Introduction
Interest in selenium (Se) nutrition for Atlantic salmon has grown as diets have shifted from Se-rich fish meal to alternative ingredients that are lower in Se and differ in Se speciation (Aas et al., 2022). Smoltification describes the period during which Atlantic salmon adapt for their transition to sea, with risk of high mortalities in commercial production during the early seawater phase (Bj��rnsson et al., 2011). The Se requirements of Atlantic salmon during this sensitive period remain unknown. Furthermore, rearing conditions may affect requirements, with an increasing use of recirculating aquaculture systems (RAS) in Norwegian smolt production (Kolarevic et al., 2014). Current growth-based recommendations for dietary Se may underestimate the Se needed for tissue homeostasis and welfare (Philip et al., 2025). Therefore, this study aimed to establish Se requirements in Atlantic salmon during smoltification, investigate its long-term effect and their interaction with the rearing environment.
Material and method
A regression design was used to evaluate the effects of graded dietary Se on Atlantic salmon smolt reared in RAS. Seven experimental diets containing graded Se levels (0.45–2.80 mg/kg, Se-yeast) were fed to duplicate tanks of 120 parr (initial BW 37.1 ± 0.5 g). The lowest and highest Se treatments were run in triplicate in both RAS and flowthrough (FT) systems, providing a factorial comparison of rearing environment. After 11 weeks in freshwater, 16 fish per tank were sampled for target tissues and whole fish. Fish from selected treatments were then transferred to sea and fed a common commercial diet (mean Se 0.79 mg/kg) to slaughter size to assess longterm effects of smoltstage Se nutrition. Regression models were fitted to growth data and tissue Se concentrations to estimate optimal dietary Se, and twoway ANOVA was used to test effects of rearing environment.
Results
Smolt reared in RAS showed a slight linear increase in final body weight with higher dietary selenium levels (p=0.01, R2< 0.01, lme), and across both RAS and FT, fish fed Se2.8 had higher specific growth rate than fish fed Se0.4 (0.61 ± 0.05 vs 0.54 ± 0.06, p=0.04, 2-w aov). The whole-body Se concentrations in RAS-reared smolt increased linearly with dietary Se, and the dietary Se concentration required to maintain initial tissue Se level was 1.23 mg/kg feed (CI 1.15-1.29, p<0.01, R2 = 0.98 , lme). Body weight differences established at the smolt stage persisted three months after transfer to sea, but initially smaller groups showed higher growth, and no significant differences in body weight were observed after six months at sea (3978 ± 1684 g; p=0.29, lme). Cataract prevalence at slaughter was significantly reduced with increasing dietary Se levels during smoltification (p=0.05, R2 = 0.05, glm).
Discussion
Growth benefits from Se supplementation were observed to range from 0.24 to 2.58 mg/kg across salmonid species (Du et al., 2021; Wang et al., 2018) without a significant effect previously reported in Atlantic salmon (Antony Jesu Prabhu et al., 2020; Lorentzen et al., 1994). In the present study, differences in final body weight were minor and dependent on rearing environment. The reduced cataract prevalence in slaughter size fish indicates a long-term effect of Se supplementation, possibly linked to antioxidant effects in the eye tissue (Bjerk��s and Sveier, 2004; Philip et al., 2023). The dietary Se level of 1.23 mg/kg required to maintain whole-body Se homeostasis was above the previously suggested requirement level (0.6-0.9 mg/kg) in Atlantic salmon parr and post-smolt stages (Philip et al., 2025).
Acknowledgment
This work received funding from the Norwegian Seafood Research Fund, FHF through grant number 901834, "Revitalise – Requirement of vitamin and trace minerals for Atlantic salmon health". The funders had no role in the study design, data collection and analysis.
References
Aas, T.S., ��sg��rd, T., Ytrest��yl, T., 2022. Utilization of feed resources in the production of Atlantic salmon (Salmo salar) in Norway: An update for 2020. Aquac. Rep. 26, 101316.
Antony Jesu Prabhu, P., Holen, E., Espe, M., Silva, M.S., Holme, M.-H., Hamre, K., Lock, E.-J., Waagb��, R., 2020. Dietary selenium required to achieve body homeostasis and attenuate pro-inflammatory responses in Atlantic salmon post-smolt exceeds the present EU legal limit. Aquaculture 526, 735413.
Bjerk��s, E., Sveier, H., 2004. The influence of nutritional and environmental factors on osmoregulation and cataracts in Atlantic salmon (Salmo salar L). Aquaculture 235, 101–122.
Bj��rnsson, B.T., Stefansson, S.O., McCormick, S.D., 2011. Environmental endocrinology of salmon smoltification. Gen. Comp. Endocrinol., Profiles in Comparative Endocrinology: Glen Van Der Kraak 170, 290–298.
Du, L., Yu, H., Li, L., Zhang, Q., Tian, Q., Liu, J., Shan, L., 2021. Dietary selenium requirement of coho salmon (Oncorhynchus kisutch W.) alevins. Aquac. Int. 29, 2291–2304.
Kolarevic, J., Baeverfjord, G., Takle, H., Ytteborg, E., Reiten, B.K.M., Nerg��rd, S., Terjesen, B.F., 2014. Performance and welfare of Atlantic salmon smolt reared in recirculating or flow through aquaculture systems. Aquaculture 432, 15–25.
Lorentzen, M., Maage, A., Julshamn, K., 1994. Effects of dietary selenite or selenomethionine on tissue selenium levels of Atlantic salmon (Salmo salar). Aquaculture 121, 359–367.
Philip, A.J.P., Fjelldal, P.G., Rem��, S., Silva, M., Holme, M.-H., Lock, E.-J., Waagbo, R., 2023. Dietary zinc, selenium and water temperature during early seawater phase influences the development of vertebral deformities and cataract in adult Atlantic salmon. Aquaculture 572, 739529.
Philip, A.J.P., Kortner, T.M., Wischhusen, P., Hamre, K., Krogdahl, ��., Skj��rven, K.H., L��kka, G., Berge, G.M., Espe, M., Sissener, N.H., 2025. Revisiting Vitamin and Trace Mineral Nutrition in Atlantic Salmon Farming With Emphasis on Fish Health. Rev. Aquac. 17, e70087.
Wang, L., Zhang, X., Wu, L., Liu, Q., Zhang, D., Yin, J., 2018. Expression of selenoprotein genes in muscle is crucial for the growth of rainbow trout (Oncorhynchus mykiss) fed diets supplemented with selenium yeast. Aquaculture 492, 82–90.