| Summary: | Facing a lack of standardized method to determine indispensable (I) amino acid (AA) maintenance requirements for fish, this study focused on the impact of selected nutritional factors on these requirements in salmon fry, in particular, the effect of dietary dispensable (D) AA, the past nutritional history and the way (and the form) in which the graded levels of the tested IAA are supplied to the fish. The framework for this study is a deductive factorial model. One of the major outputs of this research is the important role of DAA as a limiting factor under maintenance conditions. Their sparing effect on the IAA and their energetic role, allow the IAA released by protein breakdown to be used for re-synthesis. The overall balance of the AA supply, particularly DAA, must be considered for accurate measurements of IAA requirements for maintenance. The past nutritional history adaptation to a crystalline AA-rich diet affects the threonine (Thr) maintenance requirement estimation, but not the Thr retention efficiency and the Thr requirement for growth. The mechanisms involved in this phenomenon are unknown. However, our results highlight the necessity to standardize the diet composition, the feeding level and the duration of the adaptation period to crystalline-AA rich diet before any study on maintenance requirement is aimed. This research established that both graded supplementation technique and diet dilution procedure should be preferred to the feeding PF technique for the estimation of the IAA maintenance requirement for salmon fry. The Thr retention efficiency (76%) and the Thr maintenance requirement (7.3 mg/kgBW0.75/d) seemed to be independent of dietary protein level. We also highlighted the usefulness of the deletion method to determine simultaneously the N and Lys maintenance requirement (54 and 20 mg/kgBW0.75/d, respectively) and Lys pattern for both maintenance (6 g/16g N) and growth (15 g/16g N). The factorial model was discussed and was found to provide a promising tool for further research in IAA ...
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