Performance and fillet traits of rainbow trout (Oncorhynchus mykiss) fed different levels of Hermetia illucens meal in a low-tech aquaponic system

In this study, we evaluated the effects of dietary substitution of fishmeal (FM) with partially defatted Hermetia illucens meal (HI) on the growth, feed digestibility, gut morphology, and fillet quality of rainbow trout reared in a low-tech aquaponic system. A total of 173 rainbow trout (initial body weight: 156 g +/- 39.8 g) were distributed among nine experimental aquaponic units. Three diets were fed to fish (three aquaponic units per dietary treatment) over a period of 76 days, i.e. H0 diet (control), H6 diet, and H12 diet containing 0 g/kg, 62 g/kg, or 124 g/kg of HI and 200 g/kg, 150 g/kg, or 100 g/kg of FM, respectively. We found that the apparent digestibility coefficients of diets were unaffected by the inclusion level of dietary HI. At the end of the trial, trout mortality was low (2.9%) and unaffected by dietary treatment. The specific growth rate was, however, lower in fish fed the H12 diet than in those fed H0 and H6 diets after 26 days (1.07% d(-1) vs. 1.22% d(-1); P < 0.001) and at the end of the trial (0.81% d(-1) vs. 0.88% d(-1); P < 0.05). In contrast, dietary inclusion of HI appeared to have no appreciable effect on the feed conversion ratio (on average 1.53), final weight (303 g), fish condition factor (1.40), viscerosomatic index (10.9%), or hepatosomatic index (1.22%). The inclusion of HI was, nevertheless, found to promote a 10% increase in the density of goblet cells in the gut of fish fed the H12 diet compared with those receiving the H0 diet (P < 0.05). With regards to fillet traits, redness and yellowness indices were lower in fish fed the H12 diet than in those fed the H0 diet. Although dietary HI had little effect on the proximate composition and fatty acid profile of fish, the proportions of C12:0 and C14:0 increased with HI dietary inclusion. In conclusion, fish growth and fillet quality were essentially unaffected by a 25% fish meal replacement with HI in isonitrogenous and isoenergetic diets (control diet containing 200 g/kg of fish meal), whereas at a replacement rate of 50%, we detected certain effects on gut histology and fillet colour and nutritional characteristics, which warrant further investigation.