DURBIN and DURBIN: ASSIMILATION EFFICIENCY OF ATLANTIC MENHADEN 



zooplankton food (Table 1) was measured 

 gravimetrically, but it was not possible to obtain 

 an accurate measurement of chitin in the feces 

 because of their very high ash content. However, if 

 it is assumed that chitin ( 39.37'7r C and 5.889^ N by 

 weight) was not assimilated by Atlantic menha- 

 den, and the chitin C and N are then subtracted 

 from the total C and N in the food and the feces, the 

 calculated assimilation efficiency for zooplankton 

 would be increased slightly, to 91.43Vf (C) and 

 93.99^^ (N) (Table 4, columns 6, 9). 



Changes in assimilation efficiency within ex- 

 periments followed the general trend of the elimi- 

 nation of feces (Figures 2, 3). During feeding, as- 

 similation efficiency increased from initial low 

 values to a peak, which was sustained for several 

 hours after the end of feeding and thereafter de- 

 clined. The peak assimilation was reached sooner, 

 and remained elevated for longer, in the high ra- 

 tion experiments (Figures 2, 3). With the two small- 

 est rations, assimilation was still ascending when 

 the feeding period was terminated. 



Assimilation remained high as long as the fecal 

 elimination rate exceeded about 0.3 mg/g dry 

 weight per h (Figure 5). At lower elimination 

 rates, assimilation declined precipitously. The re- 

 duced assimilation efficiencies at the beginning 

 and near the end of feces production were as- 



100 





.%/•• 



40 I! 1 i 1 1 ' 



0.5 LO L5 2.0 2.5 



FECES PRODUCTION, mg (g dry wt fish)'' hr"' 



Figure 5. — Relationship between fecal elimination rate of 

 Brevoortia tyrannus and the assimilation efficiency for nitrogen. 



sociated with the presence of a mucuslike material 

 which the fish released with the feces when the 

 elimination rate was low. This material resulted 

 in high C:Si and N:Si ratios in these feces. Any 

 such input of C and N to the feces other than from 

 the food would reduce the calculated assimilation 

 efficiency. With the smaller meal sizes a greater 

 proportion of the feces were eliminated at a low 

 rate (Table 4, column 10), and the materials pro- 

 duced by the digestive tract of the fish constituted 

 a significant fraction of the total fecal material. 

 This reduced the apparent overall assimilation 

 efficiency (Table 4, columns 3, 4, 7i. If the feces 

 produced at low rates (  0.3 mg/g dry weight per h) 

 are excluded from the calculation, the dependence 

 of assimilation efficiency on ration size is reduced 

 (Table 4, columns 5, 8). 



Nitrogen Excretion 



When the fish were not feeding, their excretion 

 rates were low and changes in the concentration of 

 ammonia and DON in the tank were small. How- 

 ever, during feeding the excretion of the fish in- 

 creased rapidly, and produced a rapid and nearly 

 linear increase in the ammonia and DON concen- 

 trations (Figure 6). Excretion declined soon after 

 the fish stopped feeding (Figure 7). 



The mean ratio between DON and NH3 excreted 

 during the feeding period in all experiments was: 



0600 



TIME 



1200 

 hours 



1800 



Figure 6.— Changes in ammonia and dissolved organic nitro- 

 gen concentration in the tank water due to excretion by 12 

 Brevoortia tyrannus during and after feeding. A high ration 

 (Exp. 4) and a low ration i Exp. 10) experiment are illustrated. 



607 



