Effects of the inclusion of Hermetia illucens meal in diets for rainbow trout (Oncorhynchus mykiss) reared in a low-tech aquaponic system

The study evaluated the effects of the partial substitution of fish meal (FM) with partially defatted Hermetia illucens (HI) meal on growth, 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 into nine experimental aquaponic units (3 tanks per treatment, initial tank biomass 5.74 ± 0.44 kg m−3) and fed during 76 days with three diets containing 0%, 6.3% and 12.5% HI meal and 20%, 15% and 10% FM, respectively. Therefore, HI meal replaced 0% (HI0, control diet), 25% (HI25), or 50% (HI50) of FM, respectively. During the trial, water quality was not affected by dietary treatment: temperature averaged 19.4 °C (13.8 − 23.7 °C), dissolved oxygen 8.0 mg L−1 (6.08 − 10.2 mg L−1), pH 7.4 (6.4 − 8.5), and total ammonia nitrogen 0.13 mg L−1 (0 − 0.36 mg L−1), while daily water losses due to plant evapotranspiration averaged 1.31% d−1. At the end of the trial, trout mortality was low (2.9%) and not affected by dietary treatment. The specific growth rate was lower in fish fed HI50 diet compared to those fed HI0 and HI25 diets after 26 days (1.07% d−1 vs. 1.22% d−1; p < .001) and at the end of the trial (0.81% d−1 vs. 0.88% d−1; p < .05). However, the dietary inclusion of HI did not affect feed conversion ratio (on average 1.53), final weight (303 g), fish condition factor (1.40), viscerosomatic index (10.9%) and hepatosomatic index (1.22%). Histological analyses of the anterior gut showed no significant differences in villi height (503 μm) whereas the density of goblet cells was higher in HI50 than in HI0 trout (+11%; p < .05). Regarding fish quality, fillet redness (a*) and yellowness (b*) were lower in HI50 than in HI0 treatments (−58% and −19%, respectively; p < .001). Fillet proximate composition, total saturated, monounsaturated and polyunsaturated fatty acids, eicosapentaenoic acid and docosahexaenoic acid were not affected by diets, whereas the content of C12:0 and C14:0 increased with HI dietary inclusion and the index of atherogenicity was higher (+6%; p < .05) in HI50 treatment than in HI0 and HI25 ones. In conclusion, the rearing of rainbow trout was successful in the tested aquaponic system. Fish growth, health and fillet quality were not affected when HI meal replaced 25% FM, whereas at 50% replacement rate some effects on gut histology and fillet colour and nutritional characteristics were detected, which deserve further investigations to be elucidated.