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Duplicate groups of rainbow trout were fed low-protein (38% ) diets containing high levels (38% ) of five different carbohydrate sources (raw starch, extruded corn, extruded wheat, extruded corn starch or extruded wheat starch). A modified pair-feeding method was used to allow equal intakes of protein and of digestible energy by each group of fish. Growth parameters were followed over a period of 18 weeks; the digestibility of the diets, nitrogen and energy balances, and the respiratory and ammonia quotients of fish fed the different diets were determined. To study the long-term effects of dietary carbohydrates, plasma glucose profile and its control by bovine insulin was followed in fish fed over 30 weeks with selected test diets. The results show that inclusion of extruded cereals or extruded starch improves availability of dietary energy. High levels of carbohydrates do not adversely affect overall growth or nutrient retention efficiencies. Long-term feeding with carbohydrate-rich diets does not confer on trout any adaptive capacity to regulate postprandial glycemia levels. Respirometric measurements appear to provide interesting data on the metabolic utilization of body substrates.

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Aquaculture is facing a strategic challenge to improve feed suitability and support the global increase in fish production. Improvements in diet formulation for sustainable nutritional strategies have focused to date on the partial substitution of marine resources by plant resources but will now include other alternative feedstuffs. Growth trials and body composition data provide valuable indicators of fish nutritional status, while omics technologies may contribute to a better understanding of fish nutrition and help to demonstrate how feed and nutrients act in fish metabolism. Metabolomic approaches give an insight into fish metabolism through a non‐targeted analysis of metabolites in tissues or biofluids that involve multiple factors affecting fish, such as nutrition. In this review, we highlight the outcomes of publications in metabolomics applied to fish nutrition. We explain the concept of metabolomics and discuss specific technical considerations related to sample type, sampling and sample preparation. We show how metabolomic studies help to elucidate the impact of nutrition on fish fillet composition and fish metabolism. Finally, we describe the potential applications of metabolomic approaches for the non‐invasive monitoring of fish nutritional status.

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Compared with fish of a slow-growing strain, fast-growing rainbow trout exhibited significantly smaller white fibre diameters, throughout development from hatching to 24 cm body length, although possessing similar total number of fibres. In contrast, in red muscle, no differences were observed in fibre diameter between the two strains, but the fast growing fish showed a significantly higher number of red fibres. The differences in growth rate between the two strains were related to the mean white fibre diameter and were found to be matched by proportional adjustments in recruitment of new fibres to the growing muscle. Thus, the largest and fastestgrowing strain showed evidence of sustained higher recruitment of muscle fibres that endowed this strain with the potential to maintain rapid somatic growth for longer and accomplish greater muscle growth. 1999 The Fisheries Society of the British Isles

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Brown trout (Salmo trutta) (2.7 g initial mean weight) were reared in freshwater for 8 months at water velocities of <0.1 (control group), 1 or 2 body lengths per second (BL s−1) (exercise groups). Growth (body weight, body length and body width), muscle structure (muscle fibre diameter and width of myosepta) and flesh quality parameters (dry matter, muscle pH, collagen content and solubility, instrumental evaluation of texture) were measured at the end of the experiment. The body weight of fish at 1 BL s−1 was 22% higher than the control group. Muscle development was stimulated at 1 and 2 BL s−1, leading to a higher condition factor, greater body height and width, and muscle fibre hypertrophy (55 vs. 59.5 μm fibre diameter in the control and 2 BL s−1 groups respectively). Connective tissue and collagen were only slightly affected by exercise (no difference in collagen solubility, but a greater proportion of γ trimer and fewer α chains in the control compared with the exercised group). Flesh quality was affected, with greater dry matter content and lower post‐mortem pH in the 1 BL s−1 group compared with the control and 2 BL s−1 groups. The mechanical resistance of the raw fillets was slightly but significantly increased by exercise. The exercise‐related changes in muscle structure and texture are discussed.