In order to address the problems of pollution from commercial farms, the bioavailability of nutrients in commercial feeds available in Portugal was evaluated in three size (40 g, 100 g and 180 g) groups of rainbow trout (Oncorhynchus mykiss) over a one year period and under natural production conditions. The principal factors influencing apparent digestibility values were diet quality, temperature and dissolved oxygen. Most probably suspended solids also affected digestibility especially during the summer months. The apparent digestibilities of crude protein, organic matter and dry matter were strongly inversely correlated with dietary fibre level in the 3 size groups of farmed trout. In addition, significant positive correlations were obtained between lipid level in commercial feeds and the organic matter digestibility coefficients. A significant correlation was also obtained between all apparent digestibility coefficients and the feed protein level for all size groups of fish studied in the field. The need for, and importance of, digestibility studies is demonstrated through estimates of nutrient flow and waste production in two different situations illustrating the effect of dietary quality.

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