Dietary Vitamin D3 Requirement of Juvenile Rockfish (Sebastes schlegelii)

Rockfish (Sebastes schlegelii) is a near-shore, carnivorous coldwater fish, primarily distributed in the East China Sea, Korean peninsula, Japan, and the Sea of Okhotsk. Owing to its delicious meat, strong disease resistance, rapid growth, and other characteristics, it has become one of the main cultured fish in deep water cages in northern China. A high-quality and efficient formula diet for rockfish has not been developed because of nutrient requirement deficiencies. As vitamin D3 is the most important form of vitamin D and only exists in animals, cultured fish can only obtain vitamin D3 from their diet. Vitamin D3 can maintain the mineral homeostasis in fish, participate in the endocrine system, and affect the proliferation and apoptosis of immune cells. As such, this experiment was conducted to investigate the effects of dietary vitamin D3 on growth performance, body composition, and immune function of juvenile rockfish and determine the optimum dietary vitamin D3 requirement. Six kinds of isonitrogenous and isolipidemic diets with 707, 1254, 1740, 2513, 4519, and 8671 IU/kg of vitamin D3 were prepared by adding 0, 500, 1000, 2000, 4000, and 8000 IU/kg of vitamin D3 into the basic diet. Each diet was randomly assigned to triplicate groups of juveniles with initial bodyweights of (20.95±0.05) g for eight weeks. The fish were challenged with Vibrio harveyi for 48 hours after feeding. Results showed that, with the increase of vitamin D3 contents, both the weight gain rate (WGR) and specific growth rate first increased and then decreased, reaching a maximum value in the 2513 IU/kg group, which was significantly higher than that in the 707 IU/kg group. The feed conversion rate first decreased and then increased and was significantly lower in the 2513 IU/kg group than other groups. Both viscerosomtic index and hepatosomatic index decreased significantly and were significantly higher in the 707 IU/kg group than in other groups. There were no significant differences in the condition factor and survival rate among all groups. The crude fat of whole fish and muscle showed an increasing trend, reaching a maximum value in the 4519 IU/kg group, which was significantly higher than 707 IU/kg group. The crude muscle protein first decreased and then increased; the 8671 IU/kg group was significantly higher than the 1740 IU/kg and 2513 IU/kg groups. The moisture, crude protein, and crude ash of whole fish, as well as the muscle moisture and crude ash, had no significant differences in concentration among all groups. The spine crude ash showed an increasing trend; the 1740~8671 IU/kg groups had significantly higher contents than the 707 IU/kg group. The calcium contents increased with the increase of dietary vitamin D3 levels. Total antioxidant capacity (T-AOC) and superoxide dismutase activities first increased and then decreased, reaching a maximum in the 2513 IU/kg group. The malondialdehyde (MDA) contents first decreased and then increased; the 2513 IU/kg group had significantly lower content than the 707 IU/kg group. Complement 3 (C3) contents first increased and then decreased; the 707 IU/kg group had significantly lower content than the other groups. Alanine aminotransferase and aspartate aminotransferase first decreased and then increased, reaching minimum values in the 2513 IU/kg group. The highest cholesterol and triglyceride values were observed in the 4519 IU/kg group. The calcium (Ca2+) contents first increased with the increase of dietary vitamin D3 levels, with the 707 IU/kg group showing significantly lower content than other groups. The phosphorus and complement 4 (C4) contents were not affected by dietary vitamin D3. After the challenge, T-AOC, MDA, C3, and C4 were significantly elevated; T-AOC, C3, and C4 in the serum had maximum values in the 2513 IU/kg group, and MDA reached its minimum value in the 2513 IU/kg group. With the increase of dietary vitamin D3 content, HSP70, MYD88, IRAK4, TRAF6, and TLR2 in the liver, and head kidney increased first and then decreased before and after the challenge. Gene expression was significantly elevated after the challenge. Taking WGR as the evaluation index, a broken line regression analysis showed that the optimum dietary vitamin D3 requirement was 2 223.45 IU/kg diet for juvenile rockfish S. schlegelii with the body weight of (20.95±0.05) g