Response Characteristics of Defatted Mealworm ( <i>Tenebrio molitor</i> ) Instead of Fish Meal to the Intestinal Immunity and Microflora of Juvenile Pikeperch ( <i>Sander lucioperca</i> )

The aim of this study was to investigate the impact of varying levels of defatted mealworm as a substitute for fish meal on the growth, intestinal health, and intestinal microflora of juvenile pikeperch ( Sander lucioperca ). Juveniles were fed diets containing 54% protein and 12% lipid over an 8‐week period, with defatted mealworms replacing 0%, 25%, 50%, 75%, and 100% of fish meal, respectively (designated as MW0, MW25, MW50, MW75, and MW100). The results showed that the MW25 group achieved the highest weight gain rate and specific growth rate, with no significant difference from the MW0 group. Moreover, the MW25 group exhibited the lowest feed conversion ratio. This group also exhibited the highest trypsin activity, whereas lipase activity showed no significant variation across all treatment groups. Intestinal villus height and width were not significantly affected by mealworm inclusion. These findings suggest that the inclusion of mealworm did not impact the digestive and absorptive capacity of the pikeperch intestine. However, gene expression related to growth ( ghr , igf1 , igf2 ) showed a consistent decline as mealworm inclusion increased ( p &lt; 0.05). The expression of il10 was highest in the MW25 group ( p &lt; 0.05), whereas il-1β and tgf-β expressions peaked in the MW50 group. Additionally, increasing mealworm levels led to a reduction in intestinal microflora diversity and abundance, with a decreased presence of Staphylococcus and Lactobacillus , which negatively affected the intestinal microflora structure. In conclusion, replacing no more than 25% of fish meal with mealworm is recommended to optimize growth performance and preserve intestinal microflora structure in juvenile pikeperch.