Background: Smoltification is a critical physiological transition in Atlantic salmon production, during which fish must adapt from freshwater to seawater while maintaining growth, osmoregulatory capacity, and metabolic stability. This study evaluated whether a novel dietary feed solution, MIXA (MIXSCIENCE, France), could support growth performance and physiological robustness during the freshwater phase and after seawater transfer.
Material and Methods: Atlantic salmon parrs were fed for 12 weeks in a 2 × 2 factorial design combining two basal formulations, a practical smoltification diet and a non-smoltification diet, with or without MIXA at 200 g/ton. The trial included a 6-weeks freshwater phase followed by a 6-weeks seawater phase after direct transfer to full-strength seawater. Growth performance, feed utilisation, survival, plasma biochemical markers, free amino acids, nitrogen-related metabolites, and molecular indicators of gill osmoregulation and intestinal integrity were assessed before and after seawater transfer.
Results: During the freshwater phase, growth performance was mainly driven by the basal formulation: fish fed the smoltification diet showed higher final body weight, weight gain, specific growth rate, protein efficiency ratio, and improved feed conversion compared with fish fed the non-smoltification diet. MIXA induced a modest but significant increase in final body weight, while survival remained 100% in all treatments. In freshwater, MIXA also produced a clear higher metabolic signature, significantly reducing circulating concentrations of most essential amino acids, several non-essential amino acids, and nitrogen-related metabolites including taurine, SAM, SAH, homocysteine, cystathionine, ornithine, GABA, hydroxyproline, and beta-alanine. After seawater transfer, the growth effect of MIXA became more evident, with significant increase in final body weight and weight gain during the seawater phase. The same reduction in plasma free amino acids and nitrogenous metabolites persisted 48 hours after transfer, whereas classical plasma biochemical indicators remained only moderately affected, suggesting good physiological tolerance.
Growth performance throughout the entire duration of the trial (104 of feeding) – Freshwater and Saltwater phases
p-value
PC
PC MIXA
NC
NC MIXA
Formula (F)
Additive (A)
F x A
Survival, %
99.4 ± 1.2
100.0 ± 0.0
99.4 ± 1.2
100.0 ± 0.0
1.000
0.183
1.000
IBW, g
65.4 ± 0.6
64.7 ± 1.5
64.8 ± 1.0
65.1 ± 0.8
0.812
0.624
0.352
FBW, g
176.0 ± 2.6
177.8 ± 2.2
162.8 ± 1.8
173.2 ± 2.3
<0.001
<0.001
0.002
Weight gain/fish, g
110.6 ± 2.5
113.1 ± 2.4
98.0 ± 2.8
108.2 ± 2.6
<0.001
<0.001
0.013
SGR, %/day
0.95 ± 0.01
0.97 ± 0.02
0.89 ± 0.03
0.94 ± 0.02
<0.001
0.002
0.103
FCR
0.97 ± 0.01
0.97 ± 0.01
0.99 ± 0.01
0.99 ± 0.02
0.037
0.951
0.822
FI, %ABW/day
0.85 ± 0.01
0.87 ± 0.03
0.81 ± 0.02
0.86 ± 0.01
0.014
0.007
0.230
PER
2.22 ± 0.03
2.21 ± 0.02
2.18 ± 0.02
2.18 ± 0.06
0.043
0.867
0.764
Values are means ± standard deviation (n=4).
Discussion/Conclusion: The practical smoltification formulation was the main determinant of freshwater performance, whereas MIXA provided an additional growth benefit that was particularly visible after transfer to seawater. The consistent decrease in circulating amino acids and nitrogen-related metabolites suggests that MIXA modulated amino acid turnover, methylation-related pathways, and nitrogen metabolism during smoltification and early seawater acclimation. Overall, these findings support the potential of targeted dietary feed solutions to improve post-transfer growth while maintaining physiological stability in Atlantic salmon undergoing smoltification.