Impact of Dietary Co-Supplementation with Schizochytrium sp. and Lavender Essential Oil on Growth, Digestive Physiology, and Intestinal and Hepatic Histology in Common Carp

This study aimed to evaluate the effects of dietary supplementation with marine microalgae (Schizochytrium sp.) and lavender (Lavandula intermedia) essential oil on growth performance, biochemical composition, digestive enzyme activity, and histological parameters in common carp (Cyprinus carpio). A total of 480 fish were randomly assigned to four dietary treatment groups for 8 weeks: a control group (SL0; 0% Schizochytrium sp., 0% lavender oil), and three experimental groups supplemented with 1% Schizochytrium sp. + 0.5% lavender oil (SL1), 2% Schizochytrium sp. + 0.5% lavender oil (SL2), and 4% Schizochytrium sp. + 0.5% lavender oil (SL4). Growth performance data including final weight, weight gain, and specific growth rate were significantly higher in the SL2 group (p < 0.05), which also exhibited the highest survival rate. Compared to the initial values, significant differences were observed in the whole-body biochemical composition among groups (p < 0.05), with SL2 showing the lowest protein and ash contents, and SL1 the lowest lipid level. The biochemical composition of the diets showed significant differences in protein, lipid, and ash contents among the experimental groups (p < 0.05). Histomorphometric analysis showed significant improvements in villus height, tunica muscularis thickness, and villus width in the SL2 group. Histological evaluation of the liver and intestine demonstrated dose-dependent lipid accumulation, with the 4% group showing increased lipid droplets and mild hepatocyte alterations. While protease activity did not differ significantly among treatment groups (p> 0.05), protease inhibition was significantly reduced in the SL2 and SL4 groups (p < 0.05), indicating improved digestive efficiency. Overall, the SL2 group enhanced growth, survival, and intestinal structure without compromising enzyme activity in common carp, suggesting its potential as a functional feed additive in sustainable aquaculture.