FISHERY BULLETIN: VOL. 79. N0.4 



reached the same stage of elimination (i.e., 50^, 

 90%) at about the same time (Table 3, columns 



3,5). 



Particulate Si in the feces was used to trace the 

 passage of phytoplankton through the gut, since Si 

 was not digested by the fish. Elimination of the 

 experimental meal was rapid; 50% of the Si from 

 the food was recovered in the fecal pellets within 

 an average of 5.7 h after the midpoint of the feed- 

 ing period (the time at which 50% of the food had 

 been ingested) (Figure 4; Table 3, column 3). Be- 

 cause of the exponential decline in feces produc- 

 tion following feeding, Si from the second half of 

 the ration was egested more slowly, particularly 

 the final 10%. Ninety percent of the Si in the food 

 was recovered within a mean of 7.8 h after the end 

 of feeding (Figure 4; Table 3, column 5 ) and a mean 

 of 94.3% was recovered by 10 h after the end of 

 feeding. Much of the fecal material eliminated 



during the next 31 h appeared to have sloughed 

 from the gut since the C:Si ratios were higher in 

 these samples. The amount of Si released during 

 the interval between 14 and 41 h after the end of 

 feeding was small, corresponding on the average 

 to the food ingested during the final 10 min of the 

 7-h feeding period. 



Assimilation Efficiency 



The assimilation efficiency was high and simi- 

 lar for both phytoplankton and zooplankton ( Table 

 4, columns 3, 4, 7). In the phytoplankton experi- 

 ments, there was a slight positive trend between 

 assimilation and increasing meal size, except in 

 the largest ration experiment. In this experiment, 

 assimilation appeared to be reduced. 



It is not known whether Atlantic menhaden are 

 able to digest chitin. The chitin content of the 



Figure 4. — Cumulative fecal silicon 

 eliminated by 12 Breuoortia tyrannus in 

 Experiments 4, 7, and 10 during and 

 after the 7-h feeding period. 



1200 1800 2400 0600 



TIME, hours 



1200 



1800 



Table 4. — Assimilation efficiency of Breuoortia tyrannus, and the percentage of the total feces which were eliminated at a rate «0.3 

 mg/g dry weight per h. A) Overall assimilation efficiency. B) Assimilation during the period when fecal production was 3=0.3 mg/g dry 

 weight per h. C) Calculated assimilation after subtraction of the chitin C and N from the total. Column numbers, in parentheses, are for 

 text reference. 



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