Antioxidant nutrition in Atlantic salmon (Salmo salar) parr and post-smolt, fed diets with high inclusion of plant ingredients and graded levels of micronutrients and selected amino acids

Abstract

The shift from marine to plant-based ingredients in fish feeds affects the dietary concentrations and bioavailability of micronutrients, amino acids and lipids and consequently warrants a re-evaluation of dietary nutrient recommendations. In the present study, an Atlantic salmon diet high in plant ingredients was supplemented with graded levels of nutrient premix (NP), containing selected amino acids, taurine, cholesterol, vitamins and minerals. This article presents the results on the antioxidant nutrients vitamin C, E and selenium (Se), and effects on tissue redox status. The feed ingredients appeared to contain sufficient levels of vitamin E and Se to cover the requirements to prevent clinical deficiency symptoms. The body levels of α-tocopherol (TOH) in parr and that of Se in parr and post-smolt showed a linear relationship with dietary concentration, while α-TOH in post-smolt seemed to be saturable with a breakpoint near 140 mg kg^-1. Ascorbic acid (Asc) concentration in the basal feed was below the expected minimum requirement, but the experimental period was probably too short for the fish to develop visible deficiency symptoms. Asc was saturable in both parr and post-smolt whole body at dietary concentrations of 190 and 63-89 mg kg^-1, respectively. Maximum whole body Asc concentration was approximately 40 mg kg^-1 in parr and 14 mg kg^-1 in post-smolt. Retention ranged from 41 to 10% in parr and from -206 to 12% in post-smolt with increasing NP supplementation. This indicates that the post-smolts had an extraordinarily high consumption of Asc. Analyses of glutathione (GSH) and glutathione disulphide (GSSG) concentrations and the calculated GSH based redox potentials in liver and muscle tissue, indicated only minor effects of diets on redox regulation. However, the post-smolt were more oxidized than the parr. This was supported by the high consumption of Asc and high expression of gpx1 and gpx3 in liver. Based on the present trials, the recommendations for supplementation of vitamin C and E in diets for Atlantic salmon are similar to current practices, e.g. 150 mg kg^-1 of α-TOH and 190 mg kg^-1 Asc which was the saturating concentration in parr. Higher concentrations than what would prevent clinical deficiency symptoms are necessary to protect fish against incidents of oxidative stress and to improve immune and stress responses. There were no indications that the Se requirement exceeded the current recommendation of 0.3 mg kg^-1.

Keywords: Antioxidant nutrients; Atlantic salmon; Nutrient requirement; Redox regulation; Selenium; Vitamin C; Vitamin E.

Figure 1. Whole body concentrations (mg kg⁻¹ wet weight) of redox dependent micronutrients in Atlantic salmon parr (Trial 1) and post-smolt (Trial 2) in response to dietary supplementation of micronutrients and selected amino acids at supplementation of 0–400% NP.

Data were first fitted to second order polynomials and the fit was then compared to a first order polynomial fit, which was chosen when p > 0.05 for the second order equation. The p given in figures with first order and second order fits indicate that the slope is significantly different from 0 or that second order fits are significantly better than first order fits, respectively. n.s., not significant.

Figure 2. Retention (% of eaten) of redox dependent micronutrients in Atlantic salmon parr (Trial 1) and smolt (Trial 2) in response to dietary supplementation of micronutrients and selected amino acids at 0–400% NP.

Data were first fitted to second order polynomials and the fit was then compared to a first order polynomial equation fit. α-Tocopherol in Trial 2 had significantly best fit to the second order equation (p < 0.05). Vitamin C retention fitted a logarithmic equation better than a second order polynomial (p < 0.001). The p given in figures with first order fits indicates that the slope is significantly different from 0. n.s., not significant.

Figure 3. Muscle reduced and oxidized glutathione (GSH and GSSG, μmoles kg⁻¹ wet weight) and the GSH based redox potential (mV) in Atlantic salmon parr (Trial 1) and post-smolt (Trial 2).

The dietary supplementation of micronutrients and selected amino acids was graded from 0–400% NP, where 100NP is the assumed requirement.

Figure 4. Liver reduced and oxidized glutathione (GSH and GSSG, μmoles kg⁻¹ wet weight) and the GSH based redox potential (mV) in Atlantic salmon parr (Trial 1) and post-smolt (Trial 2).

The dietary supplementation of micronutrients and selected amino acids was graded from 0–400% NP, where 100NP is the assumed requirement.

Figure 5. Muscle TBARS (nmoles g⁻¹ ww) in Atlantic salmon parr (Trial 1) and post-smolt (Trial 2).

The dietary supplementation of micronutrients and selected amino acids was graded from 0–400% NP, where 100NP is the assumed requirement.

Figure 6. PCA bi-plots on expression of redox dependent genes related to dietary supplementation of micronutrients and selected amino acids at 0–400% NP.