DURBIN and DURBIN: ENERGY AND NITROGEN BUDGETS FOR ATLANTIC MENHADEN 



91.3, and 87.7%, respectively). The high values for 

 zooplankton were consistent with results from other 

 fishes (Gerking 1955; Pandian 1967; Beamish 1972; 

 Kelso 1972). Few measurements of carbon, nitrogen, 

 or caloric assimilation exist for marine herbivorous 

 fishes. Menzel (1959) found that Holacanthus as- 

 similated 85% of the nitrogen and 77.7% of the 

 calories from two species of macroalgae. The lower 

 assimilation inHolacanthus may have been related to 

 the type of food. However, there do not appear to be 

 any comparable studies with marine phytophageous 

 fishes, which would indicate whether the high as- 

 similation efficiency of the Atlantic menhaden is 

 typical of this trophic group. 



Energy Losses 



RESPIRATION.— The major energy outputs by 

 the Atlantic menhaden are respiration and excre- 

 tion. Respiration by the menhaden was divided into 

 feeding (T fK ) and nonfeeding components {T rK ). 

 SDA was not included as a separate component, but 

 for reasons discussed earlier was included as part of 

 the feeding respiration rate. SDA is thought to be a 

 fixed proportion of the energy content of the food ra- 

 tion, and in carnivorous fishes has been estimated at 

 about 12.7-16% (Muir and Niimi 1972; Beamish 

 1974; Pierce and Wissing 1974; Schalles and Wis sing 

 1976). Partitioning T fK into its components, T sK and 

 T SDA K , would have caused some minor changes 

 within the energy budget, but would not have sig- 

 nificantly affected the predictions of growth rate and 

 growth efficiency. The most important change would 

 be in a case analagous to Figure IB, where food con- 

 centration increases while s and h remain constant. 

 Here, the ingested ration automatically increases in 

 proportion to c because Atlantic menhaden filter a 

 constant proportion of particles from the water. T fK 

 in this illustration is constant, which reflects the fact 

 that its major component T sK is constant. However, if 

 SDA were included separately we would actually ex- 

 pect to see a small linear increase in T fK because 

 T SDA k should presumably increase in proportion to 

 the ration R K . 



For Atlantic menhaden, the metabolic cost of feed- 

 ing appears to be high (Durbin et al. 1981). This is 

 because of the very rapid increase in respiration rate 

 per unit increase in foraging speed. This rate of in- 

 crease was about 2.5 times greater than has been ob- 

 served in other (nonfilter feeding) species during 

 forced long-term swimming (Beamish 1978). Thus 

 even minor changes in the foraging speed can have a 

 significant impact on metabolic expenditures and the 

 overall energy balance. 



The energy budget demonstrates that for an active 

 species such as the menhaden, it is not possible to use 

 a constant multiplier of the standard metabolism, as 

 recommended by Winberg (1956), to estimate meta- 

 bolic expenditures in the field. Not only is the 

 suggested multiplier of 2 times the standard rate too 

 low (in our studies the routine rate was 3.4 times the 

 estimated standard rate, and the average feeding 

 rate 2.3-4.8 times routine, or about 8-17 times stan- 

 dard), but also the relative size of the respiration 

 component within the overall energy budget is also a 

 variable, changing according to the values of s, c, 

 and h. 



EXCRETION.— Excretion, the other major energy 

 output, is similarly a variable. In contrast to respira- 

 tion which depends on swimming speed and foraging 

 time, excretion depends on the amount of food eaten. 

 Excretion, therefore, will follow no constant relation- 

 ship to respiration in the energy budget (Model I). 

 The linear relationship between ration size and ex- 

 ogenous nitrogen excretion is similar to results in 

 other studies (Gerking 1971; Savitz et al. 1977), 

 although the proportion of nitrogen excreted will de- 

 pend on the balance of amino acids in the food rela- 

 tive to the requirements of the fish. 



Growth Rate and Growth Efficiency 



The rates of energy intake and expenditure deter- 

 mine the amount of energy which is available for 

 growth. Atlantic menhaden must invest considerable 

 time and energy in feeding. The Model I energy and 

 nitrogen budgets show that if foraging speed remains 

 constant, then growth will increase linearly with in- 

 creasing ration size, regardless of whether this is 

 brought about by an increase in food concentration or 

 foraging time. Consequently, gross growth efficiency 

 increases asymptotically with increasing ration size. 

 Model II demonstrates that given the actual swim- 

 ming behavior of the menhaden, the relationship be- 

 tween ration size and growth is in fact very nearly 

 linear at moderate-high plankton densities were s ~ 

 constant, but becomes significantly curvilinear at 

 lower plankton levels because of the decreasing for- 

 aging speed. With the reduction in foraging speed, 

 the energy balance changes because proportionally 

 less of the ingested ration is used to support meta- 

 bolism, which leaves more energy available for 

 growth. 



Ivlev's (1960) bioenergetic model of the bleak, 

 Alburnus alburnus, showed that in this particulate- 

 feeding planktivore, growth increased asymptotically, 

 rather than linearly, with increasing food concen- 



193 



