Spatial optimization in recirculating aquaculture systems: Cortisol-mediated growth modulation and polyunsaturated fatty acid Enrichment in Carassius auratus var. Pengze

This study systematically evaluated the effects of tank volume optimization in a recirculating aquaculture system (RAS) on growth performance, physiological responses, and nutritional quality of Carassius auratus var. Pengze. Three spatial experimental groups (0.3 m³（Large）, 0.15 m³（Medium）, 0.075 m³（Small）), with 4 parallel tanks in each experimental group, and all the rearing tanks are part of one RAS with shared water treatment processes. Juveniles (initial body weight 20.13 ± 0.52 g) were stocked at 2.68 kg/m³ density for 100 days to assess space-size effects on zootechnical parameters and physiological indices. Key findings revealed: (1) Group S exhibited higher weight gain rate and greater specific growth rate compared to Group L ( p < 0.05), with minimized weight coefficient of variation. Growth phase analysis revealed that the Group S dominated the early stage (0–60 days), while the Group L excelled in the later phase (80–100 days); (2) The Group S exhibited the lowest serum cortisol levels (Group S < Group L < Group M; p < 0.05), while displaying the highest concentrations of lipid metabolism markers (triglycerides and cholesterol); (3) Proximate muscle composition analysis revealed that Group S exhibited higher crude fat content and superior levels of health-beneficial fatty acids, including highly unsaturated fatty acids (HUFAs) and n-3 polyunsaturated fatty acids (PUFAs), compared to Group L ( p < 0.05). These findings suggested that restricted culture volume (0.075 m³) optimizes growth performance in juvenile Carassius auratus var. Pengze (∼20 g), while potentially inhibiting development in larger specimens (>40 g). Space constraint appear to modulate cortisol secretion, and lipid metabolism, providing critical insights for RAS optimization in phased culture systems. • Tank size in RAS affects behavior of Carassius auratus var. Pengze. • 0.075 m³ RAS enhances juvenile Carassius auratus var. Pengze (20 g) growth but inhibits larger specimens (>40 g). • Space limits regulate cortisol & lipid metabolism, revealing RAS optimization keys for staged culture.