Abstract:Turbot (Scophthalmus maximus) is an important mariculture species in northern China because of its high economic value. As a cold-water carnivorous fish, its rapid growth relies on high dietary protein in the form of fish meal. Fish meal is a major component of aquatic feed because of its highly digestible protein, balanced amino acids, and good palatability. However, global fish meal production is insufficient, and its price has soared due to overfishing and climate change. There is an urgent need to find high-quality protein sources to replace fish meal in the diet. Many experimental studies on replacing fish meal with protein sources have identified shortcomings. Plant protein sources contain anti-nutritional factors and defects in the amino acid profile, and have poor palatability; animal protein sources are unstable in their nutrient composition. Much work remains to be done to identify new protein sources. Insects are the largest organism community in ecosystems, and offer a new kind of protein source with great potential. The European Commission has recently approved the use of protein derived from insects in aquatic feed. Hermetia illucens is a saprophytic insect that can consume livestock manure and domestic waste to produce high-value animal feedstuff. Its larvae can be used as an alternative protein source to fish meal because they are rich in amino acids and polyunsaturated fatty acids. In addition, H. illucens is fed on decaying organic matter such as food waste, animal excrement, and animal and plant carcasses; the breeding cost is low, and the adults do not disturb humans. It can be added to the feed of sea bass (Dicentrarchus labrax), rainbow trout (Oncorhynchus mykiss), catfish (Pelteobagrus fulvidraco), large yellow croaker (Larimichthys crocea), grass carp (Ctenopharyngodon idellus), california bass (Micropterus salmoides), and Scophthalmus maximus (turbot). Most research has investigated the defatted dry powder of H. illucens. However, there are no reports on the replacement of fish meal with full-fat H. illucens in turbot. Therefore, in this study, we aimed to evaluate the application of H. illucens larvae as turbot feed. This experiment was conducted to investigate the effects of replacement of fish meal protein with full-fat H. illucens larvae meal on the growth performance, feed utilization, serum biochemical indexes, and skin color of turbot, in order to provide important information for turbot culture. Four treatments were designed for this study. Four isonitrogenous and isoenergetic diets were formulated with H. illucens larvae meal by substituting it into fish meal at 0% (FM), 20% (HI20), and 40% (HI40); the fourth experimental diet (HI40AA) was HI40 supplemented with methionine and lysine. A total of 1200 turbot [initial body weight: (26.57±0.54) g] were randomly distributed into four experimental diets, each group with three replicates, and each replicate with 100 juvenile turbot. The fish were fed twice daily to apparent satiation. The water temperature was kept at 11.0℃~14.4℃ and the water was changed twice daily. After the 56 d feeding trial, the results showed that H. illucens larvae meal significantly influenced weight gain and specific growth rate, which were significantly lower in the HI40 and HI40AA groups than in the HI20 and FM groups (P<0.05); no significant differences were observed in the feeding rate, feed conversion ratio, hepatosomatic index, viscerosomatic index, condition factor, or body composition, including crude protein and ash, among all groups (P>0.05). The serum cholesterol and high-density lipoprotein in the HI40 and HI40AA groups were significantly lower than those in the HI20 and FM groups (P<0.05), and the content of serum low-density lipoprotein in the FM group was significantly higher than that in the other groups (P<0.05). The values of L* (lightness) and a* (redness) of turbot dorsal skin color in the HI40 and HI40AA groups were significantly lower than those in the FM and HI20 groups (P<0.05). The value of b* (yellowness) of turbot dorsal skin color in the FM group was significantly lower than those in the other groups (P<0.05); it was higher in the HI40 group than in the HI20 and HI40AA groups (P<0.05). No significant differences were observed in skin color parameters in the ventral area of the turbot (P>0.05). In conclusion, the replacement of dietary fish meal with H. illucens did not affect the growth performance and physiological parameters of turbot when the replacement level was no more than 20%, which is therefore the recommended substitution level of fish meal by full-fat H. illucens larvae meal in the diet of juvenile turbot.
