FISHERY BULLETIN: VOL. 81, NO. 2 



all changes in body constituents. A comparison of the 

 two budgets will enable us to determine the relative 

 efficiency with which the Atlantic menhaden utilize 

 the total energy content, as distinct from the nitrogen 

 content, of their food for growth. Each of the budgets 

 is presented in two forms, a general model (I) and a 

 special case of this model (II) which incorporates ad- 

 ditional details on the behavior of the fish in response 

 to the abundance of food. Symbols used in the 

 budgets are as follows: 



c,n 



R K , R N 



K 



r.K 



T,k 



s,K 



n 



SDA, K 





L 

 e 



178 



Total daily food intake ("ration") 

 (kcal/g dry weight per day; mg N/g 

 dry weight per day) 



Total daily oxygen consumption 

 (kcal/g dry weight per day) 



Total daily routine oxygen consump- 

 tion (kcal/g dry weight per day) 



Total daily oxygen consumption dur- 

 ing feeding (kcal/g dry weight per 

 day) 



Oxygen consumption attributable to 

 swimming activity (kcal/g dry 

 weight per day) 



Oxygen consumption due to the heat 

 increment from food (kcal/g dry 

 weight per day) 



Total daily nitrogen excretion (kcal/g 

 dry weight per day; mg N/g dry 

 weight per day) 



Total daily endogenous nitrogen ex- 

 cretion (kcal/g dry weight per day; 

 mg N/g dry weight per day) 



Total daily exogenous nitrogen ex- 

 cretion (kcal/g dry weight per day; 

 mg N/g dry weight per day) 



Total daily losses in the feces (kcal/ 

 g dry weight per day; mg N/g dry 

 weight per day) 



Total daily growth (kcal/g dry weight 

 per day; mg N/g dry weight per 

 day) G 



Gross growth efficiency = -p^ ar >d 

 Gn k 



R N 



Assimilation efficiency (dimension- 

 less) 



Volume searched during feeding (1/ 

 fish per hour) 



Volume swept clear during feeding 

 (1/fish per minute) 



Food particle length (/im) 



Filtration efficiency of the gill rakers 

 (dimensionless) 



>G, OPT 



S K, OPT 



Swimming speed during feeding 

 ("foraging speed") (cm/second) 



Concentration of plankton in the wa- 

 ter (kcal/1; mg N/1) 



Duration of the daily feeding period 

 ("foraging time") (hours/day) 



Foraging speed which maximizes 

 growth rate at a given concentration 

 of plankton 



Foraging speed which maximizes 

 gross growth efficiency for a given 

 foraging time. 



Atlantic menhaden are highly specialized plank- 

 tivores which feed on suspended particulate material 

 (phytoplankton, zooplankton, and detritus). During 

 feeding, an Atlantic menhaden swims with its mouth 

 open and gill opercula flared, causing the comblike 

 gill rakers, which otherwise lie flat inside the mouth, 

 to swing inward and form a fine-meshed screen 

 across the throat (Peck 1894). Water entering the 

 mouth is filtered through the rakers before exiting 

 through the gill arches. Adult Atlantic menhaden do 

 not pursue individual prey (Durbin and Durbin 1975). 

 Instead they filter the column of water that lies direct- 

 ly ahead. Although the menhaden are size-selective, 

 this merely reflects the mesh size of the gill rakers 

 and does not represent active selection for specific 

 types of prey. 



Laboratory studies have shown that Atlantic 

 menhaden change their swimming and feeding 

 behavior according to the concentration of food in the 

 water (Durbin et al. 1981). In the absence of food the 

 fish swam at a characteristic speed of 0.47 body 

 lengths/s, with a routine respiration rate of 0.1 mg 

 2 /g wet weight per h. The menhaden increased their 

 swimming speed and respiration rate severalfold 

 during feeding. Foraging speed increased asymp- 

 totically with increasing food concentration, while 

 respiration rate increased exponentially with in- 

 creasing foraging speed. The fish initiated and ter- 

 minated feeding at distinct threshold concentrations 

 of plankton that were inversely related to particle 

 size. Exogenous nitrogen excretion in the Atlantic 

 menhaden was proportional to the nitrogen content 

 of the ration (Durbin and Durbin 1981). Digestion 

 rates were rapid, and assimilation efficiency was 

 high. The menhaden were evidently adapted for the 

 efficient processing of large amounts of particulate 

 material which is ingested during prolonged periods 

 of continuous feeding. 



These observations provide the basis for the 

 development of the energy and nitrogen budgets and 

 will be discussed in more detail below. In accordance 



