Fast Growth at a Physiological Cost: Comparing Odontobutis potamophilus in Recirculating Aquaculture and Rice–Crayfish–Fish Culture

The freshwater sleeper (Odontobutis potamophilus) is a high-value freshwater fish endemic to China, farmed mainly in rice–crayfish–Odontobutis potamophilus integrated culture (RCO) and recirculating aquaculture systems (RAS). How these models differ in growth, physiological status, and feeding remains unclear. The same batch of fry was reared for three months (92 days) in six paddy fields (RCO, 3 individuals/m2, no artificial feed, with access to natural food organisms and stocked crayfish) and six RAS tanks (6 individuals/m2, fed Cirrhinus molitorella and pellet feed twice daily). Growth indicators were calculated from individual data of the sampled fish (n = 138 per model). Per group, 150 fish were sampled for stomach contents, 138 for morphometrics, and six for serum alanine aminotransferase (ALT), glutamic–oxaloacetic transaminase (AST), and glucose (GLU) assays; muscle nutritional composition was also determined. Relative weight gain rate (RWG) and specific growth rate in wet weight (SGRw) were significantly higher in RAS, with positive allometric growth in both models. Muscle crude protein was higher in RCO, whereas crude fat and serum ALT, AST, and GLU levels were higher in RAS, suggesting a heavier metabolic burden on the liver in RAS, which awaits confirmation with a broader panel of liver function markers. RCO stomachs contained nine food categories but had a 38% vacuity rate, whereas RAS fish consumed only C. molitorella (86% occurrence, 14% vacuity). Because stocking density and feeding regimen differed simultaneously between the two models, the observed differences reflect the overall effects of each culture model rather than the effect of the rearing system alone. These findings reveal a growth-metabolism trade-off between the two culture models as currently practiced: RAS produced faster growth accompanied by higher serum transaminase levels and body fat, whereas RCO maintained lower metabolic markers with slower growth. Moderate supplementary feeding in RCO and dietary diversification in RAS may reconcile yield and fish health.