DURBIN and DURBIN: ASSIMILATION EFFICIENCY OF ATLANTIC MENHADEN 



fish conserve C and calories relative to N and ash 

 from its food. Exogenous N in excess of body re- 

 quirements is excreted. The 0:N ratios indicate 

 that proteins are used as a metabolic fuel by both 

 feeding and nonfeeding Atlantic menhaden with 

 subsequent excretion of N. This is consistent with 

 results from other teleosts ( Watts and Watts 1974 ). 

 If an 0:N ratio of 7.4 indicates that pure protein is 

 being burned, then the mean value of 28.2 in fish 

 unfed for 36 h indicates that 7.4/28.2 = 26.27. of 

 the 0-2 consumed is being used for protein 

 catabolism. The increase in 0:N ratios im- 

 mediately after the beginning of feeding indicates 

 that the fish are metabolizing proportionally more 

 carbohydrate or lipid to support the increased 

 swimming speed until significant quantities of 

 food have been assimilated and become available 

 as an energy source. During the feeding period of 

 the three high ration experiments, the 0:N ratios 

 declined to or below that associated with the com- 

 bustion of pure protein. The decline in the 0:N 

 ratios during feeding can be caused by two pro- 

 cesses, which can act simultaneously: the fish are 

 obtaining a large proportion of their energy di- 

 rectly from the breakdown of the C skeletons of 

 amino acids absorbed from the food, with sub- 

 sequent excretion of the N; and the proportions of 

 the various amino acids taken in the food are being 

 balanced to meet the requirements of protein 

 synthesis; excess a-amino acids are excreted 

 (Watts and Watts 1974). While it is impossible to 

 separate the two processes in the present experi- 

 ments, 0:N ratios below 7.4 are an indication that 

 both are occurring. 



Kutty (1978) has calculated the ammonia quo- 

 tient ( AQ = NH3 excreted/Oa consumed) from the 

 data of Brett and Zala (1975), from which 0:N 

 ratios can be determined. In that study the fish 

 were fed a single meal over a brief time interval, 

 and the peaks in O2 consumption and N excretion 

 were separated by several hours. However, the 

 trend in the 0:N ratios was similar to the present 

 study, in that they declined from high values ( = 

 40.0) in the unfed fish to a minimum of about 8.3 

 during the digestion and assimilation of the food, 

 then gradually increased to the prefeeding level. 



ACKNOWLEDGMENT 



We would like to thank Harold Loftes, skipper of 

 the Ocean State, and Charles Follett, skipper of 

 the Cindy Bett, for their assistance in obtaining 

 Atlantic menhaden. We also thank Theodore 



Smayda for the use of his laboratory facilities, 

 Thomas Smayda and Peter Verity for their assis- 

 tance during the experiments, and the National 

 Science Foundation for support of this research 

 under grant OCE 7602572. 



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