The bioavailability of nutrients in commercial feeds available in Portugal was evaluated in three size groups (40g. l00g and 180g) of rainbow trout (Oncorhynchus mykiss) on two private farms. Dietary crude fibre level was used as the digestibility marker. A series of laboratory trials was also carried out to investigate the influence of various biotic and abiotic factors on apparent digestibility coefficients and to provide corroboration for the field survey.
The principal factors influencing apparent digestibility values during the field survey were diet quality, temperature and water-dissolved oxygen. The apparent digestibilities of crude protein, organic matter and dry matter were strongly inversely correlated with dietary fibre level in the three size groups of farmed trout. In addition, significant positive correlations were obtained between lipid level in commercial feeds and the organic matter digestibility coefficients. Under controlled conditions, dietary lipid level (7%, 14% and 21%) had a marked effect on digestibility, the highest digestibility values being obtained with the 21% lipid diet (87.77%, 76.55% and 70.46%) compared to 84.59%, 63.25% and 57.74% with the 7% lipid diet for crude protein, organic matter and dry matter, respectively. A signification correlation was also obtained between all apparent digestibility coefficients and the feed protein level for all size groups of fish studied in the field. In the laboratory, however, only at 21®C was there a marked effect and significantly higher digestibility values were obtained with the higher protein (45%) diet (85.98%, 83.74%, 80.37% and 84.54% for protein, organic matter, dry matter and energy, respectively) as compared to the lower protein (30%) diet (74.16%, 71.66%, 65.54% and 68.70% for protein, organic matter, dry matter and energy, respectively).
General increases in apparent digestibility values of more than 10% were obtained between the lower (10°C or 15°C) and the higher (21 °C or 22°C) experimental temperatures, much higher than values previously reported.
Food intake was shown to be directly dependent on dissolved oxygen (D.O.) and for one unit decrease of D.O. food intake decreased by about 0.5% or 0.25% body weight/day whether fish were subjected to an abrupt decrease of water O2 level or to prolonged hypoxia. Furthermore, apparent digestibility values were significantly increased when fish were subjected to prolonged hypoxia.
Feeding frequency (1, 2 or 4 meals/day) did not influence digestibility, whereas time of day and fish size did have a significant effect.
Mean digestibility values estimated either from chromic oxide or crude fibre analysis were significantly different. The difference was, however, very small and bearing in mind the advantages of an internal marker as compared to an added marker, this study has shown that crude fibre is a useful tool for feed evaluation, especially in practical situations.
The need for, and importance of, digestibility evaluations is discussed and summarised in the form of a conceptual model.