Microparticulate diets as first food for gilthead sea bream larva (Sparus aurata): study of fatty acid incorporation

Recent advances have led to the development of inert diets for first-feeding marine fish larvae. The purpose of the present study was to evaluate fatty acid (FA) incorporation into larva using practical-type and semipurified diets as first food for larvae. Experimental microparticulate diets were fed to gilthead sea bream larvae, right from the mouth opening to day 21 post hatching. Five basal diets were compared, using fish meat or casein as main protein sources, and fish protein concentrate or casein hydrolysates and algae powder (Schizochytrium). Diets contained soybean lecithin to provide polar lipids and this also provided a high linoleic acid (18:2n-6) content. n-3 HUFA (1.5-1.7% DM) were mainly in dietary neutral lipids, with a part in polar lipids in diets containing fish meal or fish hydrolysates. Fatty acid incorporation was studied by analysing FA content of larvae at the beginning and at the end of the trial. A semipurified diet (casein + Schizochytrium powder) led to the best mean survival rate of 25% at day 21. While length increments were low, basal diets containing fish meat gave higher growth in size than casein-based diets. Whole body total fatty acid content differed between treatments and was higher in larvae fed casein-based diets than in those fed fish meal-based diets. As a result, fatty acid profiles shown showed various differences between treatments but not clearly related to dietary fatty acids. No clear sign of desaturation nor elongation of fatty acids was observed; the presence of 22:5n-6 (from Schizochytrium) into some diets did not seem to induce retroconversion to 20:4n-6 by larva. Fatty acid profiles incorporated into larvae were compared to those of diets in order to display common tendencies: the incorporation of n-6 fatty acids was higher than their relative proportion in the diets, even for 18:2n-6 despite high dietary supply; incorporation of 20:4n-6 was two- to fourfold higher than the proportion in diets (irrespective of the level of 22:5n-6). Among saturated FA, high stearate but low myristate incorporation was observed. Taking into high level of n-3 HUFA in the larvae before first feeding, n-3 long-chain PUFA were not selectively incorporated into larvae during feeding, with a negative tendency for EPA, and variable incorporation of DHA between treatments.