The salmon industry increasingly relies on plant-based ingredients to reduce dependency on marine resources. However, incorporating these plant-based alternatives into fish diets presents challenges, particularly in supplying essential long-chain polyunsaturated fatty acids (LC-PUFA), such as eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA). These fatty acids are crucial for fish health, growth, and reproduction.
Researchers from the Institute of Aquaculture at the University of Stirling (UK), University of Aberdeen (UK), the Veterinary Research Institute at the National Research Centre (Egypt), and Shantou University (China) studied nutritional programming and specifically evaluated the optimal duration of the dietary “stimulus” in Atlantic salmon (Salmo salar) to see if it could be shortened compared to a previous study that used a three-week “stimulus.”
Challenges and Solutions in n-3 LC-PUFA Production in Aquaculture
EPA and DHA are recognized for their health benefits in both fish and humans. In fish, these fatty acids are crucial for maintaining anti-inflammatory responses, tissue integrity, and overall health, including immune function. In particular, DHA is essential for synaptic and retinal membranes in nervous tissues. A deficiency in these fatty acids during development can negatively impact performance, tissue integrity, and stress responses in fish.
While Atlantic salmon can endogenously biosynthesize EPA and DHA, this capability is limited and depends on the presence of sufficient levels of alpha-linolenic acid (ALA; 18:3n-3) in the diet. Research has shown that net production of n-3 LC-PUFA is possible in salmon fed plant-based diets, though the rate of DHA biosynthesis in these fish is often insufficient to reach desired levels.
Nutritional Programming: A Novel Approach
An innovative solution to this problem is the application of early nutritional intervention, known as “nutritional programming” (NP). This concept involves feeding fish a predominantly plant-based diet for a short period early in their development, inducing greater efficiency in nutrient absorption and utilization when fed a similar diet in later stages. This mechanism leverages the organism’s developmental plasticity to create long-term functional changes.
According to the researchers, the NP concept has proven effective in several commercially important fish species, such as gilthead seabream (Sparus aurata), rainbow trout (Oncorhynchus mykiss), and European seabass (Dicentrarchus labrax). In the case of Atlantic salmon, previous studies have shown that a three-week stimulus phase with plant-based diets low in n-3 LC-PUFA from the first exogenous feeding resulted in greater growth and better nutrient retention, including EPA and DHA, during the freshwater growth phase.
However, a significant challenge is that fish fed plant-based diets during the stimulus phase tend to be smaller at the end of this period compared to those fed standard marine diets. This smaller size could be a limiting factor in commercial trials, as fish size influences subsequent performance.
Experimental Design
Juvenile Atlantic salmon were fed experimental plant-based diets (VS) or a standard marine-based control diet (MS) during different phases of their development. The VS diet contained 5% marine meals and 0% fish oil, while the MS diet contained 82% marine meals and 4% fish oil.
Fish were exposed to the VS diet for one (V1) or two weeks (V2) during the initial “stimulus” phase, followed by a standard marine-based diet during the “intermediate” growth phase. A final “challenge” phase involved feeding all fish a plant-based diet (VC) for six weeks.
Finally, the scientists evaluated the fish’s growth, proximate composition, fatty acid profiles, and gene expression.
Key Findings
The main findings of the study published in the journal Aquaculture are:
- Nutritional Programming and Growth: Although fish fed VS diets were smaller during the stimulus phase, there were no significant differences in overall growth or body composition at the end of the trial.
- Hepatosomatic and Viscerosomatic Indices: Fish fed VS diets for one week had smaller liver and visceral masses compared to those fed VS diets for two weeks.
- Fatty Acid Retention: Fish fed VS diets for two weeks showed a higher net gain of DHA during the challenge phase compared to those fed VS diets for one week.
- Gene Expression: Gene expression analysis revealed a downregulation of n-3 LC-PUFA biosynthesis genes in the pyloric caeca of fish fed VS diets, suggesting possible post-transcriptional modifications.
Conclusion
Nutritional programming holds great promise for optimizing plant-based diets in Atlantic salmon aquaculture. In this regard, the study results suggest that a short-term exposure to a plant-based diet during the early stages of life may not be sufficient to initiate nutritional programming in Atlantic salmon. However, the better performance of fish fed the plant-based diet for one week indicates that other factors, such as post-transcriptional modifications, may influence their response to plant-based diets.
Further research is needed to elucidate the underlying mechanisms of these findings and develop strategies to optimize the use of plant-based ingredients in Atlantic salmon aquaculture. The results of this study add to other research lines related to selective breeding of salmon to increase fatty acid levels or the enhancement of these compounds in fish feeds.
Contact
Stuart McMillan
Institute of Aquaculture, Faculty of Natural Sciences, University of Stirling
Stirling FK9 4LA, UK
Email: stuart.mcmillan@stir.ac.uk
Reference (open access)
McMillan, S., Martin, S. A., Król, E., Norambuena, F., Baumgärtner, S., Gong, X., Tawfik, M. M., Glencross, B., Taylor, J. F., Tocher, D. R., & Betancor, M. B. (2024). Can nutritional programming in Atlantic salmon (Salmo salar) be optimised with a reduced stimulus period? Aquaculture, 585, 740686. https://doi.org/10.1016/j.aquaculture.2024.740686
