The demand for sustainable aquaculture feeds has led to the exploration of alternative protein sources, including insect meals. This study investigated the effect of replacing dietary fishmeal (FM) with black soldier fly larvae meal (BSFLM) on the nutritional composition and heavy metal content of fillet tissues from farmed dusky kob (Argyrosomus japonicus) juveniles. Four isonitrogenous, isolipidic, and isocaloric experimental diets were formulated by replacing FM with BSFLM at 0, 33, 66, and 100%. After a 96-day feeding period (starting from 131 ± 0.33 mm/fish standard length), three replicates of an approximately 200 g pooled sample of skinless fillets (from five specimens) were collected from each dietary group for analyses of proximate composition, amino acids (AAs), fatty acids (FAs), trace elements (iron, zinc, copper, chromium, cobalt, and nickel), and heavy metals (lead, mercury, cadmium, and total arsenic). Proximate composition (moisture, crude fat, crude protein, and ash) was determined by oven-drying, Soxhlet extraction, the Dumas method, and furnace incineration, respectively. Amino acids were measured using high-performance liquid chromatography, and FA were analysed by gas chromatography-mass spectrometry following transesterification. Trace elements and heavy metals were quantified using inductively coupled plasma-mass spectrometry.
No significant differences (p>0.05) were found in the fillet’s proximate composition among fish fed the four diets. While the content of most analysed amino acids (histidine, serine, arginine, glycine, aspartic acid, glutamic acid, threonine, alanine, proline, tyrosine, phenylalanine, leucine, lysine, and methionine sulfone) remained unaffected, the essential AA valine was significantly lower in fish fed the 100% BSFLM diet compared to the control and 33% BSFLM diets, and isoleucine decreased at 66% and 100% BSFLM. No significant differences (p>0.05) were observed in the content of FAs such as docosahexaenoic acid and the essential FAs linoleic acid and linolenic acid—except for the non-essential monounsaturated fatty acid palmitoleic acid, which was reduced as BSFLM inclusion increased. Mercury and arsenic concentrations also decreased with higher BSFLM inclusion, whereas trace elements and other heavy metals showed no significant differences.
Overall, these findings suggest that BSFLM-based feeds are a viable alternative to FM-based diets from a nutritional standpoint, providing stable key nutrients in the fillet and reducing mercury and arsenic levels, despite modest decreases in certain essential amino acids at higher inclusion levels.