To increase the sustainability of trout farming, the industry requires alternatives to fish-based meals that do not compromise animal health and growth performances. To develop new feeds, detailed knowledge of intestinal morphology and physiology is required. We performed histological, histochemical, immunohistochemical and morphometric analysis at typical time points of in vivo feeding trials (50, 150 and 500 g). Only minor changes occurred during growth whereas differences characterized two compartments, not linearly distributed along the intestine. The first included the pyloric caeca, the basal part of the complex folds and the villi of the distal intestine. This was characterized by a significantly smaller number of goblet cells with smaller mucus vacuoles, higher proliferation and higher apoptotic rate but a smaller extension of fully differentiated epithelial cells and by the presence of numerous pinocytotic vacuolization. The second compartment was formed by the proximal intestine and the apical part of the posterior intestine complex folds. Here we observed more abundant goblet cells with bigger vacuoles, low proliferation rate, few round apoptotic cells, a more extended area of fully differentiated cells and no pinocytotic vacuoles. Our results suggest that rainbow trout intestine is physiologically arranged to mingle digestive and absorptive functions along its length.

Summary A 12‐week feeding trial was undertaken to evaluate the effects of partial replacement of marine raw materials in the diet with soybean meal and beef tallow on growth and product quality of European eel (Anguilla anguilla). Based on growth, feed efficiency and chemical composition, soybean meal was found to be an unsuitable ingredient for eel diets as a substitute for fishmeal (25% on a protein basis), probably because of the low digestibility of its carbohydrate content. However, beef tallow can be used to replace 50% of fish oil without reduction in growth, provided that digestible carbohydrates are present in the diet. No major effects of protein and lipid sources in the diet were found on fillet chemical composition. Sensory analysis revealed no significant differences between fish fed the control and the experimental diets, with the exception of salty taste which was significantly higher in fish fed combined soybean meal and beef tallow compared with fish fed the control diet.

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Twenty-two samples of commercial ®sh meals from Norway, Chile and Peru were analysed for cholesterol and oxysterols using gas chromatography. Cholesterol content ranged from 25.2 to 64.8 g kg ±1 total lipids. Detectable levels of the oxysterols 7b-hydroxycholesterol and 7-ketocholesterol were found and their identity was con®rmed by gas chromatography-mass spectrometry. Samples of ®sh meal exhibited wide variability in oxysterols content, 7b-hydroxycholesterol ranging from 3.9 to 105.6 mg kg ±1 total lipids (0.4±9.4 mg kg ±1 dry matter) and 7-ketocholesterol from 2.0 to 56.2 mg kg ±1 total lipids (0.2±5.0 mg kg ±1 dry matter). The formation of 7b-hydroxycholesterol and 7-ketocholesterol in one sample of Norse LT94 ®sh meal stored at room temperature was also studied. Oxysterol content increased during the ®rst 42 days of storage by about 350% and then decreased with further storage. The low amount of oxysterols measured indicates a limited degree of cholesterol oxidation in commercially available ®sh meals.

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