Musculoskeletal Assessment of Rainbow Trout (Oncorhynchus mykiss) Reared in a Photoelectrocatalysis (PEC)-Based Recirculating Aquaculture System

This study explored the innovative application of photoelectrocatalysis (PEC), an advanced evolution of conventional photocatalysis, as a water remediation technology in recirculating aquaculture systems (RASs) for adult rainbow trout (Oncorhynchus mykiss, 150–200 g). The research focused on its impact on fish muscle and skeletal health using a multidisciplinary approach. Water quality parameters were registered and monitored, including nitrate, nitrite, and ammonia concentrations, and calcium content was measured at the end of the 1-month trial. To identify potential skeletal anomalies, imaging analyses including X-ray and computed tomography (CT) were performed. Additionally, histochemical analysis with Picrosirius Red (PSR) staining was used to evaluate collagen distribution in the vertebrae. Muscle density was assessed by CT scans and histological analysis to study metabolism and the size/diameter of white muscle fibers. Finally, genes related to myogenesis and antioxidant enzymes were studied to detect muscle growth and oxidative stress, respectively. The PEC showed lower nitrite and nitrate levels and higher dissolved oxygen (DO) compared to the control (CTR), likely due to photoelectrocatalytic activity, with overall more stable water chemistry. At the end of the trial, a reduction in calcium content in the water was observed (p < 0.05), without affecting the calcium content in the vertebral bones. Skeletal analyses revealed no anomalies, and collagen distribution in the bone matrix appeared normal. Muscle morphology indicated proper development, with a slight hypertrophic trend in the PEC group. Oxidative stress analysis suggested no adverse effects from PEC treatment, as antioxidant enzyme levels were similar between groups. In summary, the absence of skeletal anomalies and adequate muscle growth, associated with the improvement of water quality in the PEC, suggests that this new remediation technology is a valid, sustainable aquaculture approach, combining environmental innovation and fish health.