumulation exceeded the rate of weight increase. 



Accumulation of Cs'" by muscle, hver, heart 

 and spleen of croaker was followed during a 

 period of 29 days, the last three tissues being 

 grouped for each determination. Muscle accumu- 



lated the radionuchde at a uniform rate, reaching 

 a concentration 4.5 times that of sea water after 

 29 days (fig. 2). Accumulation occurred more 

 rapidly in liver, heart, and spleen than in muscle, 

 but the rate decreased as the experiment pro- 

 gressed. These tissues had a concentration factor 

 of 9 at the end of 29 days. 



Accumulation from the digestive tract 



Accumulation and tissue distribution of Cs'^' 

 by croaker following oral administration of single 

 doses was determined over a 4-day period. 

 Values were based on averages of four fish per 

 time interval. Six hours after the dose was given 

 only 15.4 percent remained in the digestive tract 

 (table 1). The fact that the intestine did not 

 contain more than 5 percent of the dose at any 

 time plus the early appearance of the radionuclide 

 in the organs and tissues indicated rapid absorp- 

 tion. Hood and Comar (1953) reported similar 

 high absorption of Cs''' through the rumen walls 

 of cattle. 



Tissue concentration of Cs'" in the croaker 

 dosed orally (fig. 3) was similar to that in croaker 

 immersed in radioactive sea water. In both ex- 

 periments, internal organs had rapid rates of 

 accumulation initially, while muscle tissue had a 

 slower rate. However, in the experiment in 

 which croaker were kept in radioactive sea water, 

 a constant supply was available, so that the 



10 



15 

 DAYS 



20 



2S 



30 



Figure 2. — Accumulation of Cs"' by croaker from sea water. 

 Upper curve: heart, spleen, and liver. 

 Lower curve: muscle. 



