Influences of dietary fatty acid profile on growth, body composition and blood chemistry in juvenile fat cod (Hexagrammos otakii Jordan et Starks)

This study was conducted to investigate the influence of dietary lipid source and n-3 highly unsaturated fatty acids (n-3 HUFA) level on growth, body composition and blood chemistry of juvenile fat cod. Triplicate groups of fish (13.2 ± 0.54 g) were fed the diets containing different n-3 HUFA levels (0-30 g kg⁻¹) adjusted by either lauric acid or different proportions of corn oil, linseed oil and squid liver oil at 100 g kg⁻¹ of total lipid level. Survival was not affected by dietary fatty acids composition. Weight gain, feed efficiency and protein efficiency ratio (PER) of fish fed the diets containing squid liver oil were significantly (P < 0.05) higher than those fed the diets containing lauric acid, corn oil or linseed oil as the sole lipid source. Weight gain, feed efficiency and PER of fish increased with increasing dietary n-3 HUFA level up to 12-16 g kg⁻¹, but the values decreased in fish fed the diet containing 30 g kg⁻¹ n-3 HUFA. The result of second-order polynomial regression showed that the maximum weight gain and feed efficiency could be attained at 17 g kg⁻¹ n-3 HUFA. Plasma protein, glucose and cholesterol contents were not affected by dietary fatty acids composition. However, plasma triglyceride content in fish fed the diet containing lauric acid as the sole lipid source was significantly (P < 0.05) lower than that of fish fed the other diets. Lipid content of fish fed the diets containing each of lauric acid or corn oil was lower than that of fish fed the diets containing linseed oil or squid liver oil only. Fatty acid composition of polar and neutral lipid fractions in the whole body of fat cod fed the diets containing various levels of n-3 HUFA were reflected by dietary fatty acids compositions. The contents of n-3 HUFA in polar and neutral lipids of fish increased with an increase in dietary n-3 HUFA level. These results indicate that dietary n-3 HUFA are essential and the diet containing 12-17 g kg⁻¹ n-3 HUFA is optimal for growth and efficient feed utilization of juvenile fat cod, however, excessive n-3 HUFA supplement may impair the growth of fish.