Effects of a novel mixture of natural additives on mucosal and blood biomarkers in European sea bass ( <i>Dicentrarchus labrax</i> ) fry under transport stress

The transport of live fish is a critical phase in aquaculture production, often associated with high levels of stress that can negatively impact animal welfare and performance. This study had a dual objective: (1) to assess the effectiveness of Dorabrin CRX P®, a dietary supplement containing diatomaceous earth and polyphenolic extracts, in mitigating stress in European sea bass (Dicentrarchus labrax) fry during pre-transport handling and short-term transport, and (2) to evaluate the suitability of skin mucus as a non-invasive matrix for monitoring physiological stress indicators. The fish that received feed supplemented with 0.8% of Dorabrin CRX P® for 10 days prior to transport exhibited significantly lower levels of plasma cortisol, glucose, lactate, cholesterol and haematocrit compared to the control group, suggesting the supplement’s effectiveness in modulating both hormonal and metabolic responses to stress. In parallel, skin mucus analysis confirmed its value as a non-invasive matrix: cortisol, glucose, and lactate levels in mucus increased following stress exposure, with significantly higher values in the control group, showing strong positive correlations with corresponding plasma levels. Total protein concentration in mucus also rose after handling and transport, though no correlation was found with plasma proteins, likely reflecting distinct physiological roles. Overall, the results demonstrate that dietary supplementation with Dorabrin CRX P® can effectively mitigate acute stress responses in sea bass fry, and support the use of skin mucus as a reliable, non-invasive matrix for assessing stress biomarkers in aquaculture settings.