DURBIN and DURBIN: ENERGY AND NITROGEN BUDGETS FOR ATLANTIC MENHADEN 

 8 



20 40 00030 00060 



FORAGING SPEED PLANKTON CONCENTRATION 



(c, kcol//) 



8 16 



FORAGING TIME 

 (h, hours/day ) 



(s, cm/sec) 



FIGURE 6. — Model I nitrogen budget for the Atlantic menhaden. Panels are as in Figure 1. 



ist not only in the maximum or asymptotic values of 

 the two growth efficiencies, but in the x-intercepts of 

 the curves as well. The x-intercepts are of particular 

 interest since they define the minimum requirements 

 ( s min» c min» ^min) f° r the fish to obtain a maintenance ra- 

 tion in nitrogen or calories. In Figure 8 A, for instance, 

 where c = 0.0030 kcal/1 and h = 14 h/d, s min K is 

 <s min N . The Atlantic menhaden would be able to 

 show positive growth in calories at a foraging speed of 

 about 7 cm/s, whereas positive growth in nitrogen 

 would require a higher foraging speed of about 14 

 cm/s. However, in Figure 8B, C, c„^ N is <c mmK and 

 ^ mm.N i s< ^min,A> respectively. Thus in these examples 



the menhaden would show positive growth in ni- 

 trogen at a lower food concentration, and with a short- 

 er foraging time, than they would in calories. 



Atlantic menhaden can exercise direct control over 

 the variables s and h, but they may or may not have 

 any impact on the environmental variable c. Thus it is 

 of interest to consider how a change in the values of s 

 and h will affect the minimum plankton concentra- 

 tion required for the Atlantic menhaden to obtain a 

 maintenance ration in calories and nitrogen. The curve 

 in Figure 9, calculated from the Model I budget, 

 shows the combinations of s and h at which c min K = 

 c min,N- For all combinations of s and h, which fall below 



189 



