FISHERY BULLETIN: VOL. 81. NO. 2 



simple terms, i.e., an ellipsoid with cross-sectional 

 area equal to that of the fish's open mouth, and length 

 equal to the distance travelled by the fish per unit 

 time. The volume searched is equal for all types of 

 prey. With other species of filter-feeding fishes, a 

 slight modification of this basic formula may be 

 necessary, according to the mode of feeding. For ex- 

 ample, a number of species (northern anchovy, 

 Leong and O'Connell 1969; alewife, Janssen 1978; 

 gizzard shad, Drenner et al. 1978) are described as 

 rhythmically opening and closing the mouth during 

 feeding, apparently producing a suction which draws 

 in particles located outside the perimeter of the 

 mouth. Here, the cross-sectional area of the volume 

 searched is somewhat larger than the mouth area; 

 also, a correction factor is needed to account for the 

 proportion of the time the fish's mouth is closed and 

 not actually filtering. Nevertheless, the basic sim- 

 plicity of the volume searched by a filter feeder is in 

 marked contrast to the case of a predatory fish or par- 

 ticulate planktivore. Since these fishes visually lo- 

 cate and capture their prey, the volume searched is 

 complex and depends on a variety of factors, includ- 

 ing the visual capacity and adaptations of the fish, the 

 inherent visibility and behavioral characteristics of 

 the prey, and the nature of the underwater visual en- 

 vironment (quantity and quality of the illumination, 

 clarity of the water). Thus the volume searched by a 

 particulate feeder is different for different types of 

 prey, and even if a fish were to swim at constant speed 

 and feed on a single prey type, the volume searched 

 will continually change according to variables such as 

 the time of day, and the depth at which the fish swims 

 (Durbin 1979). 



FORAGING SPEED.— Foraging speed affects 

 both the energy intake and expenditure terms in the 

 energy budget, but only the energy intake in the ni- 

 trogen budget. Foraging speed is the principal deter- 

 minant of the volume searched for food, since the 

 cross-sectional area of the mouth in an Atlantic 

 menhaden of a given size is constant. Foraging speed 

 in the Atlantic menhaden increases asymptotically 

 with increasing food concentration. Because of this 

 there will be two critical levels of abundance for each 

 prey species: c t , the threshold concentration at which 

 the menhaden are stimulated to feed, and t\, the con- 

 centration at which foraging speed becomes approx- 

 imately independent of food concentration. With 

 Ditylum, the value of c, was about 4.5 jxg chlorophyll 

 a/1 (0.0027 kcal/1), and the fish swam at an average 

 speed of 41.3 cm/s. From Figure 2 it is seen that when 

 c = 0.0027 kcal/1, the fish swimming at 41.3 cm/s 

 would obtain a maintenance ration in slightly more 



than 7 h. At higher food concentrations the required 

 feeding time would be much less, generally <4 h. 

 These results suggest that Atlantic menhaden feed- 

 ing onDitylum will swim at their "preferred" speed as 

 long as the concentration is sufficiently high to en- 

 able the fish to meet their daily energy requirements 

 in <8 h of feeding. At lower food concentrations the 

 fish conserve energy by swimming more slowly dur- 

 ing feeding. Whether these results are fortuitous 

 and apply only to Ditylum, or instead imply a funda- 

 mental relationship between foraging speed and 

 foraging time which is applicable to different food 

 types, cannot be determined from present informa- 

 tion. 



FILTRATION EFFICIENCY.— The effective vol- 

 ume searched will be determined by the filtration ef- 

 ficiency {e). As described earlier (Equation (6)) 

 filtration efficiency is fairly high for zooplankton- 

 sized particles, but in the range of phytoplankton- 

 sized particles declines sharply to a minimum size 

 threshold of about 1 3 jum. This means that the Atlan- 

 tic menhaden cannot directly exploit the < 20 ju.m size 

 fraction of phytoplankton, which forms the greater 

 part of the total phytoplankton biomass on their sum- 

 mer feeding grounds (Durbin et al. 1975). Menhaden 

 exploit this food resource indirectly, however, by 

 feeding upon the zooplankton. 



ASSIMILATION EFFICIENCY.— The efficiency 

 with which food is assimilated further modifies the 

 energy intake by the Atlantic menhaden and will af- 

 fect the predicted growth rate and growth efficiency 

 in the model. If assimilation changes with different 

 meal sizes or rates of feeding, then the proportion of 

 ingested energy which is available for metabolism 

 and growth will also change. Most investigators have 

 found that assimilation efficiency is independent of 

 ration size (Gerking 1955; Menzel 1960; Pandian 

 1967; Birkett 1969; Iwata 1970; Beamish 1972; 

 Kelso 1972; Staples and Nomura 1976). However, 

 Elliott (1976) and Solomon and Brafield (1972) 

 found a slight decrease in assimilation efficiency as 

 meal size increased. (In the latter study the authors 

 suggest that the change may have been an artifact 

 arising from the incomplete recovery of a small 

 amount of fecal material in the tank.) For the Atlantic 

 menhaden we assumed a constant assimilation ef- 

 ficiency with different ration sizes. 



The mean assimilation efficiencies observed for the 

 Atlantic menhaden feeding on phytoplankton were 

 quite high (86.4% for carbon, 92.4% for nitrogen, and 

 89.5% for calories). For Atlantic menhaden feeding 

 on zooplankton the values were similarly high (86.7, 



192 



