Can nutritional programming in Atlantic salmon (Salmo salar) be optimised with a reduced stimulus period?

New strategies are required to enhance the efficient assimilation and bioconversion of plant-based ingredients in Atlantic salmon (Salmo salar) diets, especially relating to the essential long-chain polyunsaturated fatty acids (LC-PUFA), eicosapentaenoic acid (EPA; 20:5n-3) and docosahexaenoic acid (DHA; 22:6n-3). Our study investigated nutritional programming and specifically evaluated the optimal duration of dietary ‘stimulus’ and whether it could be reduced compared to a previous study using a three- week ‘stimulus’. Fish were fed an experimental ‘stimulus’ vegetable-based diet (V_S, 5% marine meals [MM]/0% fish oil [FO]) or a standard marine-based control (M_S, 82% MM/4% FO) for either one (V1) or two weeks (V2 and M) from first exogenous feeding. All groups were then fed a standard marine based formulation, for an ‘intermediate’ grow-out phase to the end of 16 weeks post-first feeding, prior to a ‘challenge’ phase of six weeks when all fish were fed a vegetable-based diet (V_C, 10% MM/0% FO). Compared to M, fish from both V1 and V2 groups were significantly smaller at the end of the ‘stimulus’ phase, but there were no statistical differences in overall growth, proximate or fatty acid compositions at the end of the trial. However, hepatosomatic and viscerosomatic indices were significantly lower in V1 compared to V2 fish and there was an overall trend of improved performance in V1 fish throughout the ‘intermediate’ and ‘challenge’ phases. During the ‘challenge’ phase, M fish displayed a greater net gain of DHA than V1 fish, whilst V2 was a net consumer of all n-3 LC-PUFA over the same period. Compared to M, n-3 LC-PUFA biosynthesis genes in pyloric caeca were downregulated in both experimental groups indicating possible post-transcriptional modification of this pathway in either V1 or V2, considering the differences in DHA retention levels between groups. Taken together, the results suggested that nutritional programming was not initiated by a one- or two- week ‘stimulus’. However, more studies are required to elucidate the mechanism behind enhanced performance of V1 fish.