Digestibility in selected rainbow trout families and relation to growth and feed utilisation

Experiments have been carried out aimed at clarifying variations in the digestibility of dietary nutrients in rainbow trout families and studying how differences in digestibility may be related to growth and feed utilisation at various growth rates. The digestibility of protein, lipid, carbohydrates (nitrogen-free extracts, NFE) and dry matter was analysed in two experiments involving eight rainbow trout families [Ab, Ba, Cd, Dc (first study); V, X, Y, Z (second study)]. In the first experiment rainbow trout were reared for 128 days at 13.0A degrees C, and in the second experiment, they were reared for 84 days at 16.8A degrees C. In both experiments, the fish were fed ad libitum and reared from an initial weight of 70-100 g to a final weight of 500-700 g. When the fish reached a weight of approximately 200 g, some individuals were moved to another experimental system in which the digestibility of protein, lipid, nitrogen-free extracts and dry matter was measured. Taken as a whole, our results indicate that selective breeding still offers a large potential for improved growth and feed utilisation in rainbow trout strains. In the first study, family Dc showed a higher specific growth rate (SGR) than the other three families (P <0.05), and family Ba showed a lower feed conversion ratio (FCR) than family Ab (P <0.05); there were no observed differences in digestibility despite some differences in growth. In the second study, family Y grew faster than all of the other families (P <0.05), and family Z grew faster than families V and X (P <0.01). A comparable pattern was seen for FCR, with family Y utilising feed better than family V (P <0.05), and families V, Y and Z performing better than family X (P <0.001). Protein digestibility was higher in the two fastest growing families (Y and Z) than in the slower growing family X (P <0.05), while lipid digestion was higher in family Y than in family V (P <0.05). A comparison of the results from both experiments revealed that protein digestibility in particular was closely related to the SGR and the FCR at high growth rates. However, despite the advantageous protein digestibility on fish growth, analysis of the protein retention efficiency (PRE) showed that when protein was ingested in relatively large amounts, as in the fastest growing families, the "excess" nitrogen was excreted and therefore did not contribute to protein deposition in the fish body. Hence, the potential weight gain offered by improved protein digestibility does not materialise when the protein intake is above a certain level. Other factors must therefore explain the positive relation between fast growth and high protein digestibility.