A nutritional study consisting of four feeding experiments was conducted in which Atlantic salmon (Salmo salar) were first fed experimental diets at swim‐up (0. 18 g) and at weights of 1.0, 1.7, and 7.5 g. The objectives of the study were to (1) determine if the size at which Atlantic salmon are first fed lecithin affects their growth, survival, and body composition, and (2) compare the responses of Atlantic salmon to different sources of dietary lecithin. Fish fed 4% commercial food‐grade lecithin (i.e., 100% soy lecithin) or 8% feed‐grade lecithin (i.e., a corn–soy lecithin mixture containing over 50% corn lecithin and 5% insoluble impurities) from swim‐up grew faster than fish fed no lecithin but fed sufficient choline. Although fish fed supplemental lecithin from swim‐up grew equally well regardless of source and level of lecithin, they grew almost 50% faster than fish fed the basal diet with no supplemental lecithin or choline. Fish started on the experimental diets at 1.0 and 1.7 g grew faster when fed lecithin, but supplemental choline had no effect. Fish first fed lecithin at 7.5 g showed no growth advantage over fish fed the basal diet. Feeding 4% food‐grade or 8% feed‐grade lecithin enhanced survival when first fed to 0.18‐g fry, but had no effect when fed to larger fish. Feeding food‐grade lecithin (with or without choline) and choline without lecithin increased body fat in 0.18‐ and l.0‐g fish; 1.7‐g fish stored more fat when fed lecithin, but not when fed choline alone. As with growth and survival, there was no significant effect on body fat when supplemental lecithin or choline was fed to 7.5‐g fish. The body protein level decreased in the 0.18‐ and 1.0‐g Atlantic salmon fed lecithin or choline, but not in the 1.7‐ or 7.5‐g fish.
