Effects of saline-alkaline water culture on growth performance and muscle quality formation in mandarin fish (Siniperca chuatsi).

Inland saline-alkaline waters are an underused resource for expanding aquaculture where freshwater is limited. However, it remains unclear how saline-alkaline culture alters fish muscle quality and which molecular responses accompany these changes. This study focused on the mandarin fish (Siniperca chuatsi) as the research subject and conducted a 42-day aquaculture experiment. Two groups were set up: the freshwater group and the saline-alkaline water (SAW; alkalinity 0.7 g/L, salinity 6 psu) group. The results showed that mandarin fish reared in saline-alkaline water exhibited significantly lower hepatosomatic (HSI) and viscerosomatic indices (VSI) (P < 0.05). However, interestingly, the weight gain (WG) did not significantly decrease (P > 0.05). Furthermore, the moisture content of the muscles in the SAW group decreased, whereas the contents of crude lipid and crude protein increased (P < 0.05). In the SAW group, the contents of amino acids such as Cys, Tyr, Phe, etc. increased significantly (P < 0.05). On the other hand, the SAW group had higher protein nutritional evaluation indicators such as chemical score, amino acid score, and essential amino acid index compared to the FW group. The SAW group exhibited higher levels of DHA (docosahexaenoic acid) and EPA (eicosapentaenoic acid) + DHA, together with an increased n-3/n-6 PUFA (polyunsaturated fatty acid) ratio (P < 0.05). The texture and histological analyses revealed that the muscle hardness, adhesiveness, and chewing ability in the SAW group significantly increased, whereas myofiber abundance declined and the diameter distribution shifted toward larger size classes (P < 0.05). The expression of genes (myod1, myog, and myhc) related to muscle growth and protein hydrolysis inhibitors (cast) was upregulated. Transcriptomic analysis further revealed that in SAW muscle, metabolic pathways associated with oxidative phosphorylation, fatty acid degradation, and PPAR signaling were upregulated, whereas amino acid metabolism pathways related to glycine, serine, and threonine metabolism, propionate metabolism cysteine and methionine metabolism were downregulated. GSEA (gene set enrichment analysis) results indicated that the proteasome and endoplasmic reticulum protein processing pathways were upregulated. In conclusion, mandarin fish can maintain normal growth within an appropriate salinity-alkaline range. Concurrently, mandarin fish showed reduced muscle moisture, increased crude protein and crude lipid, higher amino acid contents and AAS, CS, and EAAI scores, elevated DHA, EPA + DHA, and n-3/n-6 PUFA ratio, and greater hardness, adhesiveness, gumminess, and chewiness, with unchanged springiness and fewer but larger myofibers.