Analysis of Correlation between Daily Rhythm of Digestion and Metabolism of Grouper (Epinephelus lanceolatus♂ × Epinephelus fuscoguttatus♀) and Water Quality Parameters in Recycling Aquaculture System

Abstract Daily rhythm, a crucial regulatory factor in fish growth and development, controls the internal functions and regulation of organisms in response to environmental cues. Daily rhythm influences not only metabolism but also water quality factors in recycling aquaculture systems (RAS). However, studies on the daily rhythm of hybrid groupers in the RAS are limited. Therefore, this study aims to investigate the daily rhythm metabolism of hybrid grouper in RAS and explore its effect on water quality. Feeding time was used as the starting point in this study. Initially, dissolved oxygen (DO), total ammonia nitrogen (TAN), and Nitrite nitrogen (Nitrite-N) levels in the RAS were measured. This was followed by assessing 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, a combined analysis was conducted on these aspects. The results showed that DO was negatively correlated with OCR, pepsin and lipase activities, and GK gene expression. TAN showed a positive correlation with OCR and the gene expression levels of the target of the rapamycin. Nitrite-N showed a positive correlation with OCR and the gene expression levels of TOR. We also speculated an internal relationship between fish daily rhythms and water quality parameters, which showed a certain degree of synchronicity. These findings were beneficial in optimizing RAS performance through adjustments in management practices and reducing stress on fish.