DURBIN and DURBIN: ENERGY AND NITROGEN BUDGETS FOR ATLANTIC MENHADEN 



The total respiration rate during feeding increased 

 exponentially with increasing foraging speed (s, cm/ 

 s), where 



T^—IO 002948 *- 15342 (mg 2 /g wet weight 



per h) (15) 



7}.* = 2.994 (10002948 s-1.5342) ( m g 2 /g 



dry weight per h). (16) 



Converting to calories 



and Watts 1974). The caloric equivalents of these 

 compounds are: 5.51 X 10 3 kcal/mg urea-N (Elliott 

 and Davison 1975), 13.32 X 10~ 3 kcal/mg creatine- 

 N, and 41.3 X 10" 3 kcal/mg trimethylamine-N 

 (Weast 1977). The mean value for these compounds 

 was 20.04 X 10" 3 kcal/mg DON. 



Endogenous nitrogen excretion, E 1)K .— The endoge- 

 nous excretion rate equals 10.72 jug N/g dry weight 

 per h (Durbin and Durbin 1981). The daily endoge- 

 nous nitrogen excretion was therefore 



T fK = 0.00958 (10o-o2948.-i.5342) (kcal/g dry 



weight per h). (17) 



The daily energy expenditure for respiration during 

 feeding is therefore 



T t K = 0.00958 h 10 002948s - 

 dry weight per d). 



1.5342 



) (kcal/g 



(18) 



Total daily respiration, T K . — Combining Equations 

 (14) and (18) we obtain an expression for the total 

 respiratory expenditure per day as a function of the 

 foraging speed (s, cm/s) and the foraging time {h, h/ 

 d): 



T K =h [0.00958 (10 ,l0294 » -15342) - 0.000974] 



+ 0.02338 (kcal/g dry weight per d). (19) 



NITROGEN EXCRETION, E K (KCAL/G DRY 

 WEIGHT PER DAY).— Energy is lost through the 

 excretion of nitrogenous compounds. In the absence 

 of food the fish excreted nitrogen at a low rate (basal 

 or endogenous excretion, E b N ). Nitrogen excretion 

 increased as a result of feeding (exogenous excretion, 

 E fN ). The total daily nitrogen excretion (£ N ) is 

 thus: 



E s = £ b N + E f< N (mg N/g dry weight per d). (20) 



The energy equivalent of the excreted nitrogen was 

 determined as follows: Of the total nitrogen excreted 

 by menhaden, 69.6% was in the form of ammonia and 

 30.4% was in the form of dissolved organic nitrogen 

 (DON) (Durbin and Durbin 1981). The caloric equi- 

 valent of ammonia nitrogen is 5.94 X 10 3 kcal/mg 

 NH 3 - N (Elliott and Davison 1975). The individual 

 compounds comprising the DON excreted by Atlan- 

 tic menhaden were not determined. For the purpose 

 of the energy budget, the DON was assumed to con- 

 sist of equal parts of urea, creatine, and trimeth- 

 ylamine, the major organic nitrogen compounds 

 which are known to be excreted by teleosts (Watts 



E bN = 0.257 (mg N/g dry weight per d). 

 Converting to calories 



(21) 



E b< K = 0.0026282 (kcal/g dry weight per d). (22) 



Exogenous nitrogen excretion, E f K . — The exogenous 

 nitrogen excretion of menhaden fed D. brightwelli 

 was directly proportional to the total nitrogen con- 

 tent of the ration, R s (mg N/g dry weight per d) (Dur- 

 bin and Durbin 1981): 



E fili =0.616 # N - 0.020 (mg N/g dry weight 



per d). (23) 



Converting to calories 



E fK = 0.006299 R s - 0.0002045 (kcal/g dry 



weight per d). (24) 



The nitrogen content of a ration of D. brightwelli, i? N 

 (mg), may be converted to kilocalories, R K (kcal), ac- 

 cording to the following relationship 



R K = 0.06158 R s 



(25) 



(Durbin and Durbin 1981). Thus if the daily ration is 

 expressed in kilocalories rather than nitrogen, the 

 daily exogenous nitrogen excretion becomes 



E UK = 0.1023 R K - 0.0002045 (kcal/g dry 



weight per d). (26) 



Total daily nitrogen excretion, E K .— Combining 

 Equations (21) and (23) we obtain an expression for 

 the total daily nitrogen excretion rate 



E N = 0.616 R N + 0.237 (mg N/g dry weight 



perd). (27) 



Combining Equations (22) and (26), the daily ni- 

 trogen excretion rate is expressed in calories 



181 



