Influence of different dietary 18:3n-3/18:2n-6 ratio on growth performance, fatty acid composition and hepatic ultrastructure in Eurasian perch, Perca fluviatilis

It has been shown that numerous fish species require n-3 polyunsaturated fatty acids (PUFA) for normal growth, while the growth-enhancing effect of dietary n-6 PUFA differ substantially among species. The present study aimed to evaluate the effect of different n-3 to n-6 ratios, underlying the effect of linoleic (LA) and linolenic (LnA) levels, on growth performance, tissue fatty acid composition and hepatic ultrastructure of Eurasian perch. Four diets were formulated using 3 different lipid sources (cod liver, safflower and linseed oils) in variable proportions in order to prepare diets with different LnA/LA ratios: 0.43; 0.03; 3.03 and 0.6 for CO, SO, LO and SLO diets, respectively. Significant results confirm the statement that an increase proportion of 18:2n-6 and 18:3n-3 in the diet of Eurasian perch, or a partial substitution of fish oil by vegetable oil, does not appear to compromise growth performance of juvenile perch. Modification of lipid utilisation by a modulation of desaturation and elongation activity in liver tissue of perch is discussed. Analyses of tissue FA profiles indicate that a minimum level of EPA and DHA has to be incorporated into the diets to comply with the EFA requirement of this species and that the overabundance of 18:2n-6 observed in SO fed fish, may cause an apparent deficiency in 18:3n-3. Consequently, the quantity of 18:2n-6 and 18:3n-3 in the SLO diet (0.64 LnA/LAratio) seems to be more suitable than in the SO diet (0.03 LnA/LA ratio) to promote ArA biosynthesis. However fish with a tissue FA profile the most similar to that of fish fed cod liver oil were fish fed the LO diet (3.03 LnA/LA ratio), suggesting the higher activity of Δ6 desaturase rather than Δ5 desaturase.