This study was conducted to evaluate the effects of replacing fish meal (FM) with methanotroph (Methylococcus capsulatus, Bath) bacteria protein (MBP) in the diets of the juvenile American eel (Anguilla rostrata). Trial fish were randomly divided into the MBP0 group, MBP6 group, MBP12 group, and MBP18 group fed the diets with MBP replacing FM at levels of 0, 6%, 12%, and 18%, respectively. The trial lasted for ten weeks. There were no significant differences in weight gain or feed utilization among the MBP0, MBP6, and MBP12 groups (except for the feeding rate in the MBP12 group). Compared with the MBP0 group, the D-lactate level and diamine oxidase activity in the serum were significantly elevated in the MBP12 and MBP18 groups. In terms of non-specific immunity parameters in serum, the alkaline phosphatase activity was significantly decreased in the MBP18 group, and the complement 3 level was significantly elevated in the MBP12 and MBP18 groups. The activities of lipase and protease in the intestine were significantly decreased in the MBP12 and MBP18 groups. Compared with the MBP0 group, the total antioxidant capacity and activities of superoxide dismutase, catalase, and glutathione peroxidase in the intestine were significantly decreased in the MBP18 group, while the malondialdehyde level was significantly increased. The villus height, muscular thickness, and microvillus density were significantly decreased in the MBP12 and MBP18 groups. There were no significant differences in the foresaid parameters between the MBP0 group and the MBP6 group. The intestinal microbiota of the MBP6 group was beneficially regulated to maintain similar growth and health status with the MBP0 group. The adverse effects on the intestinal microbiota were reflected in the MBP18 group. In conclusion, MBP could successfully replace 6% of FM in the diet without adversely affecting the growth performance, serum biochemical parameters, and intestinal health of juvenile American eels.

The economic loss of intestinal inflammation by histamine in diet has caused concern in eel culture; it is urgent to develop a natural feed additive to alleviate the negative effects of high-level dietary histamine. Oligomeric proanthocyanidins (OPCs), as the natural antioxidant to scavenge free radicals, were supplemented to alleviate the effects of dietary histamine on parameters of growth, serum biochemistry, and liver metabolites of juvenile American eels in the present study. Juvenile American eels with an initial body weight of 10.84 ± 0.16 g per fish were divided into a control group fed commercial diet, a HIS group fed commercial diet supplemented 300 mg/kg histamine, and an OPC group fed commercial diet supplemented 300 mg/kg histamine and 300 mg/kg OPC. The trial period was 11 weeks. Compared with the HIS group, the OPC group showed that dietary OPC supplementation could alleviate growth retardation; decrease activities of glutamic-pyruvic transaminase, glutamic-oxaloacetic transaminase, acid phosphatase, and alkaline phosphatase; and lower levels of immunoglobulin and complement 3 in serum. With the upregulation of L-cysteine and dihydrouracil involving cysteine and methionine metabolism, beta-alanine metabolism, sulfur metabolism, pantothenate and CoA biosynthesis, and pyrimidine metabolism in the liver, OPC supplementation could alleviate the effects of high-level dietary histamine on growth and serum biochemical parameters. There were no significant differences of growth performance and serum biochemical parameters between the HIS group and control group. These results indicated that OPC might act as a potential feed additive to effectively counteract the negative effects of dietary histamine on juvenile American eel.

As growth-promoting feed additives, compound acidifiers (CAs) have been widely reported in many farmed fish species, whereas there is little information on the application of CAs in the eel diet. The present trial was conducted to evaluate the supplementation effects of CAs on growth performance, serum biochemical parameters, and body composition of the juvenile American eel (Anguilla rostrata). The CAs supplementation levels in the diet of American eel were 0, 2.0, 3.0, 4.0, and 5.0 g/kg, respectively. The trial lasted for 12 weeks. The most beneficial effects of CAs on promoting growth performance, decreasing levels of serum lipid, modifying parameters of the liver function and immune function in the serum, and increasing the utilization of calcium and phosphorus in the diet were observed in the fish fed with 4.0 g/kg CAs. Dietary CAs supplementation had no significant effects on the body composition of whole fish, whereas 5.0 g/kg of dietary CAs supplementation exerted negative effects on growth performance, serum biochemical parameters, and utilization of calcium and phosphorus. The recommended level of CAs in the diet was 3.5–3.7 g/kg to achieve the optimal weight gain and feed conversion ratio for the juvenile American eel.

As a functional feed additive, grape seed proanthocyanidin extract has received a lot of attention due to its biological activity in the health of aquatic animals, but its high cost limits the application of this feed additive in the diet of many fish species. It is thus urgent to develop a new resource of proanthocyanidin extract. We aimed to investigate the effects of dietary supplementation with peanut skin proanthocyanidins (PSPc) on growth parameters and lipid metabolism of juvenile American eel (Anguilla rostrata). Four hundred and fifty juvenile eels were randomly divided into five groups fed diets with five PSPc supplementation levels. The trial lasted for 8 weeks. Dietary PSPc supplementation significantly improved weight gain and feed utilization, and the best growth performance was found in the group fed with 900 mg/kg PSPc. PSPc supplementation significantly affected the crude protein level of whole fish and serum lipid parameters, and the best lipid-lowering effect was found in the fish fed with 900 mg/kg PSPc. Dietary PSPc supplementation increased lipolytic enzyme activities and decrease lipid synthase levels in the liver. The lipid metabolites affected by 900 mg/kg PSPc in the liver were mainly upregulated phosphatidylethanolamine in autophagy, downregulated ceramides in sphingolipid metabolism, upregulated phosphatidylcholine and phosphatidylethanolamine, downregulated 2-lysophosphatidylcholine in glycerophospholipid metabolism, and upregulated phosphatidylcholine in linoleic acid metabolism. In conclusion, an appropriate level of PSPc might effectively improve growth performance and regulate the lipid metabolism of the juvenile American eel, and 900 mg/kg PSPc is recommended in the diet of this fish species.

The present trial was conducted to evaluate the supplementation effects of Macleaya cordata extract (MCE) on growth performance, serum biochemical parameters, and intestinal health of the juvenile American eel (Anguilla rostrata). The 480 juvenile American eels (10.93 ± 0.06 g) were randomly divided into four groups. They were fed on diets supplemented with MCE levels of 0, 25 mg/kg, 50 mg/kg, and 100 mg/kg for ten weeks, respectively. The 50 mg/kg or 100 mg/kg MCE could significantly improve growth performance, and increase the activities of acid phosphatase and alkaline phosphatase, as well as the level of high-density lipoprotein cholesterol. These levels of MCE also decreased the levels of D-lactate acid, triglyceride, and total cholesterol and the activities of aspartate aminotransferase, alanine aminotransferase, and diamine oxidase. The antioxidant ability, muscular thickness, and fold height of the intestine were enhanced by 50 mg/kg or 100 mg/kg MCE. There was no significant difference in the above parameters of groups fed with 50 mg/kg or 100 mg/kg of MCE. The beneficial effects on the intestinal microbiota were demonstrated in the group fed with 50 mg/kg MCE. In conclusion, the 50 mg/kg MCE could be used in the diet to improve the growth performance and health status of the juvenile American eels.