Growth-promoting potential and immunostimulatory of poly-β-hydroxybutyrate in common carp (<i>Cyprinus carpio</i>) fingerlings culture.

The natural polymer poly-β-hydroxybutyrate (PHB) is converted to β-hydroxybutyric acid, which is similar to short-chain fatty acids, via microbial fermentation and host enzyme breakdown. This study investigated the impact of different PHB concentrations (Control, 1.00, 3.00, and 5.00% substitution) on growth performances and fish welfare in common carp fingerlings. After a 60-day trial, fish fed on diet containing 1.00% PHB exhibited significantly higher weight gain and improved feed conversion efficiency compared to the control group. Furthermore, analysis of enzymatic activity showed elevated levels of total protease and amylase in PHB-fed treatments compared to the control. Red blood cell counts, hemoglobin, and hematocrit levels remained unaffected and a significant increase in white blood cell count was observed in fish fed on diets containing 1.00 and 3.00% PHB compared to the control group. Furthermore, fish fed on diets containing 1.00 and 3.00% PHB demonstrated significantly higher total protein levels and lower glucose concentrations as well as reduced hepatic enzyme activities (aspartate aminotransferase and alanine aminotransferase) compared to both the control and 5.00% PHB groups. Assessment of antioxidant and immune parameters revealed significantly increased complement hemolytic activity and immunoglobulin M levels coupled with decreased malondialdehyde concentrations in the plasma of PHB-fed fish compared to the control group. In conclusion, dietary supplementation with PHB, especially at the 1.00% level, enhanced growth performance and improved nutritional and health indicators in the fingerlings. These findings suggested that PHB had the potential to be a valuable dietary additive for this species.