DURBIN and lU'RBIN: ASSIMILATION EFFICIENCY OF ATLANTIC MENHADEN 



assimilation efficiency in this experiment was also 

 somewhat reduced. However, even with this high 

 feeding rate, the behavior of the Atlantic menha- 

 den did not change significantly during the course 

 of the feeding period, and the fish gave no indica- 

 tion of approaching satiation ( Durbin et al. 1981). 



In summary we conclude that the calculated 

 gastric evacuation by Atlantic menhaden calcu- 

 lated from Elliott and Persson's (19781 model 

 agrees well with our experimental measurements 

 of elimination rates following a meal. The calcu- 

 lated lag between the gastric evacuation of a par- 

 ticle and its elimination in the feces was similar 

 for all ration sizes and also agreed well with the 

 estimates of the time of the beginning of fecal 

 elimination (Table 3, column 2). These results in- 

 dicate that R ' should be a good estimator of R. 

 However, the stomach evacuation rates of Atlantic 

 menhaden need to be determined directly, both in 

 order to verify the model predictions, and to 

 explore the reasons for the systematic deviations 

 of the observed elimination from that predicted as 

 food ration size changes. 



Finally, with regard to methods employed for the 

 study offish digestion, present results with Atlan- 

 tic menhaden indicate that when digestion is an 

 exponential process, measurements of the time to 

 "100*^ evacuation" are of limited value. This is 

 because they cannot be used to determine the ex- 

 ponential evacuation rate R. The evacuation of 

 the final portion of a meal is extremely protracted 

 and may even be nonexponential. With Atlantic 

 menhaden, for example, the food eliminated be- 

 tween 8 and 41 h after the end of feeding corre- 

 sponded on average to the food ingested during the 

 final 35 min of feeding; feces eliminated during 

 hours 14-41 corresponded to the final 10 min of 

 feeding. This makes selection of an end point, to be 

 taken as 100*^, quite difficult and arbitrary. If the 

 final stages are nonexponential, then obviously 

 the estimate of/? would be biased. The final prob- 

 lem is computational: in an exponential process, 

 the stage of 100% digestion is mathematically 

 never reached, and it is necessary to approximate 

 1007f with another value, such as 98, 99, or 99.9% . 

 Although the choice of any of these values would 

 be purely arbitrary, each provides a very different 

 estimate of the value of R . 



Assimilation Efficiency 



The Atlantic menhaden was very efficient at 

 absorbing N, C, and calories from both phyto- 



plankton and zooplankton food. These high as- 

 similation efficiencies are in general agreement 

 with those reported for carnivorous fish (Gerking 

 1955; Menzel 1960; Pandian 1967; Beamish 1972; 

 Kelso 1972). Few studies have examined the as- 

 similation efficiency of herbivorous fishes. C. M. 

 Moriarty and D. J. W. Moriarty (1973) and D. J. W. 

 Moriarty and C. M. Moriarty ( 1973) found that the 

 maximum mean C assimilation ofTilapia nilotica 

 varied according to food type, being highest for the 

 diatom Nitzschia (79%), somewhat lower for two 

 bluegreen algae, Microcystis (70% ) and Anabaena 

 (75%), and least for the green alga Chlorella 

 (49%). The average maximum C assimilation of 

 Haplochromis nigripinnis for Microcystis was 

 71%. Menzel (1959) reported that Holacanthus 

 bermudensis assimilated 85% (range 82-91%) of 

 the N and 77.7% (range 72-84%) of the calories 

 from two macroalgae, Monostroma and En- 

 teromorpha. Plant materials described from the 

 gut contents of Atlantic menhaden are planktonic 

 and resuspended benthic diatoms, and detrital 

 particles presumably derived from marsh grasses 

 ( Peck 1894; Darnell 1958; Peters and KjeLson 1975; 

 Jeffries 1975). The high assimilation efficiency for 

 D. brightwelli indicates that Atlantic menhaden 

 should have high assimilation efficiency for other 

 diatoms also. The ability of Atlantic menhaden to 

 assimilate detrital material has not been experi- 

 mentally determined. Planktonic green and blue- 

 green algae are much less important in the marine 

 environment than in freshwater; moreover they 

 are generally too small to be filtered by Atlantic 

 menhaden i Durbin and Durbin 1975' and are not 

 a significant food. Thus the comparatively low 

 assimilation efficiency which has been reported 

 for some freshwater herbivores fed green and blue- 

 green algae is not relevant to Atlantic menhaden or 

 to most other marine phytoplankton-feeding fishes, 

 which eat mainly diatoms and dinoflagellates (i.e., 

 see Durbin 1979 and references therein). 



Ration size has generally been shown to have 

 little or no effect on assimilation efficiency (Ger- 

 king 1955; Pandian 1967; Beamish 1972; Kelso 

 1972; Solomon and Brafield 1972), although Elliott 

 (1976) found that assimilation in brown trout, 

 Salmo trutta, decreased as ration level increased. 



Our results show a slight increase in assimila- 

 tion efficiency with increasing meal size. This ef- 

 fect, as well as the changes in assimilation effi- 

 ciency during the course of a feeding period, may 

 have two possible causes: 1) the addition to the 

 fecal pellets of significant quantities of materials 



613 



