Influence of 18:2n-6/20:5n-3 ratio in diets on growth and fatty acid composition of juvenile abalone, Haliotis discus hannai Ino

A 120-day feeding trial was conducted to examine the effects of the ratio of dietary linoleic acid (LA, 18:2n-6) to eicosapentaenoic acid (EPA, 20:5n-3) on the growth and fatty acid composition of juvenile Haliotis discus hannai (initial shell length 10.23±1.48mm; initial body weight 0.13±0.05g) in a recirculation water system. Five semipurified diets with 35gkg^-1 total lipid were formulated to contain graded LA/EPA ratios (1:0, 0.75:0.25, 0.5:0.5, 0.25:0.75, and 0:1, respectively). Twenty-five juveniles were stocked in a rearing unit, a plastic basket (20× 20×10cm), as a replicate, and there were three replicates for each dietary treatment. The results showed that abalone survival rates were generally high (90.1-98.3%) and independent of the dietary treatments. However, abalone growth was significantly affected by LA/EPA ratio (P< 0.05). The LA/EPA ratio of 0.25:0.75 (Diet 4) produced the highest weight gain rate (WGR, 416.3%), closely followed by the ratio of 0:1 (Diet 5, 412.9%), the ratio of 0.5:0.5 (Diet 3, 399.7%) and the ratio of 0.75:0.25 (Diet 2, 372.1%), but no significant differences were observed among these treatments. The abalone fed the diet without 20:5n-3 (Diet 1) had the lowest WGR (Diet 1, 363.8%), which was significantly lower than that of Diet 4. Fatty acid profiles in abalone body reflected those of dietary lipids, especially for the polyunsaturated fatty acids. The contents of arachidonic acid (AA; 20:4n-6) in abalone tissues were positively correlated with dietary level of 18:2n-6 (P<0.05). Similar correlation was also observed between the level of docosahexaenoic acid (DHA, 22:6n-3) in abalone tissues and the level of dietary EPA. It is suggested that abalone, H. discus hannai, have the capacity to synthesize 20:4n-6 from 18:2n-6, and maybe 22:6n-3 from 20:5n-3.