Sammendrag
The objectives of the study were to examine if early cod larvae could incorporate long-chain highly unsaturated fatty acids like docosahexaenoic acid (22:6 n − 3, DHA) from dietary phospholipids (PL) more efficiently than from dietary triacylglycerides (TAG), and to investigate if because of this, PL from copepods is a better DHA source than PL from rotifers for larval cod. Two diets of Brachionus plicatilis Nevada were compared in one first feeding experiment. Their lipid composition was similar, but %DHA of PL fatty acids was different; 2.1% for Brachionus-Low and 9.4% for Brachionus-High. A second experiment compared the Brachionus-High diet (9.4% DHA in PL) and cultivated nauplii of Acartia tonsa that contained 30% DHA of PL-fatty acids. The total lipid per fry dry weight (DW) and the lipid class composition were similar for all larval groups at 17 days post-hatching (dph). The %DHA of total fatty acids in PL of newly hatched larvae (2 dph) was 26%, and the values for fed groups varied from 17 to 30% DHA of PL fatty acids. The dominant fatty acids in the PL of larvae and PL of the diets were well correlated (r2 = 0.84, p < 0.05), specifically for %DHA (r2 = 0.90). The %DHA in larval PL was also significantly positively correlated to larval DW at 17 dph, whereas the relationship to survival was positive, but not significantly correlated across the three diets. We conclude that cod larvae exhibited a low capacity for de novo PL synthesis based on dietary TAG, the normal pathway of adult fish and humans. The 17 dph cod larvae instead synthesised their PL through re-acylation of digested dietary PL. It then follows that only a dietary PL composition close to the PL composition of larval cod, like in PL of many copepods, can secure high DHA incorporation in larval PL, and ultimately high larval growth and survival. We suggest that the DHA requirement of cod larvae cannot be easily met with B. plicatilis Nevada and call for new approaches of larval feeding.