Insect proteins are considered as suitable low environmental impact alternatives to fishmeal for sustainable aquafeeds. Among the different insect species, Hermetia illucens has attracted research and industrial interest due to its ability to grow well on organic side streams, its high protein content and favorable amino acid profiles. Its lipid content although high is characterized by a lack of EPA and DHA that are essential to fish nutrition and thus a defatted form of Hermetia meal might be of better use in fish diets. Hence, two feeding trials were conducted to investigate the effects of the partial fishmeal replacement by increasing levels of a full-fat (up to 276 g/kg) and a defatted (up to 174 g/kg) H. illucens meal on feed intake, growth, feed utilization and nutrient compositions of gilthead seabream (Sparus aurata). Results showed that both the fat content and the inclusion level of H. illucens meal are critical for the success of fishmeal replacement in the diets of S. aurata as they strongly affect feed consumption. A lower palatability of H. illucens meal was observed when included at high dietary levels with the defatted form being more readily accepted by fish. The defatted H. illucens meal is more suitable than the full-fat type to replace fishmeal, with a dietary level of about 81–104 g/kg supporting the highest feed consumption, the highest growth, an unaffected proximate composition and a better feed utilization by S. aurata.

Aquaponic systems integrate aquaculture and hydroponics into a closed-loop model aimed at sustainable food production. However, these systems still generate residual waste, notably aquaculture biosolids and plant residues, that must be effectively managed. This study explores the integration of the black soldier fly ( Hermetia illucens L. (Diptera: Stratomyidae); BSF) into aquaponics as a means of transforming organic waste into high-value insect biomass and organic fertilizer, thereby closing the nutrient loop. Two trials were conducted using BSF larvae reared on wheat bran-based diets containing varying inclusion levels (0%, 25%, 50%, 75%, and 100%; calculated on a dry weight basis) of either hydroponic residue (HR) or fish feces (FF). Key performance indicators, i.e., larval survival, final individual larval weight, specific growth rate (SGR), feed conversion ratio (FCR), and efficiency of conversion of ingested food (ECI), were evaluated. Significant reductions in larval growth, feed conversion efficiency, and nutrient utilization were observed at hydroponic residue (HR) inclusion levels > 75% and fish feces (FF) inclusion levels > 50% (in both cases p < 0.001). Post hoc analysis (Dunn’s test) confirmed that the performance of larvae, in terms of larval growth and survival and feed conversion ratio, when reared on 100% FF and HR was significantly worse than that of those on lower inclusion diets, indicating the reduced suitability of these substrates for BSF larvae, probably due to a combination of nutritional limitations and unfavorable physical substrate properties. These findings demonstrate that BSF larvae can process aquaponics-derived waste at low to moderate inclusion levels, whereas higher inclusion levels negatively affect larval growth and feed utilization.