DURBIN and DURBIN: ENERGY AND NITROGEN BUDGETS FOR ATLANTIC MENHADEN 



been shown (Fig. 4) thats Gi0PT increases with increas- 

 ing food concentration. This is because with increas- 

 ing c, the rate of energy intake increases per unit of 

 energy expenditure. An increase in food particle size 

 affects the ingestion rate in a manner analagous to an 

 increase in particle abundance, and thus we find that 

 s G 0PT increases with increasing particle size as well 

 (Fig. 13). s G0PT is most strongly affected by food par- 

 ticle size in the range of 20-60 ftm, moderately af- 

 fected within the range of 60-300 jum, and relatively 

 unaffected by further increases in particle size above 

 about 300 /Am. In other words, s G 0PT is strongly size- 

 dependent in the range of phytoplankton particles, 

 less so in the range of microzooplankton, and is for 

 practical purposes independent of particle size in the 

 range of copepodites and late- stage nauplii. This pat- 

 tern, of course, reflects the filtration efficiency curve 

 of the gill rakers (Equation (6)). 



PARTICLE LENGTH l/im| 



0009 Kcol// 



Chi o(mg/m ! 

 1200 1600 2000 *Pl d^ weight (mg/m 3 ) 

 PLANKTON CONCENTRATION 



FIGURE 13.— The effect of particle size on the relationship between 

 the foraging speed which maximizes the Atlantic menhaden's 

 growth rate (s G 0PT ) and plankton concentration. 



Figure 13 illustrates the need for information on 

 how the Atlantic menhaden responds to mixtures of 

 different-sized particles. For example, do the men- 

 haden respond to the total biomass of particles, or do 

 they key in on certain size classes, ignoring the 

 remainder even though they may filter these particles 

 simultaneously with the larger prey? We have seen 

 that Atlantic menhaden feeding on a single food type 

 will alter their energy expenditures according to the 

 abundance of food, such that they maximize their 

 growth rate at each level of food abundance. However 

 there is a need for further investigation of their feed- 

 ing behavior on different sizes and mixtures of par- 

 ticles to determine the degree to which they act as 

 "optimal foragers" in a mixture of plankton 

 species. 



Application of the Atlantic Menhaden 

 Models to the Field 



The energy and nitrogen budgets have been derived 

 in terms of three controlling variables, each of which 

 can be determined from direct field measurements: 

 The foraging speed (s), the concentration of plankton 

 (c), and the foraging time (h). Foraging speed can be 

 measured in the field using acoustic techniques, and 

 this procedure can be used to verify our predictions 

 of swimming speed based on laboratory inves- 

 tigations of the relationship between s and c. If con- 

 firmed in the field, these laboratory studies will 

 enable us to eliminate s as an independent variable 

 and define the budgets simply in terms of c and h. 

 However, as mentioned previously, before we can use 

 this approach in the field, where the fish feed on a 

 variety of particle sizes, additional laboratory work is 

 needed to quantify the foraging speed- food-concen- 

 tration relationships for different types and sizes of 

 plankton. The foraging time {h) could be determined 

 from diel surveys of stomach contents to determine 

 gut fullness and the state of digestion of the food (the 

 latter is an indicator of how recently the food was in- 

 gested). If h proves to be relatively invariant, or under 

 simple control of an external variable such as day 

 length, it may ultimately become possible to describe 

 the energy and nitrogen budgets of the Atlantic 

 menhaden solely as a function of the average concen- 

 tration of different-sized plankton in the water. 



The effects of body size and temperature also need 

 to be considered in applying the models to the field. 

 Hettler (1976) has investigated the effects of body 

 size, temperature, and salinity on routine metabo- 

 lism in juvenile Atlantic menhaden. The influence of 

 these variables on the swimming and feeding behavior 

 of the Atlantic menhaden, and on the other com- 

 ponents of the energy budget, must be investigated 

 as well, before a general energy and nitrogen budget 

 for the Atlantic menhaden can be described. 



Another point to consider in applying the present 

 energy budget to the field is that Atlantic menhaden 

 in nature may have additional energy expenditures 

 beyond those of the laboratory fish, principally the 

 costs of predator avoidance, spawning activity, and 

 the energy cost of migration. The first two activities 

 would increase respiratory expenditure, and corre- 

 spondingly reduce the amount of surplus energy that 

 is available for growth. It is not clear to what extent 

 seasonal migrations of the Atlantic menhaden (Nichol- 

 son 1971, 1978) represent an additional energy cost, 

 however, since it is possible that the Atlantic men- 

 haden continue feeding as they move along their 

 migratory routes. In addition, the seasonal migration 



197 



