Abstract
<title>Abstract</title> <p> Reducing dependence on fishmeal and fish oil in carnivorous marine fish feeds is a key challenge for sustainable aquaculture. This study evaluated the effects of five isoproteic (45%) and isolipidic (20%) experimental diets on the growth performance, fillet composition, intestinal histomorphology, brush border membrane enzyme activity and gene expression, and gut microbiota of juvenile European seabass ( <italic>Dicentrarchus labrax</italic> ) over a 17-week feeding trial. The diets included a plant-based control, two plant-based diets with 10% crude protein from partially defatted <italic>Hermetia illucens</italic> pupae meal or this combined with 30% poultry by-product meal, a fishmeal-based control and a fishmeal-based diet supplemented with <italic>H. illucens</italic> meal. Fish-based diets consistently supported superior growth performance, feed conversion, and fillet n-3 polyunsaturated fatty acid deposition compared to plant-based alternatives. Although no significant inflammatory changes were observed in the gut across treatments, plant-based diets induced subtle mucosal alterations, including enterocyte vacuolisation and epithelial lamina detachment, which were partially mitigated by the inclusion of insect meal and poultry by-products. Brush border membrane enzyme activities and gene expression patterns in juveniles contrasted markedly with those previously reported in subadult seabass fed identical diets, indicating developmental stage-specific responses. Gut microbiota composition was not significantly affected by diet in juveniles. These findings demonstrate that nutritional strategies validated in subadult European seabass cannot be directly extrapolated to juveniles, highlighting the need for age-specific feed optimisation to support the transition toward sustainable aquafeeds. </p>