Acute and cumulative cortisol responses to impulsive underwater sounds in gilthead seabream (Sparus aurata)

Environmental acoustic disturbances represent a growing but poorly understood stressor in marine aquaculture systems. Here, we investigated the physiological stress response of gilthead seabream (Sparus aurata) exposed to impulsive underwater sounds under controlled laboratory conditions. The acoustic stimuli originated from underwater recordings obtained in offshore seabream cages during storm conditions and contained prominent, randomly occurring impulsive events. To approximate sea-cage farming conditions, fish were maintained at a high stocking density similar to that used in commercial cages and subjected to repeated acoustic exposures. Stress responses were assessed using three complementary cortisol measurements representing different temporal scales: plasma cortisol as an indicator of acute individual stress, water cortisol as a non-invasive measure of group-level stress, and cortisol accumulation in caudal fin tissue as a potential indicator of cumulative exposure. Plasma cortisol concentrations remained stable during control trials, with no detectable diurnal variation between morning and afternoon sampling, whereas acoustic exposure resulted in significant elevations following stimulation. Water cortisol showed a similar response, with increased concentrations during acoustic exposures compared with control conditions, supporting its use as a non-invasive indicator of group stress. Fin cortisol concentrations increased significantly following repeated exposures and subsequently returned toward baseline, consistent with the accumulation and clearance of cortisol in fin tissue over longer timescales. These findings demonstrate acute and cumulative cortisol responses to impulsive underwater sounds in gilthead seabream and show that plasma, water, and fin cortisol provide complementary measures of physiological stress across different temporal scales.