Short-term behavioural responses of juvenile Nile tilapia (Oreochromis niloticus) to increasing stocking density quantified by YOLO-based three-dimensional tracking

In this study, the effects of different stocking densities (1–10 individuals in 144 L; 6.9–69.4 fish m-3, 0.09–0.92 kg m-3) on swimming activity, vertical positioning and inter-individual spatial relationships in juvenile tilapia (Oreochromis niloticus) were examined by automated object detection and multi-object tracking based on the YOLO deep learning algorithm. Swimming distance, speed, mean depth, layer use and nearest neighbour distance were obtained from three replicates and evaluated by repeated measures analysis of variance with replicate as a block effect, Duncan’s multiple range test and linear regression. Swimming distance rose from 60.5 ± 23.4 m with one fish to 99.7 ± 33.2 m with two and then plateaued (127.6 ± 11.1 m with ten; F9,18 = 4.46; p = 0.0034), and swimming speed behaved likewise (3.86 ± 1.41 to 8.52 ± 0.73 m min⁻¹; F9,18 = 4.87; p = 0.0021); in both, only the single-fish level differed from the groups. Mean depth fell from 20.4 ± 5.4 to 12.5 ± 3.5 cm (F9,18 = 3.70; p = 0.0088) and upper layer use rose from 39.9 ± 18.5% to 65.9 ± 15.1% (F9,18 = 3.83; p = 0.0074), both levelling off at about seven individuals. Nearest neighbour distance fell from 37.27 ± 4.39 to 11.45 ± 0.55 cm (F8,16 = 56.44; p < 0.0001; R² = 0.846); compared with a random spatial arrangement it matched expectation up to four fish but fell to 0.66–0.69 of it from seven onwards (p ≤ 0.014), so the decrease reflects active aggregation and not geometric crowding alone. Over short observation windows and at biomass densities far below production scale, an increasing number of individuals in a fixed volume thus alters both individual movement and social spatial organisation in yavru tilapia, inter-individual distance being the most responsive measure.