Dietary protein/energy ratio (mg/ kJ)Figure 2. Graphic description of the relationship between dietary proteinlenergy ratios
and growth rate in Sarotherodon mossambicus.Assimilation of Digested FoodDigestion breaks macromolecules in the diet into progressively smaller
subunits. Assimilation occurs when these subunits are small enough to pass
through the gut wall. Because assimilation is the point in the nutritional
process at which energy and materials gathered from the habitat are actually
incorporated into and become an integrated part of the consumer, a measure
of assimilation is the single most valuable quantitative descriptor of the diet.
Assimilation is usually quantified in terms of assimilation efficiency (AE):
(amount assimilated + amount ingested) x 100. AE's of tilapia feeding on
various diets have been determined by two techniques. The mass-balance
technique estimates the amount assimilated as amount ingested minus the
amount defecated. A disadvantage of this approach is that precise estimates
of ingestion and defecation are often difficult to obtain. In addition, this
approach can only be &ed under controlled experimental conditions that
necessarily differ from the natural feeding habitat. Since secretion of gastric
acid is very sensitive to disturbance (Fish 1960; Moriarty 1973), efficiency
of assimilation can be altered by experimental conditions. Nonetheless, the
mass-balance technique has produced valuable results when used with fish
acclimated to laboratory conditions and precise quantification that is possible
using 14c-labeled algae.
The second technique for determination of AE uses some refractile
compound in the diet as an undigested reference. Some workers have added
a reference compound such as Cr203 (Furukawa and Tsukahara 1966; Bowen