Dietary Supplementation With CoQ10 and Sel‐Plex Mitigates the Effects of Elevated Water Temperature on Performance, Digestive Physiology, Immune Competence, and Antioxidant Balance in Rainbow Trout

Background/Aims Elevated water temperatures can impair fish growth, immunity, and physiological performance. This study aimed to evaluate the effects of dietary coenzyme Q10 (CoQ10) and organic selenium (O‐Se; Sel‐Plex) supplementation on growth, digestive enzymes, innate immunity, antioxidant capacity, and the expression of stress‐ and immune‐related genes ( hsp70-α and il-1β ) in the liver of rainbow trout ( Oncorhynchus mykiss ) under thermal stress, measured using real‐time quantitative PCR (RT‐qPCR) with β‐actin as the reference gene. Methods A total of 225 rainbow trout (21 ± 0.5 g) were stocked in 150‐L recirculating tanks at either optimal (15°C) or elevated (20°C) water temperatures. Fish were fed diets supplemented with CoQ10 (0 or 20 mg/kg) and/or Sel‐Plex (0 or 5 mg/kg) for 60 days. Growth indices, digestive enzyme activity, hepatic antioxidant capacity, immune parameters, and the expression of hsp70-α and il-1β were assessed. Results Elevated temperature significantly reduced growth, feed efficiency, and immune status. Dietary supplementation with CoQ10 or O‐Se improved hepatic antioxidant capacity, digestive enzyme activity, and innate immunity, with the greatest effects observed when both supplements were combined. Cosupplementation also mitigated heat‐induced upregulation of HSP70‐α and IL‐1β and reduced liver malondialdehyde (MDA) levels. Conclusion Combined dietary CoQ10 and O‐Se supplementation alleviate the adverse effects of elevated water temperatures on rainbow trout by enhancing growth, physiological performance, antioxidant defenses, and immune responses.