DURBIN and DURBIN: ASSIMILATION EFFICIENCY OF ATLANTIC MENHADEN 



the N in the ingested ration, and 65.5% of the N in 

 the assimilated ration, were excreted. 



If the total basal N excreted per day (0.257 mg 

 N/g dry weight per d) is incorporated into Equa- 

 tions (3) and (4), the relationship between the total 

 N excreted per day and the ingested N ration be- 

 comes: 



£^ = 0.616/?^ + 0.237 mg N/g dry weight 



'N 



per d 

 r = 0,99 



(5) 



and the relationship between total daily N excre- 

 tion and the assimilated N ration becomes (Figure 

 10): 



E 



N 



0.655p/?N + 0.241 mg N/g dry weight 



per d 

 r = 0.99. 



(6) 



Equation (6) can be used to calculate, on a daily 

 basis, the efficiency with which Atlantic menha- 

 den retain N for growth as a function of N in the 

 ration. This is analagous to calculating the net 

 growth efficiency K2'- 



K. 



G 

 pR 



(7) 



where G = growth, grams/day 



assimilation efficiency 

 ration, grams/day. 



P 

 R 



2 I— 



/^ 



y^ E^ = 0.655 pR ^-1-0.244 



1 2 



Assimilated Ration, mg N (gm dry wt fish)'' 



Figure lO.— Relationship between the amount of nitrogen 

 assimilated from the daily ration, and the total daily nitrogen 

 excretion by Brevoortia tyrannus. 



In the present case, growth in N (Gn, milligrams 

 N/gram dry weight per day) will be equal to the N 

 in the assimilated ration (p/?N) minus the total 

 amount of N excreted per day (£n total): 



Gn = PK^-E 



N 



pR^ - (0.655 p/?^ 

 dry weight per d 



0.241) mgN/g 



(8) 



(9) 



0.345 p/?j^ 

 per d 



0.241 mg N/g dry weight 



and 



K. 



0.345p/?N -0.241 



pR 



(10) 



(11) 



N 



K2 calculated from Equation (11) for different 

 assimilated ration sizes is shown in Figure 11. At 

 an assimilated ration of 0.70 mg N/g dry weight 

 per d there would be no net gain or loss of N. If 

 Atlantic menhaden are composed of 8.03'?^^ N by 

 weight (Table 1), this maintenance ration would 

 correspond to 0.87% of their body N per day. The 

 amount of N provided in the four lowest ration 

 experiments was less than this daily maintenance 

 requirement. The asymptotic value of /C2 at high 

 ration levels was 0.345 (Figure 11). For K2 = 0.20 

 and 0.30, the Atlantic menhaden would have to 

 assimilate 1.66 and 5.36 mg N (2.1 and 6.7% of 

 their body N, respectively) per day, with resultant 

 growth rates of 0.42 and 2.01% of body N 

 per day. 



% of body N 

 10 2.0 



3.0 



ASSIMILATED RATION, mg N (gm dry wt fish)' 



Figure 11.— Calculated efficiency of the utilization of nitrogen 

 for growth by Brevoortia tyrannus. as a function of the nitrogen 

 content of the assimilated ration. 



609 



