As aquaculture expands, plant-based feeds are increasingly used, but some fish species poorly tolerate them, affecting health and growth. Probiotics can help counter these effects by improving digestion, nutrient absorption, and immunity. This study evaluated the effect of dietary incorporation of Bacillus subtilis FI99 on the intestinal health of meagre. A nutritional challenge was performed with a practical control diet and three diets higher in plant-based ingredients: one without probiotic and two with probiotic incorporated at 1 × 10⁹ CFU g⁻¹ and 5.5 × 10¹¹ CFU g⁻¹. Histomorphological analysis was used to assess intestinal health and validate previously established machine learning models in predicting fish nutritional status. No differences were observed in zootechnical performance and biometric indexes. Most effects were observed in the anterior intestinal section, where probiotics improved total area, lumen area, lumen maximum diameter, total maximum diameter, villi area, and villi + lumen area. Additionally, probiotics improved supranuclear vacuole size, eosinophilic granulocytes, and intraepithelial leukocytes presence in anterior and intermediate sections. Machine learning models could not accurately predict the nutritional status of fish. Overall, the study indicates that dietary inclusion of B. subtilis enhances the intestinal health of meagre fed plant-based diets. Machine learning models require further development for improved accuracy.

The yellow mealworm ( Tenebrio molitor ) is increasingly recognised as a novel feed ingredient. During industrial processing, larvae are often defatted to improve storage, handling and extrusion properties. Full-fat (ffTM) and defatted (dTM) mealworm meals were compared as total fishmeal replacements for rainbow trout ( Oncorhynchus mykiss ) diets. Three experimental diets were formulated to be isoproteic and isolipidic: a fishmeal-based control (FM) and two diets containing 25% ffTM (FFTM25) or dTM (DTM25). Juvenile rainbow trout (11.4 ± 1.5 g) were fed to apparent satiation twice daily. After 56 days, no impact on growth performance was observed. Feed intake decreased in the DTM25 and FFTM25 groups, while the feed conversion ratio was reduced in the FFTM25 diet, compared to the FM diet. In the anterior intestine, the DTM25 diet increased villus width and enterocyte height. Fish fed the DTM25 diet had higher whole-body lipid content, whereas fillet lipid content was higher in the FFTM25 group. The FFTM25 diet also increased fillet saturated and n-6 polyunsaturated fatty acids (PUFA), the thrombogenicity index and reduced the hypocholesterolemic/hypercholesterolemic ratio, while the n-3/n-6 PUFA ratio was highest in the FM diet. Nevertheless, all experimental treatments showed similar absolute levels of n-3 PUFA. Fillet yellowness was higher in the control and FFTM25 groups. Overall, the complete dietary replacement of fishmeal with both dTM and ffTM supported growth and anterior intestine health of rainbow trout juveniles. Nevertheless, the ffTM dietary inclusion improved feed efficiency, but reduced fillet lipid quality, indicating a trade-off between mealworm lipid use and product quality.