Developing sustainable aquafeeds and diversifying farmed fish species, with focus on that with a low dependency on marine-derived ingredients, is crucial for the modern aquaculture. In the present study, for the first time, low-fish meal diets, in which black soldier fly (Hermetia illucens) meal was used to partially replace conventional ingredients, were formulated to assess the effect on growth, gut health and transcriptomic profile of flathead grey mullet (Mugil cephalus). Four, iso-proteic, iso-lipidic and iso-energetic diets were formulated to replace the conventional protein sources of a control diet (H0) with 10, 15 and 20 % black soldier fly (Hermetia illucens) meal (H10, H15 and H20, respectively). For the feeding trial, 360 juvenile grey mullets (weight = 40.2±0.5 g), obtained by the International Marine Center (IMC, Oristano, IT), were transferred to the Institute of Marine Biological Resources and Biotechnologies, National Research Council (IRBIM-CNR, Messina, IT). After acclimatation, fish were randomly divided in 12 experimental tanks, connected to an open recirculating system located on the Sicilian coast of the Strait of Messina (Temperature, 22,05±9,12 °C; O2, 66.5±8.1 mg L-1; Salinity=, 39.07±0.41 PSU; pH, 8.62±0.42). Fish were assigned to the four dietary treatments, in triplicate and fed the experimental diets (1-3% body weight, according to the water temperature) for 138 days. At the end of the experiment, after euthanasia (MS-222 500 mg L-1), fish were individually weighted and gut, liver and spleen were sampled and processed for histological and immunohistochemical analyses.
At the end of the experiment, fish fed H15 and H20 diets showed a significantly lower final weight compared to those fed H0 and H10 diets. Moreover, fish fed diet H20 showed a significantly lower Fulton condition factor compared to the other groups. Gut histology was marginally affected by the experimental diets (fig.1 a); however, a significant increase in villi thickness and mucous cells abundance was evidenced in group H15. No alterations in liver and spleen histology were observed within the experimental groups. Transcriptomic analyses of intestine revealed changes in expression of genes involved in inflammation, cell recognition patterns and lipid uptake in response to Hermetia illucens meal inclusion. According to the results obtained in the present study, the tolerance of M. cephalus to dietary H. illucens meal is lower compared to other, strictly carnivorous farmed fish species. However, replacing conventional protein sources with 10% H. illucens meal can play a role in improving fish gut health as also suggested for the first time in this species by the transcriptomic analyses, suggesting a role of this ingredient in improving fish immune defenses, without affecting fish zootechnical performances.
Founding: This project was funded by the National Recovery and Resilience Plan (NRRP), Mission 4 Component 2 Investment 1.4 Call for tender No. 3138 of 16 December, 2021, rectified by Decree n.3175 of 18 December, 2021, of Italian Ministry of University and Research funded by the European Union–NextGenerationEU; award number: project code CN_00000033, Concession Decree No. 1034 of 17 June, 2022, adopted by the Italian Ministry of University and Research, CUPB83C22002930006, Project title “National Biodiversity Future Center-NBFC.”