DURBIN and DURBIN: ENERGY AND NITROGEN BUDGETS FOR ATLANTIC MENHADEN 



c =00030 kcol/i 

 h =14 hours/doy 



S=4I 3 cm/sec 

 h = 14 hours/doy 



c=00030 kcol// 

 S = 4I3 cm/sec 



20 40 



FORAGING SPEED 

 (s,cm/s«c) 



00030 00060 



PLANKTON CONCENTRATION 

 (Ckcol/O 



8 16 



FORAGING TIME 



(h, hours/doy) 



FIGURE 8. — Comparison of the gross growth efficiency of the Atlan- 

 tic menhaden in calories (K, K ) and nitrogen (K, »•) where A, forag- 



L 1.K 



l.N' 



ing speed(s) increases while plankton concentration (c) and foraging 

 time (h) remain constant; B, plankton concentration increases while 

 s and h are constant; and C, foraging time increases while s and c 

 are constant. 



z 

 <^ „ 



< 4 



cc 

 O 



c min, N ' c min, K 



c min, K L min, N 



10 20 30 40 50 



FORAGING SPEED (s, cm/sec) 



60 



FlGl'RE 9. — Boundary curve defining the combinations of foraging 

 time and foraging speed at which the minimum plankton concentra- 

 tion required for the Atlantic menhaden's growth in calories (c min K ) 

 is less than, and greater than, that required for growth in nitrogen 



( c min,.\)- 



this boundary, c mm A -will be lower than c min N . Atlantic 

 menhaden will be able to grow in calories at a lower 

 food concentration than they can in nitrogen. Con- 

 versely, where s and h are greater than the boundary 

 values, c mmN will be lower than c mmK . Atlantic 

 menhaden can grow in nitrogen at a lower food con- 

 centration than they can in calories. 



Next we consider how the actual foraging speeds of 

 the Atlantic menhaden compare with the boundary 

 curve in Figure 9. Figure 10A shows the foraging 

 speed in relation to food concentration. Figure 10B 

 shows that for all values of h up to 24 h/d, Atlantic 

 menhaden forage at speeds such that their minimum 

 food requirement for growth in nitrogen is lower than 

 for calories, i.e., c min _ N is <c min>A -. Thus at low plankton 

 concentrations, the growth efficiency in nitrogen is 

 greater than in calories. However, it can be seen from 

 Figure 9 that.K\ N remains > K 1K only over a narrow 

 range of food concentrations immediately above 

 c mm. v #1, k increases very rapidly above c mui K and 



soon overtakes K lN . Thus in most circumstances 

 where the fish are growing, growth efficiency in 

 calories will be considerably higher than in nitro- 

 gen. 



DISCUSSION 



Functioning of the Energy and 

 Nitrogen Budgets 



These models permit a detailed analysis of the en- 

 ergetics of the Atlantic menhaden, by showing how 

 energy intake (ingestion), as well as energy losses and 

 expenditures (feces, excretion, respiration) vary with 

 the concentration and size of the food particles, the 

 foraging speed of the fish, and the duration of feed- 

 ing. These different components of the model, and the 

 predicted growth rate and growth efficiency, are dis- 

 cussed in more detail below. 



Energy Intake 



VOLUME SEARCHED.— The volume searched 

 by the Atlantic menhaden can be described in very 



z ^. 



1 • 



p & 



li- o 



Q V> 



UJ ~~ 



UJ UJ 



£ Q- 

 m in 

 O 



50 



30 



o 



UJ 



0.2 - 



0.0030 0.0060 00090 



PLANKTON CONCENTRATION 



FIGURE 10.— A, laboratory defined relationship between Atlantic 

 menhaden voluntary foraging speed and plankton concentration. B, 

 gross growth efficiency of menhaden which forage at these swim- 

 ming speeds for different periods of time. 



191 



