Digestibility in Rainbow Trout: Comparison of the Direct and Indirect Methods of Measurement

Abstract

The present study validated a method for the automatic collection of fish feces, by comparing digestibility data obtained by the direct (total feces collection) and indirect (chromic oxide tracer) methods. Six groups of 15 rainbow trout (Salmo gairdneri) were successively fed four pelleted diets, three of which contained 30% (dry weight) of chironomid larvae, daphnids, or gammarids. Apparent digestibility coefficients were calculated by the direct and indirect methods for dry matter, crude protein, and energy content. Results were identical with both methods for the daphnid and gammarid diets. The direct method gave significantly higher digestibility coefficients for the reference diet and the chironomid diet. It is possible to obtain reliable digestibility figures by the direct method with an automatic fish feces collector for some types of diet.