ENERGY AND NITROGEN BUDGETS FOR 



THE ATLANTIC MENHADEN, BREVOORTIA TYRANNUS 



(PISCES: CLUPEIDAE), A FILTER-FEEDING PLANKTIVORE 



Edward G. Durbin and Ann G. Durbin' 



ABSTRACT 



Experimentally derived energy and nitrogen budgets for the Atlantic menhaden permit a detailed investiga- 

 tion of the food consumption rate, energy expenditures, growth rate, and growth efficiency in this filter- 

 feeding planktivore. The models were developed for adult fish (302 g wet weight, 26 cm fork length) at a 

 temperature of 20 C. Three variables are shown to control the energy and nitrogen budgets: The swimming 

 speed while the fish are feeding, the duration of the daily feeding period, and the concentration of plankton in 

 the water. 



Growth rate increased linearly, and growth efficiency increased asymptotically, with an increase in either 

 plankton concentration or the duration of feeding, provided that swimming speed remained constant. 

 However, with increasing swimming speed, growth curvilinearly increased from zero to a maximum value and 

 then declined back to zero. Growth efficiency followed a similar pattern, but reached its maximum value at a 

 slower swimming speed than that which maximized growth. The swimming speeds which maximized growth 

 rate were dependent on plankton concentration, but were independent of the duration of feeding. Converse- 

 ly, the swimming speeds which maximized gross growth efficiency depended on the duration of feeding, but 

 were independent of food concentration. Laboratory studies demonstrated that menhaden regulate their 

 swimming speeds according to the abundance of plankton in the water. Analysis of the energy budgets 

 revealed that the voluntary swimming speeds of the menhaden were very close to those which maximize 

 growth rate at different concentrations of plankton. We conclude that swimming speed in the menhaden has 

 evolved over time towards maximizing growth rate rather than growth efficiency. 



In most circumstances the growth efficiency for calories and nitrogen were significantly different. The ob- 

 served swimming speeds in the menhaden resulted in higher growth efficiency for nitrogen at low plankton 

 abundance, but higher efficiency for calories at moderate to high plankton abundance. This accounts for the 

 seasonal increase in the fat content of the menhaden during the summer, yet indicates that protein will be 

 conserved when food abundance is low. 



The study offish bioenergetics can provide consider- 

 able insight into how different biotic and abiotic fac- 

 tors interact to control food intake and growth in 

 fishes. Here we describe energy and nitrogen 

 budgets for adult Atlantic menhaden, Brevoortia 

 tyrannus, a filter- feeding planktivore which ranges in 

 inshore waters along the Atlantic coast from Florida 

 to Maine (Nicholson 1978 and references therein). 

 These budgets are based on experimental inves- 

 tigations of the physiological and behavioral re- 

 sponses of the adult Atlantic menhaden to differing 

 food conditions (Durbin and Durbin 1975, 1981; 

 Durbin et al. 1981). 



In the energy budget, the sum of somatic and re- 

 productive growth (G K ) equals the energy content of 

 the ingested ration (R K ), minus the energy losses to 

 respiration (T K ), excretion {E K ), and feces (F K ): 



R„ 



T K — E K 



(1) 



'Graduate School of Oceanography, University of Rhode Island, 

 Narragansett, RI 02882 



We have attempted to incorporate into the energy 

 budget the energetic gains and losses which occur 

 during different phases of the normal daily activity of 

 the fish. For example, the energetic expenditures 

 during periods of feeding are considered separately 

 from periods when the fish are not feeding. The en- 

 ergy budget is then used to predict food intake, 

 growth rate, and growth efficiency of Atlantic 

 menhaden under different feeding regimes. 



In the nitrogen budget, growth in nitrogen (G N ) 

 equals the nitrogen contained in the daily ration (R s ) 

 minus the daily nitrogen losses to excretion (£ N ) and 

 in the feces (F N ): 



R» 



£ N F N . 



(2) 



Manuscript accepted July 1982. 



FISHERY BULLETIN: VOL. 81. NO. 2, 1983. 



Food intake, growth rate, and growth efficiency are 

 predicted. 



The energy and nitrogen budgets measure different 

 things; the nitrogen budget is mainly for a specific 

 component (protein), while the energy (caloric) 

 budget is more inclusive and attempts to account for 



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