| Summary: | An 8-week feeding trial was designed to evaluate the potential of mealworm (Tenebrio molitor) as a locally available unconventional source of protein in a practical diet for olive flounder (Paralichthys olivaceus). Three hundred juveniles (initial weight, 33.5 ± 0.01 g) were randomly distributed into five dietary groups in triplicate (20 fish per tank), and each group was hand-fed one of the experimental diets containing a graded level of a mealworm meal (MW) replacing 0, 20, 40, 60, and 80 % of fish meal (FM) protein (designated as MW0, MW20, MW40, MW60, and MW80). The specific growth rates of juvenile flounders revealed a decreasing trend with increasing dietary MW levels. However, feed utilization efficiency tended to improve with the increasing percentage of dietary FM replacement up to 40 % but gradually decreased in higher replacement levels. The results displayed a linear and quadratic negative effect of dietary MW inclusion level on nutrient digestibility. A significant negative linear trend was detected between dietary MW content and plasma total cholesterol. Except for lipid content, which displayed a significant decreasing trend as the dietary MW content increased, no other changes were evident in the fillet proximate composition. Significant positive linear and quadratic trends were observed between fillet C18:1n-9 (oleic acid; OA) and C18:2n-6 (linoleic acid; LA) levels and dietary MW content, where the OA and LA values increased in a dose-dependent manner. The fillet Σn3 / Σn6 ratio and atherogenic index demonstrated significant decreasing trends with dietary inclusion of MW while the fillet thrombogenic index and hypocholesterolaemic/hypercholesterolaemic fatty acid ratio exhibited significant positive trends with increasing values. Myeloperoxidase activity declined with replacement of up to 40 % FM protein but thereafter gradually increased at higher replacement levels. Glutathione peroxidase activity generally was enhanced with increasing dietary MW inclusion. Dietary MW inclusion had no significant impact on the bacterial community of the juvenile flounder’s hindgut. Although replacing up to 80 % of FM with MW may promote an immunostimulatory effect, in order to avoid unfavorable effects on feed efficiency and nutritional value of the fillet lipid fraction no more than 40 % of FM is recommended to be replaced by a MW in diets for olive flounder juveniles. Our findings could promote the utilization of MW in olive flounder feed, which may not only reduce dependence on high priced and finite feed ingredients, but also enhance farmed fish health status.
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