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Does iron supplementation improve post-smolts Atlantic salmon performance and health in today’s practical diets?

  • May 29, 2024

Iron (Fe) is a crucial trace mineral involved in various biochemical processes in fish. For Atlantic salmon (Salmo salar L.), the dietary iron requirement ranges from 60 to 160 mg/kg. However, these requirements were established using purified diets with highly bioavailable iron forms, and may not reflect the needs in practical diets where interactions with antinutrients such as phytate can affect iron bioavailability.

Supplementing iron through an organic amino acid (AA) complex could enhance bioavailability and reduce oxidative interactions with other nutrients in the feed. This study, conducted by Joana Silva and colleagues, evaluates the impact of iron supplementation on the performance, pigmentation, health, and oxidative status of post-smolt Atlantic salmon fed practical diets.

The study was carried out over a 10-week period with post-smolt Atlantic salmon (mean initial weight 143 g) in a full salinity recirculating aquaculture system (RAS) at BioMar’s trial facility, ATC Patagonia. Fish were divided into duplicate groups of 100 and allocated to 14 tanks (2.5 m³ each). Seven low fish meal (FM) diets, supplemented with an organic Fe-AA complex at graded levels (208 - 443 mg/kg total Fe),

were tested. Fish were fed to satiation, and performance parameters such as growth, feed conversion ratio (FCR), and condition factors were monitored. Health-related responses, including hematocrit and hemoglobin concentrations and various plasma biomarkers, were assessed. The oxidative status of key nutrients in the feed was also analyzed. 

Table showing Blood hematocrit (HCT) and hemoglobin (Hb) concentrations were within the expected range in all dietary treatments (HCT 32 – 58% and Hb 55 – 110 g L -1 , respectively). No significant differences were observed in HCT among dietary groups while Hb concentration was significantly higher in groups fed with diets 4, 5 and 7 compared to groups fed with D1 (N=12; p<0.001).

Figure 1: Blood hematocrit (HCT) and hemoglobin (Hb) concentrations were within the expected range in all dietary treatments (HCT 32 – 58% and Hb 55 – 110 g L -1 , respectively). No significant differences were observed in HCT among dietary groups while Hb concentration was significantly higher in groups fed with diets 4, 5 and 7 compared to groups fed with D1 (N=12; p<0.001).

Results

Growth Performance

The results indicated that iron supplementation through an organic Fe-AA complex did not significantly affect the growth performance of post-smolt Atlantic salmon. The thermal growth coefficient (TGC) averaged around 3.31, and FCR averaged 0.75 across all dietary treatments. There was a trend toward lower FCR with increasing levels of iron supplementation, suggesting improved feed efficiency at higher iron levels (data not shown).

Health and Hematological Parameters

Iron supplementation significantly impacted several health-related parameters. Hemoglobin (Hb) concentrations were significantly higher in groups fed with diets containing higher levels of iron (D4, D5, D7) compared to the basal diet without iron supplementation (D1) (p<0.001) (Figure 1). Similarly, lactate dehydrogenase (LDH) levels were significantly lower in the highest iron group (D7), indicating better physiological condition under chronic stress (p<0.05) (Figure 2). Hematocrit levels remained within the expected range and did not differ significantly among the dietary groups.

Table showing A significantly higher level of lactate dehydrogenase (LDH) was observed in the plasma of fish fed with D1 compared to fish fed with D7 (N=12; p<0,05) while lactate (LCT) and all the other plasma biomarkers (cholesterol, glucose, albumin, total proteins, AST, ALT) were not significantly different among dietary treatments. There was, however, a trend to high levels of LCT in all groups (>9; LCT range 4 -9 mmol L -1 )

Figure 2: A significantly higher level of lactate dehydrogenase (LDH) was observed in the plasma of fish fed with D1 compared to fish fed with D7 (N=12; p<0,05) while lactate (LCT) and all the other plasma biomarkers (cholesterol, glucose, albumin, total proteins, AST, ALT) were not significantly different among dietary treatments. There was, however, a trend to high levels of LCT in all groups (>9; LCT range 4 -9 mmol L -1 )

Oxidative Status and Nutrient Stability

The study found no significant differences in the oxidative status of key nutrients among the different diets, indicating that the organic Fe-AA complex did not promote oxidation even at the highest supplementation levels. Astaxanthin and total carotenoid content were slightly lower in the basal diet (D1) compared to other diets, but the differences were not statistically significant. Other nutrients, such as anserine, peroxide, vitamins, and minerals, remained stable across all dietary treatments. (Figure 3)

Pigmentation

Significant improvements in fish pigmentation were observed with increasing iron supplementation. Fillets from fish fed the highest iron diet (D7) exhibited significantly higher redness and yellowness (Minolta score) compared to the basal diet group (D1) (p<0.0001). This may suggest that iron supplementation could enhance pigmentation, which is an important quality attribute in marketable fish.

Bar chart showing Astaxanthin and total carotenoids content were numerically lower in D1 than in all the other diets (p>0.05). Anserine, peroxide, vitamins and other minerals (data not shown) did not differ among all diets at start and end of the trial.

Figure 3: Astaxanthin and total carotenoids content were numerically lower in D1 than in all the other diets (p>0.05). Anserine, peroxide, vitamins and other minerals (data not shown) did not differ among all diets at start and end of the trial.

The findings demonstrate that supplementing iron through an organic amino acid complex will not compromise the oxidative stability of the feed. Enhanced hemoglobin levels and reduced LDH in plasma indicate improved health and better capacity to handle stress in post-smolt Atlantic salmon. Moreover, the improved pigmentation suggests potential benefits for the market value of the fish. 

Iron supplementation up to 230 mg/kg through an organic Fe-AA complex in low fish meal diets does not negatively impact the performance of post-smolt Atlantic salmon and can improve health status and pigmentation. This study supports the use of an organic iron AA complex in practical diets to enhance overall health status and fish quality. 

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