Enhancing defense and production efficiency in rainbow trout (Oncorhynchus mykiss) through prophylactic dietary selenium and vitamin E fortification

Abstract

The present investigation determined the effects of dietary concentrations of selenium (Se) and vitamin E (VE) alone or in combination on rainbow trout (Oncorhynchus mykiss) fingerlings over a 90-day trial. The levels of selenium and vitamin E and their combinations were chosen on the basis of requirements for this species but also for supra-nutritional assessment reported in the literature for other species and salmonids. Diets for rainbow trout cultured intensively in Pakistan may be relatively deficient in selenium and vitamin E contents because of economic considerations and poorer quality feeds. Seven experimental diets were therefore formulated with the inclusion of sodium selenite (Na₂SeO₂) and vitamin E alone and mixed at defined ratios, i.e., 3S (mg kg¹), 6S (mg kg¹), 160E (mg kg¹), 280E (mg kg¹), 1.5S/80E (mg kg¹) and 4.5S/90E (mg kg¹), and a diet without any supplementation was designated the control (Table 1), with 3 replicates per treatment group. At the end of the trial, a direct pathogen challenge study was conducted on rainbow trout exposed to Aeromonas salmonicida bacteria for 14 days. Compared with those fed the control or other diets, rainbow trout fed the 1.5S/80E and 4.5S/90E diets presented significantly (P < 0.05) greater final weights, net weight gains (NWGs), and specific growth rates (SGRs). The fish fed the 3S and 6S diets presented a nonsignificant (P > 0.05) increase in growth performance. Hematological parameters, i.e., red blood cells (RBCs), hemoglobin (Hb), the mean corpuscular value (MCV), the mean corpuscular hemoglobin (MCH), the mean corpuscular hemoglobin concentration (MCHC), hematocrit, and white blood cells (WBCs), were significantly (P < 0.05) elevated in the fish fed selenium and vitamin E alone or in combination compared with the control fish. The mortality rate was significantly (P < 0.05) high in the control, while the lowest mortality rate was observed in the fish fed higher levels of selenium and vitamin E in combination with 1.5S/80E and 4.5S/90E. Compared with the control, 3S and 6S diets, supplementation with vitamin E alone or in combination with selenium (160E, 280E, 1.5S/80E, and 4.5S/90E) improved the phagocytic activity of white blood cells in rainbow trout against stress after bacterial pathogenic challenge. In conclusion, supplementation with vitamin E significantly enhances the growth performance, hematological profile, and phagocytic activity of white blood cells against infection and reduces mortality in rainbow trout fingerlings.