[Significance of extruded feeds for trout nutrition and water protection].

Extruded feeds exhibit an improved starch digestibility and are more firmly bound due to the almost complete gelatinization of the starch. This results in fewer fines and longer water stability than pelleted feeds. Extruded pellets also have the advantage that they can soak up more oil than a conventional pellet. It is therefore possible to increase the maximum oil content to more than 20%. On the other hand extruding feeds is more expensive than steam pelleting. Gelatinized starch is a useful energy source in trout diets helping to reduce feed conversion ratios. Proportions up to 35-40% in the diet are tolerable. Using high dietary levels of both gelatinized starch and oil the non-protein energy of feed may be increased and thus a protein-sparing effect results. High-energy diets enable to reduce excretion of faeces and of nitrogen via gills. In addition a decrease of phosphorus level in feeds and thus of phosphorus excretion by fish is possible. Extruded high-energy diets therefore make a contribution to improve water quality.