Development of a technique for fish feed digestibility estimations using microtracers

The aim of this thesis was to compare the methods of feed digestibility determination in fish and subsequently develop a technique for fish feed digestibility estimations using microtracer. The review of current methods showed that none of the existing methods fulfil adequately the criterion of an ideal method when employing external markers. Therefore microtracer F-Ni was used in this study and a series of experiments was conducted to evaluate the digestibility coefficients in fish ( rainbow trout, Oncorhynchus mykiss) using microtracer F-Ni as a marker and compare its suitability with the conventional Cr203 and acid insoluble ash (AIA) markers. The particle counts of microtracer F-Ni per unit weight were validated in Experiment 1, as it was essential to establish a consistent count of particles per unit weight for the calculation of digestibility values. Both direct counting and colour development methods were used for this purpose. Results showed that microtracer F-Ni marker contained on average 64 particle per mg. The sequential uniformity in the rate of passage of the microtracer through the digestive tract were tested in Experiment 2, using similar diets with four inclusion levels to select one level for further digestibility studies. One percent inclusion level was found to be the best as the rate of increase in concentration in different parts of the digestive tract and feces were sequential and progressive for all the three days tested. Apparent digestibility coefficients (ADC) of protein and dry matter were determined and compared employing different fecal collection procedure and different collection interval using this marker with chromic oxide marker and acid insoluble ash marker in Experiment 3. Results indicated that the ADC values generated by using microtracer F-Ni were consistent and in close agreement with the ADC values calculated by using chromic oxide as marker in all the situations. The ADC determined by using AIA were inconsistent and higher than the ADC calculated by both chromic oxide and microtracer F-Ni markers. When tnicrotracer F-Ni and chromic oxide were incorporated into the same diet, ADC values calculated were higher than values obtained with individual marker inclusion. In Experiment 4, protein and dry matter digestibility of fish meal and sunflower meal ingredients were determined and compared using microtracer F-Ni, Cr203 and AIA markers. Digestibility coefficients of nutrients generated using microtracer resulted ADC values similar (P> 0.05) to those using chromic oxide as external marker. Experimental diets containing either microtracer F-Ni or chromic oxide supported excellent growth performances and feed conversion efficiency. In summary, microtracer F-Ni incorporated at 1% can be used as an external marker in fish feed digestibility studies in rainbow trout.