Optimizing Stocking Density for Indoor-Cultured Monopterus albus Fry: Insights from Cannibalism Dynamics, Oxidative and Antioxidative Response, and Transcriptome Profiling

The rice field eel (Monopterus albus) is a commercially significant freshwater fish. However, the persistent shortage of fry has therefore become a major constraint on the sustainable development of the eel aquaculture industry. The causes of mortality are generally associated with aquaculture practices, stocking density, and water quality. Therefore, there is an urgent need to optimize stocking density for M. albus fingerlings in indoor aquaculture to improve survival and support sustainable development of M. albus aquaculture. This study aimed to determine the optimal stocking density for M. albus fry (0.6–0.7 g) by evaluating growth, cannibalism, antioxidant status, and transcriptome responses. Four initial densities were tested over 80 days: 500 (D1), 1000 (D2), 1500 (D3), and 2000 (D4) individuals per square meter (inds/m2). Fry reared at 1500 inds/m2 (D3) achieved the lowest cannibalism-induced mortality and the highest survival and growth rates. Conversely, low-density groups (D1 and D2) exhibited increased spontaneous aggression and oxygen consumption rates, suggesting higher metabolic costs linked to agonistic interactions. Oxidative stress analysis revealed tissue-specific responses between liver and muscles: high density significantly increased hepatic malondialdehyde (MDA) levels and activated hepatic superoxide dismutase (SOD) and glutathione peroxidase (GPx) while activating an SOD-catalase (CAT) response in muscles. Transcriptome profiling supported these findings, showing enrichment in pathways related to neuroactive ligand–receptor interaction and tissue damage in cannibalistic low-density groups, while the D3 group showed enrichment in O-glycan biosynthesis and detoxification pathways (cytochrome P450), reflecting a metabolically healthy adaptation. In conclusion, stocking density markedly influences behavior, physiological status and molecular responses in M. albus. An intermediate density of 1500 inds/m2 optimizes survival and reduces cannibalism, providing a practical reference for improving welfare and productivity in indoor eel aquaculture.