Introduction
The Scottish farmed salmon industry continues to expand and there is a requirement to reduce the level of finite marine origin raw materials used in traditional feed formulations. One alternative is to produce feeds with higher proportions of plant-derived oils and proteins, but these diets can lead to reduced feed utilization and also have lower levels of essential omega-3 long-chain polyunsaturated fatty acids (n-3 LC-PUFA), eicosapentaenoic acid (EPA; 20:5n-3) and docosahexaenoic acid (DHA; 22:6n-3). A novel solution to this problem is a concept referred to as “nutritional programming”. This involves applying an early nutritional intervention or “stimulus” where fish are fed a predominantly vegetable-based diet for a short period, to induce more efficient uptake and utilization of nutrients from a similar diet when fed later in life. Previously, it was demonstrated that Atlantic salmon (Salmo salar) fed such diets during a three-week stimulus phase from first exogenous feeding, resulted in significantly greater growth and nutrient retention efficiency during freshwater grow out, associated with upregulation in key pathways of intermediary metabolism (Clarkson et al. 2017; Vera et al. 2017). The current study aims to elucidate whether the effects of nutritional programming are sustained during post-smolt rearing in seawater at a transcriptomic level.
Materials and methods
Four experimental dietary groups were established in triplicated treatment tanks (Figure 1). Two treatments were fed an experimental vegetable-based diet for a three-week “stimulus” period from first feeding (V), while remaining tanks were fed a standard marine-based diet (M; Figure 1). Following stimulus, all groups were then fed a standard marine ingredient diet until week 36. Thereafter, two treatment groups were re-fed a vegetable-based diet for four weeks prior to sea transfer (MVV, VVV), while remaining tanks were maintained on marine-based feeds (MMV, VMV). Following a two-week acclimatization post- sea water (SW) transfer, when fish were fed a standard marine based diet, all fish were transferred to a vegetable-based diet for a 14-week period. Liver and pyloric ceaca tissue were collected from six fish per tank immediately prior to SW transfer and at the start and end of SW challenge for molecular analysis. RNA was extracted from tissues, reverse transcribed and sequenced on an Illumina NovaSeq platform, producing an average on 49 million reads per sample, before quantifying expression based on read alignment to the S. salar genome.
Results & Discussion
Our hypothesis was that fish subjected to a vegetable-based stimulus diet would better utilize dietary nutrients in later development. Growth and retention results presented previously (McMillan et al. 2021) suggested no conclusive evidence, by those measures, that this was the case. However, there was evidence that groups fed a combination of both diets in FW adapted to the SW environment more rapidly. We also saw some evidence, from fish fed both diets in FW, that fish may better regulate their metabolism according to requirements when the n-3 LC-PUFA levels are running low. This study presents RNAseq results indicating whether there is evidence to support a continued nutritional programming effect at a transcriptomic level during the SW rearing phase. Furthermore, functional regulatory pathways will be discussed relating to the FW dietary regime, with primary focus on regulation of fatty acid metabolism at the end of FW and subsequent SW stages. Empirical evidence of nutritional programming regulating fatty acid biosynthesis to trigger endogenous production of essential EPA and DHA could determine how we formulate more resource-efficient aquafeeds and feeding strategies for Atlantic salmon in the future.
References
Clarkson, M., Migaud, H., Metochis, C., Vera, L. M., Leeming, D., Tocher, D. R. and Taylor J. F. (2017) Early nutritional intervention can improve utilisation of vegetable-based diets in diploid and triploid Atlantic salmon (Salmo salar L.), British Journal of Nutrition, doi:10.1017/S0007114517001842
McMillan, S., Glencross, B. D., Taylor J. F., Gong, X., Tocher, D, R., Gómez-Requeni, P. and Betancor, M, B. (2012) DO EFFECTS OF NUTRITIONAL PROGRAMMING IN FRESHWATER CONTINUE DURING SEAWATER REARING OF ATLANTIC SALMON (Salmo salar)? Aquaculture Europe, 6 October, Funchal.
Vera, L. M., Metochis, C., Taylor J. F., Clarkson, M., Skjærven, K. H., Migaud, H., and Tocher, D. R. (2017) Early nutritional programming affects liver transcriptome in diploid and triploid Atlantic salmon, Salmo salar, BMC Genomics (2017) 18:886
Acknowledgements
This work was completed as part of the biotechnology and biological sciences research council (BBSRC) funded project NUTRIPROG “Investigating the potential of nutritional programming to improve the utilisation of sustainable feeds in aquaculture” (BB/R018812/1) in collaboration with colleagues at the
University of Aberdeen and BioMar.