Production performance, nutrient digestibility, serum biochemistry, fillet composition, intestinal microbiota and environmental impacts of European perch (Perca fluviatilis) fed defatted mealworm (Tenebrio molitor)

Yellow mealworm (Tenebrio molitor) larvae meal (TM), one of the seven insect species approved for use in aquafeed in Europe, is a frequently investigated candidate for fish diets. This study aimed to investigate the effects of dietary defatted TM on production performance, serum biochemistry, nutrient digestibility, fillet traits, intestinal microbiota, and environmental impacts of perch (Perca fluviatilis). Four experimental diets, characterized by defatted TM inclusion levels of 0, 6.8, 13.5 and 20.3%, respectively, or 0, 25, 50, and 75% replacement for fishmeal on a w/w basis (TM0, TM25, TM50, and TM75, respectively), were fed to juvenile perch (bodyweight 20.81 ± 3.36 g, total length 117.7 ± 7.2 mm) (quadruplicated per diet) for 105 days. Inclusion levels of 6.8% or 25% fishmeal replacement by defatted TM did not show a significant effect on specific growth rate and feed conversion ratio, while further levels of 13.5 and 20.3%, or 50 and 75% fishmeal replaced by defatted TM, respectively, displayed a significant delay in the former (P = 0.01), but increase in the latter indice (P = 0.04) compared to TM0 diet. The aspartate aminotransferase activities in perch's serum increased with increasing dietary TM (P = 0.02). Protein digestibility of perch exhibited a negative correlation with dietary TM (P = 0.03). Dietary defatted TM did not, alter fillet composition of perch (P > 0.05) and modify diversity of fish gut microbiota (Chao1 index, P = 0.742; Shannon index, P = 0.557; and observed species, P = 0.522), but significantly reduced abundance of Lactobacillus (P = 0.04) and Streptococcus (P = 0.01). Diets containing more than 6.8% TM generated a comparable amount of total solid waste and solid phosphorus waste as the TM0, whereas solid nitrogen waste significantly increased with elevated TM levels (P < 0.001). The estimated environmental impacts of perch fed TM25 were comparable to TM0 for global warming potential, acidification, and land use (P > 0.05), whereas TM50 and TM75 exerted heavier burdens on energy use, eutrophication, and water use than TM0 (P < 0.001). Fishmeal replacement by TM significantly reduced the economic fish-in fish-out ratio (P < 0.001). Results of this multidisciplinary study revealed important considerations for formulating diets with insect meals and their potential effects on fish performance and environmental impacts.