Effects of Small Shelter-like Structure Types on Tank Behavioral Responses of Juvenile Black Seabream (Acanthopagrus schlegelii)

Small shelter structures may affect the spatial use, locomotor state, and group spatial relationships of juvenile black seabream (Acanthopagrus schlegelii), but behavioral responses induced by different structure types remain insufficiently examined using multiple behavioral metrics. In this study, we conducted a tank experiment to test three hypotheses: compared with a structure-free control, cuboid and cylindrical structures would increase juvenile use of the central structure-placement zone, alter locomotor state, and affect inter-individual distance. Three treatments were established, including cuboid structure, cylindrical structure, and structure-free control, and fish behavior was observed under daytime and nighttime conditions. Video tracking and image analysis were used to quantify area-standardized average occurrence rate, average swimming speed, percentage of active time, and inter-individual distance. Both structures increased juvenile use of S1, namely the central structure-placement zone within a radius of 0–20 cm from the shelter-structure center. During the daytime, the average occurrence rates in S1 were 54.1% ± 6.33% and 46.7% ± 2.32% in the cuboid and cylindrical structure groups, respectively, which were higher than those in the structure-free control group (15.2% ± 3.08%). At night, the corresponding values were 50.2% ± 0.55%, 42.8% ± 3.77%, and 13.3% ± 1.57%, respectively. Locomotor behavior and inter-individual distance also varied with structure type, observation period, and diel condition. Under tank conditions, small shelter-like structures altered the short-term spatial use, locomotor state, and group spatial relationships of juvenile black seabream. These findings provide experimental evidence for how juvenile black seabream respond to simple shelter-like structures under controlled tank conditions and may help researchers, hatchery managers, and stock-enhancement practitioners select small structures for short-term behavioral assessment and shelter enrichment.