FISHERY BULLETIN: VOL. 79, N0.4 



EXP 4 





< 

 30 



Ji 



EXP 7 



o 

 I 



rfT1Tnl^-~rn-Tn 



EXP 10 



0600 



1800 2400 



TIME, hours 



Figure 7. — Ammonia excretion rates of Brevoortia tyrannus 

 before, during, and after a 7-h period of feeding on three ration 

 sizes of Ditylum bnghtwelli. 



X 



DON-N 

 NH,-N 



0.437, a =0.088. 



(2) 



Thus 30.4'7f of the total N excretion was in the 

 form of DON, and 69.6% was in the form of 

 ammonia. 



The mean ± 957c confidence limits of the am- 

 monia excretion rate offish unfed for 36 h (corre- 

 sponding to measurement no. 1 in Durbin et al. 

 1981) was 7.46 ±2.54 ixg NHg-N/g dry weight per 

 h. Using Equation (2), the total excretion was cal- 

 culated to be 10.72 ±3.65 /xg total N/g dry weight 

 per h. 



The amount of exogenous N excretion (that de- 

 rived from the food) was calculated by subtracting 

 the basal N excretion (10.72 /xg N/g dry weight per 

 h) from the total during the period of elevated 

 excretion. Excretion rates were considered to have 

 returned to basal when they reached the upper 

 95% confidence limit on the mean prefeeding rate, 

 14.4 ixg N/g dry weight per h. Assimilation and N 

 excretion did not lag far behind ingestion of the 

 food. The time required for 50% of the exogenous N 

 excretion to occur was only 1 or 2 h after 50% of the 

 food was ingested iX = 1.4 h, Figure 8; Table 3, 

 column 4). This indicates that all of the N ingested 

 during the first 5.6 h of feeding (80% of the total) 

 was assimilated during the feeding period. 



The immediate decline in excretion rate after 

 the fish stopped feeding was in accord with the 

 decline in the elimination rate. In spite of this 

 decline, 90% of the total exogenous excretion was 

 completed within a mean of 2.4 h after the end of 

 feeding (Figure 8; Table 3, column 6). The diges- 

 tion and assimilation of D. brightwelli is therefore 

 much more rapid than its complete elimination 



1200 



1800 

 TIME, 



2400 

 hours 



0600 



Figure 8. — Cumulative total exogenous nitrogen excreted by 

 12 Brevoortia tyrannus during and after feeding on Ditylum 

 brightwelli. A high ration (Exp. 4) and a low ration (Exp. 10) 

 experiment are illustrated. 



from the gut (i.e., compare Table 3, columns 4, 6 

 with columns 3, 5). 



The total exogenous N excreted (£'n, 

 milligrams/gram dry weight) increased linearly 

 with both the total N ingested in the ration (i?N, 

 milligrams) (Figure 9) and the N assimilated from 

 the ration (pi?N, milligrams). The least squares 

 linear regressions were: 



E 



N 



r 



= 0.616fi 

 = 0.99 



N 



r = 



0.655p/?i^ 

 0.99 



0.020 



0.016 



(3) 



(4) 



where p is the assimilation efficiency for N. These 

 regressions indicate that approximately 61.6% of 



I 5,- 



0.5 10 15 2 2 5 



TOTAL RATION, mgN(gm dry wt fish)"' 



Figure 9. — Total exogenous nitrogen excreted by Brevoortia 

 tyrannus, as a function of the amount of nitrogen ingested from 

 Ditylum brightwelli. The least squares linear regression is 

 shown. 



608 



