Effects of light intensity on the behavioural response and physiological parameters of rainbow trout ( Oncorhynchus mykiss)

Light conditions play a pivotal role in enhancing the welfare of rainbow trout ( Oncorhynchus mykiss) in deeper- offshore aquaculture systems in China. However, optimal light intensity and duration for improving rainbow trout welfare remain unclear due to limited foundational knowledge. This study investigated the effects of different light intensities (control group L 0 : 0 lx; treatment groups L 1 : 42.8 lx, L 2 : 59.0 lx, L 3 : 63.5 lx, L 4 : 73.0 lx) and exposure durations (1 hour and 7 days) on the behavioural responses and physiological parameters of rainbow trout. Behavioural tests measured the frequency of fish crossing a light curtain during a 10-minute period, while physiological parameters including plasma cortisol, total protein (TP), albumin (ALB), triglyceride (TG), total cholesterol (TC), aspartate transaminase (AST), and alanine transaminase (ALT) were assessed after 1-hour and 7-day treatments. Results reveal that the rainbow trout exhibits the highest crossing frequency under L 1 , alongside significantly lower levels of cortisol, TP, TG, TC, and AST compared to higher light intensities. In contrast, higher light intensities (L2, L 3 , and L 4 ) cause increased stress, reflected by elevated plasma cortisol, protein, lipid, and enzyme levels. There are no significant differences in behaviour or physiological responses between L 0 and L 1 . These findings imply that L 1 represents a balanced light intensity that supports natural swimming behaviours with minimal stress, while higher intensities induce elevated stress. Therefore, a light intensity of 42.8 lx (L1) is recommended to enhance fish welfare in deeper-offshore aquaculture systems when artificial light is utilized.