Abstract This study investigated the stress response of cantang hybrid grouper ( Epinephelus fuscoguttatus × E. lanceolatus ) to different salinity levels using cortisol concentrations in blood and mucus as indicators, with the aim of assessing the potential of mucus sampling as a non‐invasive stress detection method. A total of 252 fish were exposed to three salinity treatments—28 ppt (T 1 ), 32 ppt (T 2 /control) and 36 ppt (T 3 )—for 24 h using a salinity shock method. Cortisol levels in blood and mucus, blood glucose and opercular movement rates were measured at 0, 0.5, 1, 2, 4, 8 and 24 h. The results showed significantly higher cortisol concentrations in both blood and mucus under salinity stress, especially at 36 ppt. Strong positive correlations were observed between blood and mucus cortisol ( R = 0.997, p < 0.001) and between blood cortisol and glucose ( R = 0.959, p < 0.001). Elevated opercular movement rates further confirmed stress responses at non‐optimal salinities. The findings suggest that mucus cortisol concentration reflects stress levels comparably to blood cortisol, supporting its use as a practical, non‐invasive biomarker for stress monitoring in aquaculture. This approach could enhance fish welfare and stress management in hybrid grouper farming under fluctuating environmental conditions.
Sulmartiwi et al. (Mon,) studied this question.