Behavioural Monitoring Underlines Habituation to Repeated Stressor Stimuli in Farmed Gilthead Sea Bream () Reared at a High Stocking Density.

A confinement stress test with 75% tank space reduction and behavioural monitoring through tri-axial accelerometers externally attached to the operculum was designed. This procedure was validated by demonstrating the less pronounced stress response in gilthead sea bream than in European sea bass (950-1200 g). Our study aimed to assess habituation to high stocking densities with such procedure in gilthead sea bream. Animals (420-450 g) were reared (June-August) in a flow-through system at two stocking densities (CTRL: 10-15 kg/m; HD: 18-24 kg/m), with natural photoperiod and temperature (21-29 °C), and oxygen levels at 5.2-4.2 (CTRL) and 4.2-3.2 ppm (HD). At the end, blood and muscle were sampled for haematology and transcriptomic analyses, and external tissue damage was assessed by image-based scoring. Four days later, fish underwent a 45 min confinement stress test over two consecutive days. HD fish showed reduced feed intake, growth rates and haematopoietic activity. Muscle transcriptome changes indicated a shift from systemic to local growth regulation and a primed muscle regeneration over protein accretion in HD animals with slight external injuries. After stress testing, HD fish exhibited a decreased recovery time in activity and respiration rates, which was shorter after a second stressor exposure, confirming habituation to high densities.