Effects of Dietary Supplementation with Enzymatically Hydrolyzed Kelp Powder on Growth, Digestion, Metabolism, and Oxidation Resistance of Juvenile Sea Cucumber (Apostichopus japonicus)

As a raw material for aquatic feed, kelp has a large output and is rich in sugars, minerals, vitamins, free amino acids, fatty acids, etc. It is not only a natural bait for sea cucumbers in natural seas but also often used as a main ingredient in compound feed for sea cucumbers. However, kelp contains non-starch polysaccharides (NSP) which are difficult to be digested by endogenous enzymes secreted by aquatic animals.Enzymatically hydrolyzed kelp (EKP) powder is made by hydrolyzing kelp powder with a compound enzyme preparation (NSP enzyme∶cellulase∶neutral protease∶flavor enzyme = 8∶12∶3∶1), and the compound enzyme is added at 3% (by weight). The ratio of material to liquid is 1∶6, and the conditions of enzymatic hydrolysis comprise a temperature of 50 ℃, pH 6, and reaction time 6 h.A feeding trial was conducted to investigate the effects of EKP on the growth, digestion, metabolism, and oxidation resistance of juvenile sea cucumbers Apostichopus japonicus (Selenka, 1867). In this experiment, fish meal, kelp meal and EKP were used as the main protein sources to prepare a basic feed with a crude protein content of 12.00%, a crude fat content of 0.40%, and an energy content of 6.20 KJ/g. A total of 540 healthy sea cucumbers with an initial average weight of (11.4±0.04) g were selected and randomly assigned to 18 cylindrical circulating buckets. They were divided into 6 experimental groups with 3 replicates in each group and 30 sea cucumbers in each replicate. Six experimental diets were formulated with the graded levels of EKP, 0% (D1, control group), 3% (D2), 6% (D3), 9% (D4), 12% (D5), and 15% (D6) dry diets. The feeding period was 56 days. Bait was fed once a day at a fixed time (16:00). The water was changed every 3 days, and a siphon was used to withdraw the residual bait and feces from the bottom of the bucket. The amount of water changed was 1/2 of the water level in the bucket. The breeding experiment was run for 1 month. During the breeding period, the water temperature was 13~17 ℃, pH was 7.5~8.2, dissolved oxygen was at least 6 mg/L, and salinity was maintained at 28~30.The results showed that: (1) The weight gain rate and specific growth rate of sea cucumbers in the D3, D4, D5 and D6 groups were significantly increased as compared to that in the control group, with the D4 group reporting the highest values. (2) The contents of water, crude protein, crude lipid, and ash in the body wall of sea cucumbers were not different among groups, but additio