Introduction: Aquaculture is rapidly expanding, making fish welfare a crucial factor for sustainable and efficient production. Stress negatively affects fish health and growth, so minimally invasive tools to monitor physiological status are needed. Skin mucus biomarkers offer a practical way to assess stress in red tilapia Oreochromis sp. fry. This study evaluated the welfare and stress status of red tilapia fry cultured in two systems: biofloc technology (BFT) and traditional ponds over 30 days, through the analysis of skin mucus biomarkers including protein, glucose, lactate, and cortisol, as well as their protein-normalized ratios . Methods : Red tilapia fry weighing 0.6±0.3g were studied. The fish were anesthetized with eugenol at a dose of 0.2 mL/L of water and the skin was subsequently scraped above the midline. Mucus was collected after 10, 20, and 30 days of culture. Mucus was extracted using an adaptation of the technique proposed by Fernández-Alacid et al. 2016 . The samples were preserved in liquid nitrogen and then transferred to dry ice to facilitate shipment to the Department of Cellular Biology, Physiology and Immunology of the Faculty of Biology of Universidad de Barcelona (Spain). The mucus was thawed and prepared by mechanical homogenization and then centrifuged. In the aqueous phase, skin mucus-associated biological markers, including protein, glucose, lactate, and the hormone cortisol, were used to determine their relationships with protein. All readings were performed in triplicate on the Tecan Nanoquant Infinite N200 equipment adapted for mucus. Results: Mucus protein content : Soluble protein levels in the skin mucus were consistently higher in fish from the biofloc system compared to pond-cultured fish, indicating a better physiological condition. Mucus glucose : At day 10, glucose levels were significantly higher in pond fish (58.27 ± 27.16 µg/mL) than in biofloc fish, reflecting stress responses and greater activity of the interrenal axis. Subsequently, glucose decreased in both systems, remaining lower in biofloc. Cortisol/protein ratio: This ratio was significantly higher in pond fish, showing increased stress levels. The cortisol/protein ratio corrects for mucus dilution variability, providing a more reliable stress marker.
Conclusions: Skin mucus-associated biomarkers (SMABs) are reliable for measuring stress in red tilapia Oreochromis sp. fry in a minimally invasive way, which is crucial to improving management practices in aquaculture. The biofloc system provides a more suitable environment for red tilapia culture, promoting lower stress and improved physiological condition as reflected by mucus biomarkers.
This research was funded by the Colombian General Royalties System (SGR) under reference number BPIN 2020000100465.