DURBIN and DURBIN: ENERGY AND NITROGEN BUDGETS FOR ATLANTIC MENHADEN. 



suggests that protein is conserved when food levels 

 are low. 



It should also be noted that since the foraging costs 

 of obtaining a ration of a particular size will vary 

 according to s, c, and h, there will not be a single 

 unique) relationship between ration size, growth 

 rate and growth efficiency in Atlantic menhaden. 



The models predict that over most of the range of 

 plankton concentrations where growth is possible, 

 growth efficiency will be higher for calories than for 

 nitrogen. These findings are consistent with field 

 observations that the fat and caloric composition of 

 the menhaden increases relative to protein during its 

 season of growth (Dahlberg 1969; Dubrow et al. 

 1976). At low plankton concentrations the fish forage 

 at speeds such that growth in nitrogen is possible 

 even when there is an overall net energy deficit. This 

 suggests that protein is conserved when food levels 

 are low. 



Optimal Foraging by Planktivores 



In a landmark study, Ware (1975) combined Ivlev's 

 (1960) data on Alburnus with Holling's (1966) preda- 

 tion model to develop a bioenergetic model of this 

 particulate planktivore, which could be used to test 

 different theories of optimal foraging. Ware was the 

 first to demonstrate the existence of s G0PT and 

 s K0PT , and showed the importance of swimming 

 speed in determining the energy balance within the 

 fish. His analysis demonstrated that the swimming 

 speeds of fish in nature can be extremely useful and 

 sensitive indicators of how different species respond 

 to and exploit changes in their food resource. An in- 

 teresting feature of Ware's (1975) model of a particu- 

 late planktivore was that as c increased, s G0PT 

 curvilinearly increased to a maximum at a single food 

 concentration, and thereafter declined, whereas 

 s kopt declined monotonically with increasing values 

 of c. These changes in s G 0PT and s K 0PT were due to 

 the effect of handling time on the rate of ingestion in 

 the Holling (1966) model. In contrast the present 

 study, which extends Ware's concepts of s G0PT and 

 s K 0PT to a filter feeder, shows that since handling time 

 is negligible in a filter feeder, s G0PT increases 

 asymptotically with increasing values of c, whereas 

 s K 0PT is solely a function of h and independent of c. It 

 is interesting that for both particulate and filter- 

 feeding planktivores, distinct foraging strategies are 

 required in order to achieve maximal growth rate or 

 growth efficiency. 



The experimental data from the Atlantic menhaden 

 make it possible to determine whether the foraging 

 behavior of this species is directed towards enhanc- 



ing some measure of ecological fitness such as growth 

 rate or growth efficiency. This may be done by com- 

 paring the growth rates and growth efficiencies 

 calculated for the observed swimming speeds of the 

 menhaden with those that would result if the fish 

 were to swim at speeds equivalent to either s GO pt or 

 s k,opt- The comparison is made with s G 0PT in Figure 

 11 for the case where h = 14 h/d and with s 



A'.OPT 



m 



Figure 12 for the case where c = 0.0030 kcal/1. 



Figure 1 1 demonstrates that the growth of Atlantic 

 menhaden which swim according to the laboratory 

 derived relationship in Equation (48) is very close to 

 the maximum possible growth at each concentration 

 of plankton. This suggests that foraging speed in the 

 adult Atlantic menhaden is a behavioral adaptation 

 to maximize growth rate. 



In contrast, at any given concentration of food the 

 observed foraging speed was always >s K0PT , which 

 resulted in submaximal values of K x K (Fig. 12). This 

 is evidence that the fish were not acting to maximize 

 growth efficiency. To maximize growth efficiency the 

 fish would have had to regulate their foraging speed 

 according to the duration of feeding. This was not ob- 

 served in Atlantic menhaden in the laboratory, where 

 foraging speed at a given concentration of food 

 remained constant for periods of up to 7 h. Further, 

 we have shown that foraging strategies which regu- 

 late swimming speed in order to maximize growth 

 rate and growth efficiency are mutually exclusive. 



Figures 4 and 11 provide an explanation for the 

 hyperbolic nature of the plankton concentration- 

 foraging speed relationships in Equation (48). s G0PT 

 changes most rapidly at low concentrations of plank- 

 ton, and it is in this region where Atlantic menhaden 

 most strongly regulate their foraging speed, s G 0PT 





o 

 a. 



h= 14 hours/day 



— S = S G.OPT 



s = observed 



s = 30cm/sec 



s= 50 cm/sec 



O 00006 O.OOI8 00030 0.0012 00054 00066 00078 00090 



PLANKTON CONCENTRATION (c.kcal//) 



FIGURE 11.— A comparison of the growth of the Atlantic menhaden 

 at different concentrations of plankton, when the fish swim accord- 

 ing to s G 0PT ; their actual voluntary speeds; and constant speeds of 

 30 and 50 cm/s. Foraging time is 14 h/d. 



195 



