W. Koven, Y.Y. Chen, M. Harel, P. Behrens, A. Tandler
The effect of dietary ratios of docosahexaenoic acid (22:6n-3, DHA) and arachidonic acid (20:4n-6, AA) on the growth and lipid composition of 22-34 day old gilthead seabream larvae was tested in triplicate in a 3 x 3 factorial aquarium experiment. The experimental setup consisted of twenty-seven 17 L aquariums stocked with 300 larvae/aquarium and equipped with a flow through system providing temperature controlled filtered 25 ppt sea water. Nine lipid emulsion enrichments treatments, containing various ratios of olive oil, high (40%) docosahexaenoic acid (DHA) oil and high (40%) arachidonic acid (AA) oil, were prepared so that three levels of the DHA oil (0, 25 and 50% of total lipid) were tested together with three levels of the AA oil (0, 25 and 50% of total lipid). The nine emulsion treatments were used to enrich one day old Artemia nauplii which were then fed to the larvae three times/day for twelve days.
The DHA:AA ratios in the emulsion and corresponding Artemia treatments were generally preserved and clearly reflected in the larval total lipid DHA:AA ratios. In addition, the DHA:EPA ratios in the larval lipid increased with higher dietary AA suggesting that increasing levels of dietary AA selectively replaced EPA in the larval tissues. Incremental dietary AA, in the absence of DHA, did not promote growth but did appear to enhance survival. Larval weight increased (p<0.05) with increasing dietary DHA within the 0% and 25% AA group but larval growth was less responsive (p>0.05) to incremental dietary DHA in the 50% AA group. This suggests that elevated levels of incorporated AA were depressing the growth promoting effects of increasing levels of DHA. This argument was also supported by the observation that although the 50% DHA oil:50% AA oil larvae exhibited the highest (p<0.05) DHA:EPA ratios, they showed poorer (p<0.05) growth than larvae fed the 50% DHA oil:0% AA oil and 50% DHA oil:25% AA oil treatments. On the other hand, larvae from the 50% DHA:25% AA treatment demonstrated the highest larval weight gain.
Dietary DHA:AA ratios of 1 or 0.5 showed the best survival but had an inhibitory effect on growth despite high larval DHA:EPA levels. On the other hand, the results implied that a dietary DHA:AA ratio of 2 promotes growth in larvae with sufficiently high lipid DHA:EPA ratios. The conclusions of the present study support a growing consensus among workers in the field that larval lipid DHA:EPA:AA ratios are more indicative of the nutritional status of the larvae of marine species than larval lipid DHA:EPA ratios.
(National Center for Mariculture, Israel Oceanographic and Limnological Research Ltd., P.O.B. 1212, Eilat 88112, Israel)