FISHERY BULLETIN: VOL. 81. NO. 2 



where the assimilated daily ration, pR K , is given by 

 pR K = 0.8954 R K (kcal/g dry weight per d) (11) 



= 0.071250 sc h (kcal/g dry weight 

 per d). 



Energy Output 



(12) 



RESPIRATION, T K (KCAL/G DRY WEIGHT 

 PER DAY).— In the absence of food, the Atlantic 

 menhaden swam at a characteristic speed of 1 2.2 cm/ 

 s (0.47 body lengths/s), with a routine respiration 

 rate of 0.10 mg 2 /g wet weight per h (Durbin et al. 

 1981). 



During feeding the fish increased their swimming 

 speed by a factor of 2.4- to 3.5-fold above the non- 

 feeding rate, depending on the plankton concentra- 

 tion in the water. Both swimming speed and 

 respiration rate increased abruptly with the onset of 

 feeding, and stabilized within a few minutes. One of 

 the more interesting aspects of the Atlantic men- 

 haden feeding behavior was that they would maintain 

 a virtually constant swimming speed throughout the 

 entire 7-h experimental feeding period, if the input of 

 food remained constant. When the food input was 

 stopped, the fish quickly consumed the remaining 

 plankton in the tank, decreasing their swimming 

 speed as the plankton concentration dropped. Thus 

 the return to the routine swimming speed following 

 feeding was quite rapid. In low-ration experiments, 

 respiration rates declined to the routine, prefeeding 

 rate almost immediately after feeding. In high-ration 

 experiments, respiration rate remained slightly el- 

 evated above baseline for 2-5 h after feeding. The 

 amount of energy expended above routine during the 

 postfeeding period was small and did not show any 

 clear relationship with food ration size. It has 

 therefore been omitted for the purpose of the en- 

 ergy budget. 



Based on these considerations, the respiratory 

 costs for the energy budget are considered separate- 

 ly for periods of feeding and nonfeeding. Thus 



T K = T r> K + 7) K (kcal/g dry weight per d) (13) 



where T K = total daily expenditure for respiration 

 T r K — routine respiration during the non- 

 feeding period 

 T f K = respiration during feeding. 



Oxygen consumption rates were converted to 

 caloric equivalents by means of oxycalorific coef- 

 ficients in Elliott and Davison (1975). The appro- 



priate coefficients were determined from the ratios of 

 oxygen consumed: nitrogen excreted by Atlantic 

 menhaden before, during, and after feeding (Durbin 

 and Durbin 1981). During feeding, Atlantic men- 

 haden swimming at their preferred speed of about 

 41.3 cm/s appeared to be catabolizing protein. An ox- 

 ycalorific coefficient of 3.20 X 10~ 3 kcal/mg 2 was 

 therefore used during periods when the fish were 

 feeding. Nonfeeding menhaden catabolized about 

 28% protein and 72% fat (where Q ox = 3.28 X 10" 3 

 kcal/mg 2 ), and the combined oxycalorific coeffi- 

 cient was 3.258 X 10" ;i kcal/mg 2 . 



Routine respiration rate, T r K . — The routine respi- 

 ration rate of quietly swimming, nonfeeding Atlantic 

 menhaden was 0.10 mg 2 /g wet weight per h = 0.2 99 

 mg 2 /g dry weight per h = 0.000974 kcal/g dry 

 weight per h (Durbin et al. 1981). Thus the daily 

 routine respiration during the nonfeeding period is 

 given by 



T rK = 0.000974 (24-/2) (kcal/g dry weight per d)(14) 



where h is the duration of the feeding period (h/d). 



Respiration during feeding, T f K . — The respiration 

 rate increased significantly during feeding. This in- 

 crease could be attributed to three sources: The 

 higher voluntary swimming speed, the possible effect 

 of excitement, and the specific dynamic effect of the 

 food (SDA). The swimming speed was clearly the 

 dominant factor, and accounted for 84.37c of the in- 

 creased respiratory rate during feeding and 73.3% 

 during the postfeeding period. Excitability was dif- 

 ficult to quantify, but our qualitative observations of 

 the behavior of the fish indicated that they were least 

 excitable during feeding and most excitable during 

 the postfeeding period when they continued to hunt 

 for food after the input to the tank had been stopped. 

 SDA is considered to represent mainly the loss of en- 

 ergy during the deamination of protein, and it ap- 

 pears to constitute a fixed proportion of the energy 

 content of a particular type of food (Muir and Niimi 

 1972). The energy cost of SDA is usually determined 

 by monitoring the metabolic rate of the fish following 

 a meal. Unfortunately in the present study we were 

 unable to measure SDA separately because of the 

 prolonged feeding period, during which ingestion 

 and digestion occurred simultaneously. However, 

 since about 80% of the ration was digested and 

 assimilated during the 7-h feeding period (Durbin 

 and Durbin 1981), most of the respiratory cost of 

 SDA was included in the measurement of the total 

 respiration rate during feeding. 



180 



