Analysis of correlation between daily rhythm of digestion and metabolism of grouper (Epinephelus lanceolatus♂ × Epinephelus fuscoguttatus♀) and water quality parameters in recycling aquaculture system

The daily rhythm is a critical regulatory factor in fish growth and development, governing internal functions and organismal responses to environmental cues. This rhythm influences not only metabolism but also water quality parameters in recirculating aquaculture systems (RAS). However, research on the daily rhythm of hybrid groupers in RAS remains limited. Consequently, this study is aimed at investigating the metabolic daily rhythm of hybrid groupers in RAS and its impact on water quality. Feeding time was designated as the starting point for this investigation. Initially, we measured the levels of dissolved oxygen (DO), total ammonia nitrogen (TAN), and nitrite nitrogen (Nitrite-N) in the RAS. Subsequently, we assessed oxygen consumption rates (OCR), digestive enzyme activities, and the expression levels of metabolism-related genes in the pearl gentian grouper (Epinephelus lanceolatus♂ × Epinephelus fuscoguttatus♀). Finally, we conducted a comprehensive analysis of these factors. The results indicated that DO was negatively correlated with OCR, pepsin and lipase activities, and GK gene expression. TAN exhibited a positive correlation with OCR and the expression levels of the target of rapamycin (TOR) gene. Similarly, Nitrite-N showed a positive correlation with OCR and TOR gene expression levels. We also speculated on the internal relationship between fish daily rhythms and water quality parameters, noting a certain degree of synchronicity. These findings can minimize stress on fish, and it can contribute to optimizing RAS performance through management adjustments.