The Effect of Replacing Fish Meal with Fermented Stickwater on Growth, Antioxidant Capacity, Protein Metabolism and Related Gene Expression of Juvenile Turbot (Scophthalmus maximus L.)

With the rapid development of the aquaculture industry and the shortage of global fishery resources, the contradiction between supply and demand of fish meal has become increasingly significant. Therefore, finding new substitutes for fish meals and reasonably reducing the amount of fish meal in formula feed has become an important research topic in aquatic feed. Stickwater is a byproduct of fishmeal processing. It contains many water-soluble molecules, such as small molecular peptides, biogenic amines, taurine, and high unsaturated fatty acids. These components are regarded as particular nutrients or bioactive substances in the fish meal. Studies have shown that stickwater can replace part of fish meal in recent years and achieve good results in fish and other aquatic animals. The fermented feed has excellent advantages for fish meal replacement. Studies have shown that after fermentation, the contents of anti-nutritional factors in plant raw materials are decreased.In contrast, the contents of small peptides and free amino acids are increased, the nutrient composition is changed, and the intermediate metabolites of microorganisms are obtained, which can further improve the replacement level of fish meal. There are few studies on the fermentation of feed materials, such as fish meal and stickwater, which do not contain anti-nutritional factors. Our laboratory's preliminary study found that stickwater combined with other animal and plant proteins could replace 40% of fish meal in the feed. This experiment fermented stickwater first, then replace fish meal in high plant protein formula feed with fermented stickwater (FSW), aims to further increase the amount of fish meal replacement, reduce the negative effect of the plant protein source on the turbot, for FSW application in feeds for turbot provide a theoretical reference.The stickwater in the experiment was a brown viscous liquid, with a water content of 48.73%, dry matter crude protein content of 64.08%, and crude fat content of 8.91%. The strains used were Bacillus subtilis and Lactobacillus. The fermentation conditions were as follows: Temperature of 37℃, the addition of 1% sugar as auxiliary material, Bacillus subtilis and Lactobacillus (1:1) added to 1% of the total mass of the stickwater, and the fermentation period was five days. After fermentation, the content of acid-soluble protein decreased significantly. Still, the crude protein, crude fat, and free amino acids showed no significant changes. It was then used for turbot culture.Healthy juvenile turbot with an average bodyweight of (30.00±0.03) g were randomly divided into six groups with three replicates and 30 fish per replicate. The trial lasted for eight weeks. Six diets consisted of a positive control diet with 50% fish meal (positive control group), a harmful control diet with 30% fish meal (negative control group), and experimental diets formulated by FSW were used to replace 2%, 4%, 6%, and 8% of the fish meal with the harmful control diet, respectively (FSW2, FSW4, FSW6, and FSW8). The results showed: There were no significant differences in the survival rate of juvenile turbot among all groups (P > 0.05). There were no significant differences in juvenile turbot's weight gain rate and protein efficiency ratio in the FSW2~FSW8 group compared with that of the positive control group. Still, they were all higher than those in the negative control group. The crude protein content of whole fish and dorsal muscle in the FSW2~FSW8 group was not significantly different from that in the positive control group. Still, it was significantly higher than that in the negative control group. The highest crude lipid content of whole fish and dorsal muscle was found in the negative control group (P < 0.05). Serum ALT, AST, and TG levels in the negative control group were significantly higher than those in the positive control group; the negative control group and FSW2~FSW8 group showed first decreasing and subsequent increasing serum ALT, AST, and T-CHO levels. At the same time, the ALT and AST levels in the liver showed an opposite trend. The serum HDL-C content in the negative control group was significantly lower than that in the positive control group. The activities of PKA, KPC, and LDH in the liver were significantly lower in the negative control group than in the positive control group (P < 0.05). Compared with the positive control group, the expression of b0at1 and pept1 in the intestine was upregulated in the negative control group. In contrast, the expression of cat1 and pat1 was not significantly different. The expression levels of boat1, cat1, pat1, and pept1 in the FSW2~FSW8 group were significantly higher than those in the positive control and negative control groups (P < 0.05).The results showed that FSW was an excellent substitute for fish meals, and the added amount could be up to 8% in the feed. In the feed with high plant protein, the fish meal can be replaced by FSW to reduce the amount of fish meal further, reduce the metabolic abnormality caused by plant proteins, and improve juvenile turbot's antioxidant capacity. Under these experimental conditions, the group supplemented with FSW achieved the same growth effect as the positive control group. In conclusion, the fish meal content of juvenile turbot feed can be reduced to 22% by adding FSW without adverse effects on the growth of juvenile turbot. This provides a theoretical reference for the fermentation process of SW and subsequent application of FSW in seawater fish