Freshness-Dependent Effects of Degreased Hermetia illucens Meal as a Fishmeal Substitute on Growth Performance, Edible Muscle Nutritional Quality, and Physiological Status of Macrobrachium nipponense.

Degreased Hermetia illucens (black soldier fly) meal has been widely used as a substitute for traditional protein sources, including fish and soybean meals, in livestock and aquaculture especially for prawn Litopenaeus vannamei and Macrobrachium rosenbergii. The oriental river prawn (Macrobrachium nipponense) is a commercially significant shrimp species prevalent in freshwater lakes and river systems across China. We explored the potential of replacing fish meal with degreased H. illucens meal in the diet of M. nipponense by assessing how different substitution ratios and the freshness of the meal influenced river prawn growth. The results showed that a 20% substitution ratio significantly increased the specific growth rate and condition factor of M. nipponense (p < 0.05), while the feed conversion ratio was significantly reduced (p < 0.05). Based on the dose-response model, the specific growth rate reaches its peak when the substitution ratio is 24.25%. At this 20% ratio, there was a significant increase in the crude protein and lipid content in M. nipponense muscle. Moreover, the concentrations of several essential amino acids, along with monounsaturated and polyunsaturated fatty acids, were notably elevated compared to the control group (p < 0.05). In the 20% substitution group, the activities of superoxide dismutase, trypsin, and lipase in the hepatopancreas, as well as acid phosphatase and lysozyme in the hemolymph, were significantly increased relative to the control group (p < 0.05). The research also assessed the impact of using moderately decomposed (T1: TVB-N 65 mg/100 g) and highly decomposed (T2: TVB-N 80 mg/100 g) degreased H. illucens meal as a 20% replacement for fish meal, in comparison to fresh degreased H. illucens meal (TVB-N 50 mg/100 g). In the T1 group, both the specific growth rate and feed conversion ratio were considerably lower than those observed in the control group (p < 0.05). In both the moderately (T1) and highly (T2) decomposed treatment groups, the crude lipid content in M. nipponense muscle was significantly reduced, while the crude ash content was notably higher than that in the control group (p < 0.05). Additionally, the T1 and T2 groups exhibited significantly decreased levels of certain essential amino acids, saturated fatty acids, monounsaturated fatty acids, polyunsaturated fatty acids, and mineral elements in the muscle (p < 0.05). In these groups, the activities of alanine aminotransferase in the hepatopancreas and hemolymph, as well as the malondialdehyde content in the hepatopancreas, were significantly elevated compared to the control group (p < 0.05). On the other hand, the activities of hepatopancreatic trypsin and lipase in the T2 treatment were significantly reduced compared to the control group (p < 0.05). In conclusion, we recommend the use of fresh degreased H. illucens meal at a substitution level of 20%-25% (with the model-derived peak near 24.25%) in M. nipponense diets.