BIOLOGICAL AND NUTRITIONAL APPROACH TO THE ENVIRONMENTAL-IMPACT OF TROUT CULTURE IN PORTUGAL

In order to address the problems of pollution from commercial farms, t he bioavailability of nutrients in commercial feeds available in Portu gal was evaluated in three size (40 g, 100 g and 180 g) groups of rain bow trout (Oncorhynchus mykiss) over a one year period and under natur al production conditions. The principal factors influencing apparent d igestibility values were diet quality, temperature and dissolved oxyge n. Most probably suspended solids also affected digestibility especial ly during the summer months. The apparent digestibilities of crude pro tein, organic matter and dry matter were strongly inversely correlated with dietary fibre level in the 3 size groups of farmed trout. In add ition, significant positive correlations were obtained between Lipid l evel in commercial feeds and the organic matter digestibility coeffici ents. A significant correlation was also obtained between all apparent digestibility coefficients and the feed protein level for all size gr oups of fish studied in the field. The need for, and importance of, di gestibility studies is demonstrated through estimates of nutrient flow and waste production in two different situations illustrating the eff ect of dietary quality.