Burbot Lota lota maculosa, a freshwater species similar to cod, has the potential for diversifying aquaculture. However, limited information exists on the dietary protein requirement of this fish species. For this reason, a 10-week feeding trial was conducted to evaluate the optimal dietary protein requirement of sub-adult burbot.
Two-hundred and sixty-four fish (141 ± 2.80 g) were randomly distributed in twenty-four 60-L closed recirculating circular tanks (11 fish/tank). Six experimental diets with increasing crude protein levels (38, 43, 48, 53, 57, and 61 %) were formulated and designated as CP38, CP43, CP48, CP53, CP57, and CP61, respectively. Four tanks were used for each diet, and the fish were hand-fed to apparent satiation twice daily.
The weight gain and specific growth rate of experimental fish increased with increasing dietary protein up to 53% (CP53) and then decreased with further increases in dietary protein. The CP38 group exhibited the lowest feed conversion efficiency, and the protein efficiency ratio and protein productive value increased up to CP53 and decreased with higher protein levels. Dietary protein levels significantly influenced fillet nutrient composition (P < 0.05) but had no significant effect on body indices (K-value, fillet yield, HSI, and VSI) (P > 0.05). Dietary protein level did not affect serum glucose content. Fish fed CP43, CP48, and CP53 diets showed the highest activities of serum glutamic-pyruvic transaminase, alkaline phosphatase, and glutamic-oxaloacetic transaminase, respectively. While the expression of insulin-like growth factor 1 (igf1) did not exhibit any defined trend ( P = 0.158), the levels of myoblast determination protein 1 (myod; P = 0.035) and myosin heavy chain (myhc; P < 0.001) were significantly influenced by the graded protein levels. Conclusively, based on a second-order polynomial regression analysis of weight gain, the optimum dietary protein level for sub-adult burbot was estimated to be 49.5% (Figure 1).