Hydrolyzed earthworm protein as a fishmeal substitute in rainbow trout diet: Effects on growth, nutrient utilization, digestive physiology, intestinal morphology, and growth-related gene expression

An eight-week feeding trial examined the effects of replacing fishmeal with earthworm protein hydrolysate (EPH) from Eisenia fetida on rainbow trout ( Oncorhynchus mykiss ). The fishmeal in a basal diet (43.29 % crude protein and 9.48 % crude lipid) was replaced with earthworm protein hydrolysate (EPH) at four levels of 5 % (EPH 5 ), 10 % (EPH 10 ), 15 % (EPH 15 ), 25 % (EPH 25 ), and earthworm protein at a level of 25 % (EP 25 ). Each experimental diet was allocated to a triplicate group of fish (initial body weight of 5.00 ± 0.10 g) in a flow-through system for eight weeks. At the end of the feeding trial, no significant differences in final body weight, weight gain ratio, average daily gain, specific growth rate, or feed conversion ratio were detected between the fish fed diets supplemented with different levels of EPH and those fed the basal diet (p > 0.05). In contrast, the fish fed the EP 25 had significantly lower final body weight, weight gain ratio, and specific growth rate as well as higher feed conversion ratio than those fed EPH 5 and EPH 10 supplemented diets (p 0.05). The amino acid analysis revealed no significant variation in total essential amino acids among the experimental treatments (p > 0.05). However, methionine and alanine levels were lower in fish fed the EPH 5 diet than those fed diets supplemented with higher levels of EPH (p < 0.05). The activity of digestive enzymes significantly improved with increasing EPH supplementation, with trypsin activity peaking in the EPH 15 diet. Histomorphological evaluations revealed greater muscle thickness and villus height in the fish fed the EPH supplemented diets compared to those fed the EP 0 and EP 25 diets. Gene expression analysis showed significant increases in the mRNA levels of growth hormone and insulin-like growth factor in the fish fed the EPH supplemented diets compared with those in the fish fed the basal diet (p < 0.05). The findings of the present study illustrated that EPH has no adverse effect on the experimental variables compared with fishmeal, indicating its appropriate potential to be used as a viable alternative protein source in the diet of rainbow trout. While substitution of EPH up to 25 % can be effectively utilized to compensate for fishmeal shortages, 10 % EPH substitution is recommended for achieving the best performance and physiological responses in rainbow trout. • EPH replaces fishmeal in rainbow trout diets without adverse effects. • Optimal EPH substitution is 10 % for growth performance of rainbow trout. • EPH supplementation increased digestive enzyme activities of rainbow trout. • Muscle thickness of rainbow trout improved with EPH-supplemented diets. • Expression of growth related genes elevated in fish fed the EPH-supplemented diet.