A Critical Biological Transition, Novel feed solutions can support salmon smoltification

by Stéphane Frouel, Aquaculture Project Manager, Mixscience, France 

Moving from freshwater to seawater requires major physiological remodelling. Atlantic salmon must adapt gill ion transport, intestinal function and endocrine regulation to maintain salt and water balance. In farming conditions, this transition has practical consequences: fish entering seawater with stronger growth and better physiological preparedness are more likely to maintain performance. 

The study compared a smoltification diet and a non-smoltification diet, each with or without Mixa at 200 g/tonne [Table 1]. Fish were followed for seven weeks in freshwater and eight weeks in seawater. Growth was recorded by tank and samples collected before transfer and 48 hours after seawater exposure were used to assess biochemical, metabolic and molecular criteria associated to physiological changes. 

Table 1. Detailed composition and nutritional profile of the experimental diets used in the freshwater and seawater phases 

Growth performance 

In freshwater, performance was driven mainly by the basal formulation [Table 2]. Fish receiving the smoltification diet reached higher final body weight, gained more weight, grew faster and converted feed more efficiently than fish on the non-smoltification diet. Protein efficiency was also improved. Mixa had a narrower effect, significantly increasing final body weight, while weight gain and feed intake showed positive but non-significant trends. Survival was 100 percent across treatments. 

Table 2. Growth performance after 42 days of feeding (end of freshwater phase) 

Values are means ± standard deviation (n=4). 

After seawater transfer, the smoltification diet maintained its advantage because fish entered this phase heavier [Table 3]. Mixa then became more evident, significantly increasing final body weight and weight gain across dietary backgrounds. Feed intake, feed conversion, survival and protein efficiency were not significantly affected, indicating that the main seawater benefit was additional biomass gain rather than improved feed efficiency. 

Table 3. Growth performance in between day 42 and day 104 (saltwater phase) 

Values are means ± standard deviation (n=4). 

The biochemical profile of plasma free amino acids helped interpret this response [Figure 1]. Before transfer, Mixa reduced circulating concentrations of several essential amino acids, including valine, methionine, lysine, leucine and arginine, as well as some non-essential amino acids. Because these changes occurred with stable or improved growth, they more likely reflected amino acid use, tissue uptake or metabolic routing than a simple deficiency. 

Nitrogen-related metabolites followed the same direction [Figure 2]. Taurine, SAM, SAH, homocysteine, cystathionine, ornithine, GABA and beta-alanine were lower in supplemented fish. These compounds are associated with osmotic balance, methylation, sulphur amino acid metabolism and nitrogen handling, suggesting a coordinated metabolic adjustment during smoltification. 

This signature persisted 48 hours after seawater transfer [Figures 3 and 4]. Supplemented fish again showed lower levels of several amino acids and nitrogenous metabolites in plasma, including taurine, SAM, SAH, homocysteine, cystathionine, ornithine and beta-alanine. In the context of later growth improvement, the pattern is consistent with more active nutrient use during early seawater adaptation. 

Classical plasma biochemical markers remained relatively stable. Protein, albumin, globulin, glucose, lipids and several enzyme activities did not indicate broad metabolic disturbance. Plasma ions were also largely unaffected, with only limited changes in calcium before transfer and potassium after transfer. Overall, the treatments did not destabilise systemic osmoregulation. 

Gill gene expression provided the clearest molecular signal [Table 4]. Before transfer, NKAα1a, NKAα1b, NKCC, growth hormone receptor and IGF1 receptor were not significantly affected. After transfer, NKCC expression increased in fish receiving MIXA, suggesting a targeted stimulation of chloride transport capacity during early seawater acclimation. 

Table 4. Gill osmoregulatory molecular markers prior (freshwater) and 48 hours after an abrupt transfer to saltwater conditions 

Values are means ± standard deviation (n=4).  

Intestinal tight-junction markers remained unchanged before and after transfer, indicating no detectable transcriptional disruption of epithelial barrier integrity during the acute transition. 

Interpreting the response 

Taken together, the results suggest two complementary effects. The smoltification formulation improved freshwater performance and helped fish enter seawater at a stronger starting point. MIXA showed its clearest added value after transfer, when physiological costs increase and nutrient partitioning becomes critical. 

The most consistent biological signature was the reduction in plasma amino acids and nitrogen-related metabolites in supplemented fish. In isolation, lower circulating pools could be viewed negatively. Here, however, they occurred alongside preserved biochemical status, stable ionic balance, unchanged intestinal markers and improved seawater growth, supporting the hypothesis of accelerated use or metabolic channelling. 

The NKCC response refines this interpretation by pointing to selective osmoregulatory adjustment rather than broad physiological disruption. Still, plasma metabolites remain indirect indicators and the 48-hour sampling point describes only the acute transfer window. 

Result 

This study shows that freshwater growth depended strongly on the smoltification formulation, while Mixa supplementation became most relevant after seawater transfer. The smoltification diet improved growth and feed utilisation before transfer, whereas Mixa increased final body weight and weight gain during the saltwater phase. Physiologically, Mixa consistently modified amino acid and nitrogen-metabolite profiles without broad biochemical, ionic or intestinal-integrity disruption. The main molecular signal was increased gill NKCC expression after seawater transfer. Overall, the findings suggest that targeted nutrition can support both performance and metabolic adaptation during the freshwater-to-seawater transition, with the strongest supplementation effect appearing under the challenge of seawater entry. 

The trial was conducted in Portugal facilities under approved ethical protocols. The authors thank SPAROS Lda for providing the experimental diets and technical support.