FISHERY BULLETIN: VOL. 70, NO. 4 



and egestion of food by the organisms under 

 study. Recently, this problem was circumvented 

 by Conover (1966a). He developed a method 

 for determining the net assimilation efficiency 

 of organic matter {U')' without quantitative 

 recovery of either the uneaten food or the feces, 

 based upon the assumption that the assimilation 

 process affects only the organic portion of the 

 food. This is a critical assumption since the ash 

 in the food is treated as an inert label to measure 

 the quantity of food ingested. If this assumption 

 is made, the general definition of assimilation 

 efficiency, 



Table 1. — List of symbols. 



U' 



(^-^) X 100 



(1) 



where / is the organic weight ingested, and N 

 is the organic weight egested, reduces to Con- 

 over's equation. 



U' 



(1 - E') (F') 



X 100. (2) 



Symbol 



Explanation 



A Net organic weight assimilated. 



A-y Net amouni of on organic moiety assimilated.' 



E Dry weight of feces. 



E' Ratio of organic weight to dry weight of feces. [N/E). 



e'y Ratio of organic moiety to dry weight of feces. 



F Dry weight of food. 



F' Ratio of organic weight to dry weight of food. (l/F). 



I' Ratio of organic moiety to dry weight of food. 



/ Organic weight ingested. 



/„ Quantity of an organic moiety ingested. 



A' Organic weight egested. 



N„ Quantity of an organic moiety egested. 



R Ratio of organic weight to oxidizoble carbon. 



r. Ratio of an organic moiety to total organic weight of 



* food. il^/I). 



r Ratio of an organic moiety to total organic weight of 



" feces. {A',^/A'). 



U' Percent ossimilation efficiency of total organic weight 



u/nt!]oo). 



{/'„ Percent net assimilation efficiency of an organic moiety. 



^ {J^/I^){]OQ). 



' When denoting a specific organic moiety the subscript A' is replaced 

 by P for protein, L for lipid, CH2O for carbohydrates, C for oxidizoble 

 carbon. 



F' = I/F = ratio of organic weight to dry 

 weight of food ingested, and E' = N/E = ratio 

 of organic weight to dry weight of material 

 egested. Symbols are defined in Table 1. Con- 

 over (1966a) and Corner, Cowey, and Marshall 

 (1967) found close agreement between assim- 

 ilation efficiencies calculated by the ratio method 

 and direct quantitative recovery of food and 

 feces. In both cases the comparison was made 

 with zooplankton feeding on phytoplankton. 



Assimilation, however, is a complex function 

 controlled by (1) the net assimilation efficiency 

 iU'), or digestibility; (2) the concentration of 

 ash; and (3) the rate of ingestion. For our 

 work, we found it helpful to modify Equation 

 2 to consider (a) the combined effect of di- 

 gestibility and organic concentration upon the 

 amount of food assimilated and (b) the net as- 

 similation rate. A further modification was re- 



" Conover u.ses the term "assimilation efficiency.'' 

 Strictly this should be "net assimilation efficiency" since 

 it measures the difference between assimilated organics 

 and previously assimilated organics which are lost in 

 the feces (peritrophic membrane, digestive secretions). 

 The two terms are used interchangeably in this paper. 



quired to consider the assimilation of specific 

 organic moieties. The reasoning and equations 

 are presented briefly below. A more detailed 

 derivation can be found in Condrey (1971) . 



FEEDING EFFICIENCY 



A comparison of the amount of organic mat- 

 ter assimilated per unit of diet ingested is: 



A/F = {F')iU7100). 



(3) 



This is a valuable type of representation for si- 

 multaneous comparison of the relative organic 

 weight assimilated on different diets, as is dem- 

 onstrated in Figure 1. The ratio is termed 

 "feeding efficiency" for lack of a better term. 

 Note that whereas U' is a ratio of organic weight 

 assimilated to organic weight ingested, the feed- 

 ing efficiency considers the effect of differing or- 

 ganic concentrations in the diet. As will be 

 shown, this may be of considerable importance 

 when comparing net assimilation of diets sim- 



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