Short-term thermal exposure alters boldness-related behaviour, exploration, shoaling organization, and innate immune-related physiology in juvenile black rockfish (Sebastes schlegelii).

Water temperature is a critical environmental factor that can rapidly alter behavioural expression and physiological status in ectothermic fishes. However, the short-term responses of juvenile black rockfish (Sebastes schlegelii) to ecologically relevant thermal variation remain insufficiently understood. In this study, hatchery-produced juvenile S. schlegelii from the same spawning batch were exposed to four temperature conditions: 6, 12, 18, and 24°C. The 18°C treatment represented the original hatchery rearing temperature and was used as the reference condition. After gradual adjustment to the target temperatures, fish were maintained at each temperature for 24h before behavioural and physiological measurements. The experiment used a between-subjects design; each individual fish was tested only once in the individual behavioural assays, and each shoal was tested only once in the collective behavioural assays. For individual behavioural assays, 60 fish were tested per temperature treatment. Boldness-related behaviour was evaluated using the latency to first enter the open area, whereas exploration-related behaviour was assessed using open-area residence time and the number of entries into the open area. Novel-object exploration was evaluated using the latency to approach the novel object and the average distance from the object. Collective behaviour was examined using independent shoals under two group-size/density treatments, namely 8- and 16-individual shoals. Four independent shoals were analysed for each temperature × group-size/density combination. Physiological status was assessed using alkaline phosphatase, lysozyme, catalase, and glutamate oxaloacetate transaminase activities. Temperature treatment altered boldness-related behaviour, exploration-related behaviour, shoaling organization, and physiological enzyme activities in juvenile S. schlegelii. Low-temperature treatments generally reduced behavioural activity and modified shoal cohesion, whereas warmer conditions within the tested range were associated with increased activity, stronger exploratory expression, and higher immune-related or antioxidant enzyme activities. The collective-behaviour results should be interpreted cautiously because the 8- and 16-individual shoals were tested in tanks of the same volume, resulting in a combined group-size/density effect. These findings indicate that short-term thermal exposure can rapidly affect behavioural expression, group organization, and physiological status in juvenile S. schlegelii, providing insight into the short-term responses of reef-associated juvenile fishes to thermal variation.