The land-based recirculating aquaculture system (RAS) has been widely applied to fish farming as a new eco-friendly culture model. This system consists of circular culture tanks on land integrated with water treatment and recycling systems. This study investigated the growth performance of juvenile gibel carp (Carassius gibelio) cultured at high stocking density (HSD, 0.3 kg/m³) and low stocking density (LSD, 0.15 kg/m³) conditions in RAS, and evaluated the comprehensive economic profit of RAS. The body weight, body length, weight gain rate, and condition factor of gibel carp in the LSD group were significantly higher than those in the HSD group (p < 0.05). The feed conversion ratio increased significantly in the HSD group (p < 0.05). A histological analysis revealed a significantly higher density of white muscle fibers in the LSD group (p < 0.05). Relative mRNA expression levels showed that ubiquitin–proteasome system (UPS)-related genes, ub, psma2, and mafbx, were significantly expressed in the HSD group, while the s6k1 expression was elevated in the LSD group (p < 0.05). The mRNA expression levels of keap1 and hsp70 in the dorsal muscle were significantly higher in the HSD group (p < 0.05). Throughout the rearing period, the water temperature remained consistent between the two density groups. The pH value gradually decreased and the dissolved oxygen levels in the HSD group were generally lower than in the LSD group. The nitrite nitrogen (NO₂⁻-N) content was higher in the HSD group. Compared to the LSD group, the return on investment was significantly lower in the HSD group. In conclusion, the water quality and growth rates of juvenile gibel carp were better in the LSD group. An appropriate stocking density improved the growth performance and aquaculture economic efficiency.

To investigate the effects of photoperiods on the growth and muscle quality indicators of grass carp (Ctenopharyngodon idella), 225 fish (109.65 ± 3.62 g) were randomly assigned into five different photoperiod groups (0L:24D, 8L:16D, 12L:12D, 16L:8D, and 24L:0D). The experiment spanned a 75-day period, after which sampling and analysis were performed. Compared with the 0L:24D and 8L:16D groups, the 12L:12D and 16L:8D groups significantly promoted the growth of grass carp (p < 0.05). The texture parameters of the muscle in the 0L:24D and 16L:8D groups were significantly greater than those in the 12L:12D group (p < 0.05). The crude protein content was significantly higher in the 12L:12D and 16L:8D groups (p < 0.05). The amino acid content and muscle fiber characteristics, as well as the mRNA levels of myostatin (mstn), myogenic factor 5 (myf5), type I collagen α1 (col1α1), and α2 (col1α2), along with the hydroxyproline and collagen contents, were all significantly influenced by the photoperiod (p < 0.05). The lysine (Lys), aspartic acid (Asp), and alanine (Ala) contents in the muscle and muscle fiber density of grass carp reached the highest levels under the 16L:8D treatment (p < 0.05). Collectively, these results indicate that a 16L:8D photoperiod is optimal for enhancing both the growth and muscle quality indicators of grass carp. The findings of this study offer valuable scientific references for the precise regulation of grass carp quality when using a photoperiod, and they are anticipated to foster the further development and optimization of strategies for improving grass carp quality.

Diet form is increasingly recognized as a welfare-relevant factor in intensive aquaculture, yet the effects of feed cooking on fish behavioral and physiological welfare remain poorly characterized. Juvenile southern catfish (Silurus meridionalis; 6.18 ± 0.52 g) were reared for 6 weeks in an indoor recirculating aquaculture system and fed either raw grass carp (Ctenopharyngodon idella) muscle (fish fed raw muscle, FR) or cooked grass carp muscle (fish fed cooked muscle, FC; 15 min ramp to ~100 °C followed by 2–3 min at ~100 °C). Locomotor activity and anxiety-like behavior were assessed using the open-field test and an annular light–dark preference assay, respectively. Flow-through respirometry further revealed a significantly lower standard metabolic rate (SMR) in FC fish than in FR fish, decreasing from 10.30 to 6.83, which represents a 33.7% reduction. Endocrine and biochemical analyses showed that cooking significantly decreased serum total triiodothyronine (T3) by 23.8%, whereas routine serum biochemical indices remained unchanged. In brain tissue, dopamine (DA) was significantly reduced by 7.2% in the FC group, and RT-qPCR analysis of dopamine-related genes further showed a significant downregulation of the rate-limiting synthesis gene th. These results indicate that cooking primarily downshifts the activity-energy axis in southern catfish and is accompanied by coordinated thyroid and dopaminergic changes. To our knowledge, this is the first integrated study to evaluate the behavioral, metabolic, and neuroendocrine effects of cooked feed in S. meridionalis, providing a short-term phenotypic baseline for assessing welfare-relevant feeding scenarios in aquaculture.