Effects of dietary vitamin C supplementation on growth performance, antioxidant status, and growth hormone and insulin-like growth factor 1 levels in juvenile walleye pollock (Gadus chalcogrammus)

Vitamin C (V_C) is an essential micronutrient that plays a critical role in growth performance and antioxidant status of fish. However, no study has examined V_C levels in the juvenile walleye pollock (Gadus chalcogrammus) diet. Thus, six isoproteic and isolipidic diets containing 3.32, 22.08, 63.67, 101.72, 145.88, and 202.81 mg kg⁻¹ V_C were prepared and 8-week feeding trial was conducted. The fish fed the 145.88 mg kg⁻¹ V_C diet exhibited the greatest growth (final weight, weight gain, and specific growth rate) and feed utilization (feed efficiency and protein efficiency ratio. Fish fed the diet supplemented with 145.88 mg kg⁻¹ Vc had significantly higher crude whole-body protein content than those fed the 3.32 and 22.08 mg kg⁻¹ Vc diets. The whole-body crude lipid content of fish fed the 145.88 mg kg⁻¹ Vc diet was significantly higher than that of fish fed diets containing 3.32–101.72 mg kg⁻¹. However, the dietary Vc content did not affect the whole-body moisture, ash contents and plasma hematological parameters of juvenile walleye pollock. Plasma superoxide dismutase activity was significantly lowered in fish fed the 202.81 mg kg⁻¹ V_C diet. Fish fed the 145.88 and 202.81 mg kg⁻¹ V_C diets displayed higher plasma growth hormone and insulin-like growth factor-1 levels. In conclusion, the growth performance and superoxide dismutase activity of juvenile walleye pollock can be enhanced via dietary V_C inclusion at levels of 145.88–202.81 mg kg⁻¹. Based on broken-line regression analysis of weight gain, the optimal dietary Vc requirement was estimated to be 168.28 mg kg⁻¹. These findings provide species-specific guidance for Vc inclusion in aquafeeds to support efficient and sustainable aquaculture of walleye pollock.